| Message ID | 20191211165651.7889-2-maz@kernel.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | KVM: arm/arm64: user_mem_abort() assorted fixes | expand |
Hi, On 12/11/19 4:56 PM, Marc Zyngier wrote: > A device mapping is normally always mapped at Stage-2, since there > is very little gain in having it faulted in. > > Nonetheless, it is possible to end-up in a situation where the device > mapping has been removed from Stage-2 (userspace munmaped the VFIO > region, and the MMU notifier did its job), but present in a userspace > mapping (userpace has mapped it back at the same address). In such > a situation, the device mapping will be demand-paged as the guest > performs memory accesses. > > This requires to be careful when dealing with mapping size, cache > management, and to handle potential execution of a device mapping. > > Cc: stable@vger.kernel.org > Reported-by: Alexandru Elisei <alexandru.elisei@arm.com> > Signed-off-by: Marc Zyngier <maz@kernel.org> > --- > virt/kvm/arm/mmu.c | 21 +++++++++++++++++---- > 1 file changed, 17 insertions(+), 4 deletions(-) > > diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c > index a48994af70b8..0b32a904a1bb 100644 > --- a/virt/kvm/arm/mmu.c > +++ b/virt/kvm/arm/mmu.c > @@ -38,6 +38,11 @@ static unsigned long io_map_base; > #define KVM_S2PTE_FLAG_IS_IOMAP (1UL << 0) > #define KVM_S2_FLAG_LOGGING_ACTIVE (1UL << 1) > > +static bool is_iomap(unsigned long flags) > +{ > + return flags & KVM_S2PTE_FLAG_IS_IOMAP; > +} > + > static bool memslot_is_logging(struct kvm_memory_slot *memslot) > { > return memslot->dirty_bitmap && !(memslot->flags & KVM_MEM_READONLY); > @@ -1698,6 +1703,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa, > > vma_pagesize = vma_kernel_pagesize(vma); > if (logging_active || > + (vma->vm_flags & VM_PFNMAP) || > !fault_supports_stage2_huge_mapping(memslot, hva, vma_pagesize)) { > force_pte = true; > vma_pagesize = PAGE_SIZE; > @@ -1760,6 +1766,9 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa, > writable = false; > } > > + if (exec_fault && is_iomap(flags)) > + return -ENOEXEC; > + > spin_lock(&kvm->mmu_lock); > if (mmu_notifier_retry(kvm, mmu_seq)) > goto out_unlock; > @@ -1781,7 +1790,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa, > if (writable) > kvm_set_pfn_dirty(pfn); > > - if (fault_status != FSC_PERM) > + if (fault_status != FSC_PERM && !is_iomap(flags)) > clean_dcache_guest_page(pfn, vma_pagesize); > > if (exec_fault) > @@ -1948,9 +1957,8 @@ int kvm_handle_guest_abort(struct kvm_vcpu *vcpu, struct kvm_run *run) > if (kvm_is_error_hva(hva) || (write_fault && !writable)) { > if (is_iabt) { > /* Prefetch Abort on I/O address */ > - kvm_inject_pabt(vcpu, kvm_vcpu_get_hfar(vcpu)); > - ret = 1; > - goto out_unlock; > + ret = -ENOEXEC; > + goto out; > } > > /* > @@ -1992,6 +2000,11 @@ int kvm_handle_guest_abort(struct kvm_vcpu *vcpu, struct kvm_run *run) > ret = user_mem_abort(vcpu, fault_ipa, memslot, hva, fault_status); > if (ret == 0) > ret = 1; > +out: > + if (ret == -ENOEXEC) { > + kvm_inject_pabt(vcpu, kvm_vcpu_get_hfar(vcpu)); > + ret = 1; > + } > out_unlock: > srcu_read_unlock(&vcpu->kvm->srcu, idx); > return ret; I've tested this patch using the scenario that you described in the commit message. I've also tested it with the following scenarios: - The region is mmap'ed as backed by a VFIO device fd and the memslot is created, it is munmap'ed, then mmap'ed as anonymous. - The region is mmap'ed as anonymous and the memslot is created, it is munmap'ed, then mmap'ed as backed by a VFIO device fd. Everything worked as expected, so: Tested-by: Alexandru Elisei <alexandru.elisei@arm.com> Just a nitpick, but stage2_set_pte has a local variable iomap which can be replaced with a call to is_iomap. Thanks, Alex
Hi Alex, On 2019-12-11 17:53, Alexandru Elisei wrote: > Hi, > > On 12/11/19 4:56 PM, Marc Zyngier wrote: >> A device mapping is normally always mapped at Stage-2, since there >> is very little gain in having it faulted in. >> >> Nonetheless, it is possible to end-up in a situation where the >> device >> mapping has been removed from Stage-2 (userspace munmaped the VFIO >> region, and the MMU notifier did its job), but present in a >> userspace >> mapping (userpace has mapped it back at the same address). In such >> a situation, the device mapping will be demand-paged as the guest >> performs memory accesses. >> >> This requires to be careful when dealing with mapping size, cache >> management, and to handle potential execution of a device mapping. >> >> Cc: stable@vger.kernel.org >> Reported-by: Alexandru Elisei <alexandru.elisei@arm.com> >> Signed-off-by: Marc Zyngier <maz@kernel.org> >> --- >> virt/kvm/arm/mmu.c | 21 +++++++++++++++++---- >> 1 file changed, 17 insertions(+), 4 deletions(-) [...] > I've tested this patch using the scenario that you described in the > commit > message. I've also tested it with the following scenarios: > > - The region is mmap'ed as backed by a VFIO device fd and the memslot > is created, > it is munmap'ed, then mmap'ed as anonymous. > - The region is mmap'ed as anonymous and the memslot is created, it > is munmap'ed, > then mmap'ed as backed by a VFIO device fd. > > Everything worked as expected, so: > > Tested-by: Alexandru Elisei <alexandru.elisei@arm.com> Thanks for that! > Just a nitpick, but stage2_set_pte has a local variable iomap which > can be > replaced with a call to is_iomap. Yeah, I noticed. I'm just trying to keep the patch relatively small so that it can be easily backported. The cleanup can come later! ;-) Cheers, M.
Hi Marc, On 11/12/2019 16:56, Marc Zyngier wrote: > A device mapping is normally always mapped at Stage-2, since there > is very little gain in having it faulted in. > > Nonetheless, it is possible to end-up in a situation where the device > mapping has been removed from Stage-2 (userspace munmaped the VFIO > region, and the MMU notifier did its job), but present in a userspace > mapping (userpace has mapped it back at the same address). In such > a situation, the device mapping will be demand-paged as the guest > performs memory accesses. > > This requires to be careful when dealing with mapping size, cache > management, and to handle potential execution of a device mapping. Reviewed-by: James Morse <james.morse@arm.com> Thanks, James
Hi Marc, On Wed, Dec 11, 2019 at 04:56:48PM +0000, Marc Zyngier wrote: > A device mapping is normally always mapped at Stage-2, since there > is very little gain in having it faulted in. It is actually becoming less clear to me what the real benefits of pre-populating the stage 2 page table are, especially given that we can provoke a situation where they're faulted in anyhow. Do you recall if we had any specific case that motivated us to pre-fault in the pages? > > Nonetheless, it is possible to end-up in a situation where the device > mapping has been removed from Stage-2 (userspace munmaped the VFIO > region, and the MMU notifier did its job), but present in a userspace > mapping (userpace has mapped it back at the same address). In such > a situation, the device mapping will be demand-paged as the guest > performs memory accesses. > > This requires to be careful when dealing with mapping size, cache > management, and to handle potential execution of a device mapping. > > Cc: stable@vger.kernel.org > Reported-by: Alexandru Elisei <alexandru.elisei@arm.com> > Signed-off-by: Marc Zyngier <maz@kernel.org> > --- > virt/kvm/arm/mmu.c | 21 +++++++++++++++++---- > 1 file changed, 17 insertions(+), 4 deletions(-) > > diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c > index a48994af70b8..0b32a904a1bb 100644 > --- a/virt/kvm/arm/mmu.c > +++ b/virt/kvm/arm/mmu.c > @@ -38,6 +38,11 @@ static unsigned long io_map_base; > #define KVM_S2PTE_FLAG_IS_IOMAP (1UL << 0) > #define KVM_S2_FLAG_LOGGING_ACTIVE (1UL << 1) > > +static bool is_iomap(unsigned long flags) > +{ > + return flags & KVM_S2PTE_FLAG_IS_IOMAP; > +} > + nit: I'm not really sure this indirection makes the code more readable, but I guess that's a matter of taste. > static bool memslot_is_logging(struct kvm_memory_slot *memslot) > { > return memslot->dirty_bitmap && !(memslot->flags & KVM_MEM_READONLY); > @@ -1698,6 +1703,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa, > > vma_pagesize = vma_kernel_pagesize(vma); > if (logging_active || > + (vma->vm_flags & VM_PFNMAP) || WHat is actually the rationale for this? Why is a huge mapping not permitted to device memory? Are we guaranteed that VM_PFNMAP on the vma results in device mappings? I'm not convinced this is the case, and it would be better if we can stick to a single primitive (either kvm_is_device_pfn, or VM_PFNMAP) to detect device mappings. As a subsequent patch, I'd like to make sure that at the very least our memslot prepare function follows the exact same logic for mapping device memory as a fault-in approach does, or that we simply always fault pages in. > !fault_supports_stage2_huge_mapping(memslot, hva, vma_pagesize)) { > force_pte = true; > vma_pagesize = PAGE_SIZE; > @@ -1760,6 +1766,9 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa, > writable = false; > } > > + if (exec_fault && is_iomap(flags)) > + return -ENOEXEC; > + nit: why don't you just do this when checking kvm_is_device_pfn() and avoid having logic in two places to deal with this case? > spin_lock(&kvm->mmu_lock); > if (mmu_notifier_retry(kvm, mmu_seq)) > goto out_unlock; > @@ -1781,7 +1790,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa, > if (writable) > kvm_set_pfn_dirty(pfn); > > - if (fault_status != FSC_PERM) > + if (fault_status != FSC_PERM && !is_iomap(flags)) > clean_dcache_guest_page(pfn, vma_pagesize); > > if (exec_fault) > @@ -1948,9 +1957,8 @@ int kvm_handle_guest_abort(struct kvm_vcpu *vcpu, struct kvm_run *run) > if (kvm_is_error_hva(hva) || (write_fault && !writable)) { > if (is_iabt) { > /* Prefetch Abort on I/O address */ > - kvm_inject_pabt(vcpu, kvm_vcpu_get_hfar(vcpu)); > - ret = 1; > - goto out_unlock; > + ret = -ENOEXEC; > + goto out; > } > > /* > @@ -1992,6 +2000,11 @@ int kvm_handle_guest_abort(struct kvm_vcpu *vcpu, struct kvm_run *run) > ret = user_mem_abort(vcpu, fault_ipa, memslot, hva, fault_status); > if (ret == 0) > ret = 1; > +out: > + if (ret == -ENOEXEC) { > + kvm_inject_pabt(vcpu, kvm_vcpu_get_hfar(vcpu)); > + ret = 1; > + } > out_unlock: > srcu_read_unlock(&vcpu->kvm->srcu, idx); > return ret; > -- > 2.20.1 > I can't seem to decide for myself if I think there's a sematic difference between trying to execute from somewhere the VMM has explicitly told us is device memory and from somewhere which we happen to have mapped with VM_PFNMAP from user space. But I also can't seem to really fault it (pun intended). Thoughts? Thanks, Christoffer
Hi Christoffer, On 2019-12-13 08:29, Christoffer Dall wrote: > Hi Marc, > > On Wed, Dec 11, 2019 at 04:56:48PM +0000, Marc Zyngier wrote: >> A device mapping is normally always mapped at Stage-2, since there >> is very little gain in having it faulted in. > > It is actually becoming less clear to me what the real benefits of > pre-populating the stage 2 page table are, especially given that we > can > provoke a situation where they're faulted in anyhow. Do you recall > if > we had any specific case that motivated us to pre-fault in the pages? It's only a minor performance optimization that was introduced by Ard in 8eef91239e57d. Which makes sense for platform devices that have a single fixed location in memory. It makes slightly less sense for PCI, where you can move things around. >> Nonetheless, it is possible to end-up in a situation where the >> device >> mapping has been removed from Stage-2 (userspace munmaped the VFIO >> region, and the MMU notifier did its job), but present in a >> userspace >> mapping (userpace has mapped it back at the same address). In such >> a situation, the device mapping will be demand-paged as the guest >> performs memory accesses. >> >> This requires to be careful when dealing with mapping size, cache >> management, and to handle potential execution of a device mapping. >> >> Cc: stable@vger.kernel.org >> Reported-by: Alexandru Elisei <alexandru.elisei@arm.com> >> Signed-off-by: Marc Zyngier <maz@kernel.org> >> --- >> virt/kvm/arm/mmu.c | 21 +++++++++++++++++---- >> 1 file changed, 17 insertions(+), 4 deletions(-) >> >> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c >> index a48994af70b8..0b32a904a1bb 100644 >> --- a/virt/kvm/arm/mmu.c >> +++ b/virt/kvm/arm/mmu.c >> @@ -38,6 +38,11 @@ static unsigned long io_map_base; >> #define KVM_S2PTE_FLAG_IS_IOMAP (1UL << 0) >> #define KVM_S2_FLAG_LOGGING_ACTIVE (1UL << 1) >> >> +static bool is_iomap(unsigned long flags) >> +{ >> + return flags & KVM_S2PTE_FLAG_IS_IOMAP; >> +} >> + > > nit: I'm not really sure this indirection makes the code more > readable, > but I guess that's a matter of taste. > >> static bool memslot_is_logging(struct kvm_memory_slot *memslot) >> { >> return memslot->dirty_bitmap && !(memslot->flags & >> KVM_MEM_READONLY); >> @@ -1698,6 +1703,7 @@ static int user_mem_abort(struct kvm_vcpu >> *vcpu, phys_addr_t fault_ipa, >> >> vma_pagesize = vma_kernel_pagesize(vma); >> if (logging_active || >> + (vma->vm_flags & VM_PFNMAP) || > > WHat is actually the rationale for this? > > Why is a huge mapping not permitted to device memory? > > Are we guaranteed that VM_PFNMAP on the vma results in device > mappings? > I'm not convinced this is the case, and it would be better if we can > stick to a single primitive (either kvm_is_device_pfn, or VM_PFNMAP) > to > detect device mappings. For now, I've tried to keep the two paths that deal with mapping devices (or rather, things that we interpret as devices) as close as possible. If we drop the "eager" mapping, then we're at liberty to restructure this in creative ways. This includes potential huge mappings, but I'm not sure the rest of the kernel uses them for devices anyway (I need to find out). > As a subsequent patch, I'd like to make sure that at the very least > our > memslot prepare function follows the exact same logic for mapping > device > memory as a fault-in approach does, or that we simply always fault > pages > in. As far as I can see, the two approach are now identical. Am I missing something? And yes, getting rid of the eager mapping works for me. > >> !fault_supports_stage2_huge_mapping(memslot, hva, >> vma_pagesize)) { >> force_pte = true; >> vma_pagesize = PAGE_SIZE; >> @@ -1760,6 +1766,9 @@ static int user_mem_abort(struct kvm_vcpu >> *vcpu, phys_addr_t fault_ipa, >> writable = false; >> } >> >> + if (exec_fault && is_iomap(flags)) >> + return -ENOEXEC; >> + > > nit: why don't you just do this when checking kvm_is_device_pfn() and > avoid having logic in two places to deal with this case? Good point. I've already sent the PR, but that could be a further cleanup. > >> spin_lock(&kvm->mmu_lock); >> if (mmu_notifier_retry(kvm, mmu_seq)) >> goto out_unlock; >> @@ -1781,7 +1790,7 @@ static int user_mem_abort(struct kvm_vcpu >> *vcpu, phys_addr_t fault_ipa, >> if (writable) >> kvm_set_pfn_dirty(pfn); >> >> - if (fault_status != FSC_PERM) >> + if (fault_status != FSC_PERM && !is_iomap(flags)) >> clean_dcache_guest_page(pfn, vma_pagesize); >> >> if (exec_fault) >> @@ -1948,9 +1957,8 @@ int kvm_handle_guest_abort(struct kvm_vcpu >> *vcpu, struct kvm_run *run) >> if (kvm_is_error_hva(hva) || (write_fault && !writable)) { >> if (is_iabt) { >> /* Prefetch Abort on I/O address */ >> - kvm_inject_pabt(vcpu, kvm_vcpu_get_hfar(vcpu)); >> - ret = 1; >> - goto out_unlock; >> + ret = -ENOEXEC; >> + goto out; >> } >> >> /* >> @@ -1992,6 +2000,11 @@ int kvm_handle_guest_abort(struct kvm_vcpu >> *vcpu, struct kvm_run *run) >> ret = user_mem_abort(vcpu, fault_ipa, memslot, hva, fault_status); >> if (ret == 0) >> ret = 1; >> +out: >> + if (ret == -ENOEXEC) { >> + kvm_inject_pabt(vcpu, kvm_vcpu_get_hfar(vcpu)); >> + ret = 1; >> + } >> out_unlock: >> srcu_read_unlock(&vcpu->kvm->srcu, idx); >> return ret; >> -- >> 2.20.1 >> > > I can't seem to decide for myself if I think there's a sematic > difference between trying to execute from somewhere the VMM has > explicitly told us is device memory and from somewhere which we > happen > to have mapped with VM_PFNMAP from user space. But I also can't seem > to > really fault it (pun intended). Thoughts? The issue is that the VMM never really tells us whether something is a device mapping or not (the only exception being the GICv2 cpuif). Even with PFNMAP, we guess it (it could well be memory that lives outside of the linear mapping). I don't see a way to lift this ambiguity. Ideally, faulting on executing a non-mapping should be offloaded to userspace for emulation, in line with your patches that offload non-emulated data accesses. That'd be a new ABI, and I can't imagine anyone willing to deal with it. Thanks, M.
On Fri, Dec 13, 2019 at 09:28:59AM +0000, Marc Zyngier wrote: > Hi Christoffer, > > On 2019-12-13 08:29, Christoffer Dall wrote: > > Hi Marc, > > > > On Wed, Dec 11, 2019 at 04:56:48PM +0000, Marc Zyngier wrote: > > > A device mapping is normally always mapped at Stage-2, since there > > > is very little gain in having it faulted in. > > > > It is actually becoming less clear to me what the real benefits of > > pre-populating the stage 2 page table are, especially given that we can > > provoke a situation where they're faulted in anyhow. Do you recall if > > we had any specific case that motivated us to pre-fault in the pages? > > It's only a minor performance optimization that was introduced by Ard in > 8eef91239e57d. Which makes sense for platform devices that have a single > fixed location in memory. It makes slightly less sense for PCI, where > you can move things around. User space could still decide to move things around in its VA map even if the device is fixed. Anyway, I was thinking more if there was some sort of device, like a frambuffer, which for example crosses page boundaries and where it would be visible to the user that there's a sudden performance drop while operating the device over page boundaries. Anything like that? > > > > Nonetheless, it is possible to end-up in a situation where the > > > device > > > mapping has been removed from Stage-2 (userspace munmaped the VFIO > > > region, and the MMU notifier did its job), but present in a > > > userspace > > > mapping (userpace has mapped it back at the same address). In such > > > a situation, the device mapping will be demand-paged as the guest > > > performs memory accesses. > > > > > > This requires to be careful when dealing with mapping size, cache > > > management, and to handle potential execution of a device mapping. > > > > > > Cc: stable@vger.kernel.org > > > Reported-by: Alexandru Elisei <alexandru.elisei@arm.com> > > > Signed-off-by: Marc Zyngier <maz@kernel.org> > > > --- > > > virt/kvm/arm/mmu.c | 21 +++++++++++++++++---- > > > 1 file changed, 17 insertions(+), 4 deletions(-) > > > > > > diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c > > > index a48994af70b8..0b32a904a1bb 100644 > > > --- a/virt/kvm/arm/mmu.c > > > +++ b/virt/kvm/arm/mmu.c > > > @@ -38,6 +38,11 @@ static unsigned long io_map_base; > > > #define KVM_S2PTE_FLAG_IS_IOMAP (1UL << 0) > > > #define KVM_S2_FLAG_LOGGING_ACTIVE (1UL << 1) > > > > > > +static bool is_iomap(unsigned long flags) > > > +{ > > > + return flags & KVM_S2PTE_FLAG_IS_IOMAP; > > > +} > > > + > > > > nit: I'm not really sure this indirection makes the code more readable, > > but I guess that's a matter of taste. > > > > > static bool memslot_is_logging(struct kvm_memory_slot *memslot) > > > { > > > return memslot->dirty_bitmap && !(memslot->flags & > > > KVM_MEM_READONLY); > > > @@ -1698,6 +1703,7 @@ static int user_mem_abort(struct kvm_vcpu > > > *vcpu, phys_addr_t fault_ipa, > > > > > > vma_pagesize = vma_kernel_pagesize(vma); > > > if (logging_active || > > > + (vma->vm_flags & VM_PFNMAP) || > > > > WHat is actually the rationale for this? > > > > Why is a huge mapping not permitted to device memory? > > > > Are we guaranteed that VM_PFNMAP on the vma results in device mappings? > > I'm not convinced this is the case, and it would be better if we can > > stick to a single primitive (either kvm_is_device_pfn, or VM_PFNMAP) to > > detect device mappings. > > For now, I've tried to keep the two paths that deal with mapping devices > (or rather, things that we interpret as devices) as close as possible. > If we drop the "eager" mapping, then we're at liberty to restructure > this in creative ways. > > This includes potential huge mappings, but I'm not sure the rest of the > kernel uses them for devices anyway (I need to find out). > > > As a subsequent patch, I'd like to make sure that at the very least our > > memslot prepare function follows the exact same logic for mapping device > > memory as a fault-in approach does, or that we simply always fault pages > > in. > > As far as I can see, the two approach are now identical. Am I missing > something? > And yes, getting rid of the eager mapping works for me. > As far as I can tell, our user_mem_abort() uses gfn_to_pfn_prot() which goes doesn a long trail which ends up at hva_to_pfn_remapped(), which might result in doing the same offset calculation that we do in kvm_arch_prepare_memory_region(), but it also considers other scenarios. Even if we analyze all that and convince oursleves it's always all the same on arm64, the two code paths could change, leading to really hard to debug differing behavior, and nobody will actively keep the two paths in sync. I'd be fine with keeping the performance optimization if we have good grounds for that though, and using the same translation mechanism for VM_PFNMAP as user_mem_abort. Am I missing something? > > > > > !fault_supports_stage2_huge_mapping(memslot, hva, > > > vma_pagesize)) { > > > force_pte = true; > > > vma_pagesize = PAGE_SIZE; > > > @@ -1760,6 +1766,9 @@ static int user_mem_abort(struct kvm_vcpu > > > *vcpu, phys_addr_t fault_ipa, > > > writable = false; > > > } > > > > > > + if (exec_fault && is_iomap(flags)) > > > + return -ENOEXEC; > > > + > > > > nit: why don't you just do this when checking kvm_is_device_pfn() and > > avoid having logic in two places to deal with this case? > > Good point. I've already sent the PR, but that could be a further cleanup. > Sure, I can have a look when we agree on the above. > > > > > spin_lock(&kvm->mmu_lock); > > > if (mmu_notifier_retry(kvm, mmu_seq)) > > > goto out_unlock; > > > @@ -1781,7 +1790,7 @@ static int user_mem_abort(struct kvm_vcpu > > > *vcpu, phys_addr_t fault_ipa, > > > if (writable) > > > kvm_set_pfn_dirty(pfn); > > > > > > - if (fault_status != FSC_PERM) > > > + if (fault_status != FSC_PERM && !is_iomap(flags)) > > > clean_dcache_guest_page(pfn, vma_pagesize); > > > > > > if (exec_fault) > > > @@ -1948,9 +1957,8 @@ int kvm_handle_guest_abort(struct kvm_vcpu > > > *vcpu, struct kvm_run *run) > > > if (kvm_is_error_hva(hva) || (write_fault && !writable)) { > > > if (is_iabt) { > > > /* Prefetch Abort on I/O address */ > > > - kvm_inject_pabt(vcpu, kvm_vcpu_get_hfar(vcpu)); > > > - ret = 1; > > > - goto out_unlock; > > > + ret = -ENOEXEC; > > > + goto out; > > > } > > > > > > /* > > > @@ -1992,6 +2000,11 @@ int kvm_handle_guest_abort(struct kvm_vcpu > > > *vcpu, struct kvm_run *run) > > > ret = user_mem_abort(vcpu, fault_ipa, memslot, hva, fault_status); > > > if (ret == 0) > > > ret = 1; > > > +out: > > > + if (ret == -ENOEXEC) { > > > + kvm_inject_pabt(vcpu, kvm_vcpu_get_hfar(vcpu)); > > > + ret = 1; > > > + } > > > out_unlock: > > > srcu_read_unlock(&vcpu->kvm->srcu, idx); > > > return ret; > > > -- > > > 2.20.1 > > > > > > > I can't seem to decide for myself if I think there's a sematic > > difference between trying to execute from somewhere the VMM has > > explicitly told us is device memory and from somewhere which we happen > > to have mapped with VM_PFNMAP from user space. But I also can't seem to > > really fault it (pun intended). Thoughts? > > The issue is that the VMM never really tells us whether something is a > device mapping or not (the only exception being the GICv2 cpuif). Even > with PFNMAP, we guess it (it could well be memory that lives outside > of the linear mapping). I don't see a way to lift this ambiguity. > > Ideally, faulting on executing a non-mapping should be offloaded to > userspace for emulation, in line with your patches that offload > non-emulated data accesses. That'd be a new ABI, and I can't imagine > anyone willing to deal with it. So what I was asking was if it makes sense to report the Prefetch Abort in the case where the VMM has already told us that it doesn't want to register anything backing the IPA (no memslot), and instead return an error to user space, so that it can make a decision (for example inject an external abort, which may have been the right thing to do in the former case as well, but that could be considered ABI now, so let's not kick that hornet's nest). In any case, no strong feelings here, I just have a vague feeling that injecting more prefetch aborts on execute-from-some-device is not necessarily the right thing to do. Thanks, Christoffer
On 2019-12-13 11:14, Christoffer Dall wrote: > On Fri, Dec 13, 2019 at 09:28:59AM +0000, Marc Zyngier wrote: >> Hi Christoffer, >> >> On 2019-12-13 08:29, Christoffer Dall wrote: >> > Hi Marc, >> > >> > On Wed, Dec 11, 2019 at 04:56:48PM +0000, Marc Zyngier wrote: >> > > A device mapping is normally always mapped at Stage-2, since >> there >> > > is very little gain in having it faulted in. >> > >> > It is actually becoming less clear to me what the real benefits of >> > pre-populating the stage 2 page table are, especially given that >> we can >> > provoke a situation where they're faulted in anyhow. Do you >> recall if >> > we had any specific case that motivated us to pre-fault in the >> pages? >> >> It's only a minor performance optimization that was introduced by >> Ard in >> 8eef91239e57d. Which makes sense for platform devices that have a >> single >> fixed location in memory. It makes slightly less sense for PCI, >> where >> you can move things around. > > User space could still decide to move things around in its VA map > even > if the device is fixed. > > Anyway, I was thinking more if there was some sort of device, like a > frambuffer, which for example crosses page boundaries and where it > would > be visible to the user that there's a sudden performance drop while > operating the device over page boundaries. Anything like that? > >> >> > > Nonetheless, it is possible to end-up in a situation where the >> > > device >> > > mapping has been removed from Stage-2 (userspace munmaped the >> VFIO >> > > region, and the MMU notifier did its job), but present in a >> > > userspace >> > > mapping (userpace has mapped it back at the same address). In >> such >> > > a situation, the device mapping will be demand-paged as the >> guest >> > > performs memory accesses. >> > > >> > > This requires to be careful when dealing with mapping size, >> cache >> > > management, and to handle potential execution of a device >> mapping. >> > > >> > > Cc: stable@vger.kernel.org >> > > Reported-by: Alexandru Elisei <alexandru.elisei@arm.com> >> > > Signed-off-by: Marc Zyngier <maz@kernel.org> >> > > --- >> > > virt/kvm/arm/mmu.c | 21 +++++++++++++++++---- >> > > 1 file changed, 17 insertions(+), 4 deletions(-) >> > > >> > > diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c >> > > index a48994af70b8..0b32a904a1bb 100644 >> > > --- a/virt/kvm/arm/mmu.c >> > > +++ b/virt/kvm/arm/mmu.c >> > > @@ -38,6 +38,11 @@ static unsigned long io_map_base; >> > > #define KVM_S2PTE_FLAG_IS_IOMAP (1UL << 0) >> > > #define KVM_S2_FLAG_LOGGING_ACTIVE (1UL << 1) >> > > >> > > +static bool is_iomap(unsigned long flags) >> > > +{ >> > > + return flags & KVM_S2PTE_FLAG_IS_IOMAP; >> > > +} >> > > + >> > >> > nit: I'm not really sure this indirection makes the code more >> readable, >> > but I guess that's a matter of taste. >> > >> > > static bool memslot_is_logging(struct kvm_memory_slot *memslot) >> > > { >> > > return memslot->dirty_bitmap && !(memslot->flags & >> > > KVM_MEM_READONLY); >> > > @@ -1698,6 +1703,7 @@ static int user_mem_abort(struct kvm_vcpu >> > > *vcpu, phys_addr_t fault_ipa, >> > > >> > > vma_pagesize = vma_kernel_pagesize(vma); >> > > if (logging_active || >> > > + (vma->vm_flags & VM_PFNMAP) || >> > >> > WHat is actually the rationale for this? >> > >> > Why is a huge mapping not permitted to device memory? >> > >> > Are we guaranteed that VM_PFNMAP on the vma results in device >> mappings? >> > I'm not convinced this is the case, and it would be better if we >> can >> > stick to a single primitive (either kvm_is_device_pfn, or >> VM_PFNMAP) to >> > detect device mappings. >> >> For now, I've tried to keep the two paths that deal with mapping >> devices >> (or rather, things that we interpret as devices) as close as >> possible. >> If we drop the "eager" mapping, then we're at liberty to restructure >> this in creative ways. >> >> This includes potential huge mappings, but I'm not sure the rest of >> the >> kernel uses them for devices anyway (I need to find out). >> >> > As a subsequent patch, I'd like to make sure that at the very >> least our >> > memslot prepare function follows the exact same logic for mapping >> device >> > memory as a fault-in approach does, or that we simply always fault >> pages >> > in. >> >> As far as I can see, the two approach are now identical. Am I >> missing >> something? >> And yes, getting rid of the eager mapping works for me. >> > > As far as I can tell, our user_mem_abort() uses gfn_to_pfn_prot() > which > goes doesn a long trail which ends up at hva_to_pfn_remapped(), which > might result in doing the same offset calculation that we do in > kvm_arch_prepare_memory_region(), but it also considers other > scenarios. > > Even if we analyze all that and convince oursleves it's always all > the > same on arm64, the two code paths could change, leading to really > hard > to debug differing behavior, and nobody will actively keep the two > paths > in sync. I'd be fine with keeping the performance optimization if we > have good grounds for that though, and using the same translation > mechanism for VM_PFNMAP as user_mem_abort. > > Am I missing something? I'm not disputing any of the above. I'm only trying to keep this patch minimal so that we can easily backport it (although it is arguable that deleting code isn't that big a deal). [...] >> > I can't seem to decide for myself if I think there's a sematic >> > difference between trying to execute from somewhere the VMM has >> > explicitly told us is device memory and from somewhere which we >> happen >> > to have mapped with VM_PFNMAP from user space. But I also can't >> seem to >> > really fault it (pun intended). Thoughts? >> >> The issue is that the VMM never really tells us whether something is >> a >> device mapping or not (the only exception being the GICv2 cpuif). >> Even >> with PFNMAP, we guess it (it could well be memory that lives outside >> of the linear mapping). I don't see a way to lift this ambiguity. >> >> Ideally, faulting on executing a non-mapping should be offloaded to >> userspace for emulation, in line with your patches that offload >> non-emulated data accesses. That'd be a new ABI, and I can't imagine >> anyone willing to deal with it. > > So what I was asking was if it makes sense to report the Prefetch > Abort > in the case where the VMM has already told us that it doesn't want to > register anything backing the IPA (no memslot), and instead return an > error to user space, so that it can make a decision (for example > inject > an external abort, which may have been the right thing to do in the > former case as well, but that could be considered ABI now, so let's > not > kick that hornet's nest). > > In any case, no strong feelings here, I just have a vague feeling > that > injecting more prefetch aborts on execute-from-some-device is not > necessarily the right thing to do. The ARMv8 ARM has the following stuff in B2.7.2 (Device Memory): <quote> Hardware does not prevent speculative instruction fetches from a memory location with any of the Device memory attributes unless the memory location is also marked as Execute-never for all Exception levels. Note This means that to prevent speculative instruction fetches from memory locations with Device memory attributes, any location that is assigned any Device memory type must also be marked as Execute-never for all Exception levels. Failure to mark a memory location with any Device memory attribute as Execute-never for all Exception levels is a programming error. </quote> and <quote> For instruction fetches, if branches cause the program counter to point to an area of memory with the Device attribute which is not marked as Execute-never for the current Exception level, an implementation can either: - Treat the instruction fetch as if it were to a memory location with the Normal Non-cacheable attribute. - Take a Permission fault. </quote> My reading here is that a prefetch abort is the right thing to do. What we don't do correctly is that we qualify it as an external abort instead of a permission fault (which is annoying as it requires us to find out about the S1 translation level). Happy to revisit this once we get a S1 PTW. M.
On Mon, Dec 16, 2019 at 10:31:19AM +0000, Marc Zyngier wrote: > On 2019-12-13 11:14, Christoffer Dall wrote: > > On Fri, Dec 13, 2019 at 09:28:59AM +0000, Marc Zyngier wrote: > > > Hi Christoffer, > > > > > > On 2019-12-13 08:29, Christoffer Dall wrote: > > > > Hi Marc, > > > > > > > > On Wed, Dec 11, 2019 at 04:56:48PM +0000, Marc Zyngier wrote: > > > > > A device mapping is normally always mapped at Stage-2, since > > > there > > > > > is very little gain in having it faulted in. > > > > > > > > It is actually becoming less clear to me what the real benefits of > > > > pre-populating the stage 2 page table are, especially given that > > > we can > > > > provoke a situation where they're faulted in anyhow. Do you > > > recall if > > > > we had any specific case that motivated us to pre-fault in the > > > pages? > > > > > > It's only a minor performance optimization that was introduced by > > > Ard in > > > 8eef91239e57d. Which makes sense for platform devices that have a > > > single > > > fixed location in memory. It makes slightly less sense for PCI, > > > where > > > you can move things around. > > > > User space could still decide to move things around in its VA map even > > if the device is fixed. > > > > Anyway, I was thinking more if there was some sort of device, like a > > frambuffer, which for example crosses page boundaries and where it would > > be visible to the user that there's a sudden performance drop while > > operating the device over page boundaries. Anything like that? > > > > > > > > > > Nonetheless, it is possible to end-up in a situation where the > > > > > device > > > > > mapping has been removed from Stage-2 (userspace munmaped the > > > VFIO > > > > > region, and the MMU notifier did its job), but present in a > > > > > userspace > > > > > mapping (userpace has mapped it back at the same address). In > > > such > > > > > a situation, the device mapping will be demand-paged as the > > > guest > > > > > performs memory accesses. > > > > > > > > > > This requires to be careful when dealing with mapping size, > > > cache > > > > > management, and to handle potential execution of a device > > > mapping. > > > > > > > > > > Cc: stable@vger.kernel.org > > > > > Reported-by: Alexandru Elisei <alexandru.elisei@arm.com> > > > > > Signed-off-by: Marc Zyngier <maz@kernel.org> > > > > > --- > > > > > virt/kvm/arm/mmu.c | 21 +++++++++++++++++---- > > > > > 1 file changed, 17 insertions(+), 4 deletions(-) > > > > > > > > > > diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c > > > > > index a48994af70b8..0b32a904a1bb 100644 > > > > > --- a/virt/kvm/arm/mmu.c > > > > > +++ b/virt/kvm/arm/mmu.c > > > > > @@ -38,6 +38,11 @@ static unsigned long io_map_base; > > > > > #define KVM_S2PTE_FLAG_IS_IOMAP (1UL << 0) > > > > > #define KVM_S2_FLAG_LOGGING_ACTIVE (1UL << 1) > > > > > > > > > > +static bool is_iomap(unsigned long flags) > > > > > +{ > > > > > + return flags & KVM_S2PTE_FLAG_IS_IOMAP; > > > > > +} > > > > > + > > > > > > > > nit: I'm not really sure this indirection makes the code more > > > readable, > > > > but I guess that's a matter of taste. > > > > > > > > > static bool memslot_is_logging(struct kvm_memory_slot *memslot) > > > > > { > > > > > return memslot->dirty_bitmap && !(memslot->flags & > > > > > KVM_MEM_READONLY); > > > > > @@ -1698,6 +1703,7 @@ static int user_mem_abort(struct kvm_vcpu > > > > > *vcpu, phys_addr_t fault_ipa, > > > > > > > > > > vma_pagesize = vma_kernel_pagesize(vma); > > > > > if (logging_active || > > > > > + (vma->vm_flags & VM_PFNMAP) || > > > > > > > > WHat is actually the rationale for this? > > > > > > > > Why is a huge mapping not permitted to device memory? > > > > > > > > Are we guaranteed that VM_PFNMAP on the vma results in device > > > mappings? > > > > I'm not convinced this is the case, and it would be better if we > > > can > > > > stick to a single primitive (either kvm_is_device_pfn, or > > > VM_PFNMAP) to > > > > detect device mappings. > > > > > > For now, I've tried to keep the two paths that deal with mapping > > > devices > > > (or rather, things that we interpret as devices) as close as > > > possible. > > > If we drop the "eager" mapping, then we're at liberty to restructure > > > this in creative ways. > > > > > > This includes potential huge mappings, but I'm not sure the rest of > > > the > > > kernel uses them for devices anyway (I need to find out). > > > > > > > As a subsequent patch, I'd like to make sure that at the very > > > least our > > > > memslot prepare function follows the exact same logic for mapping > > > device > > > > memory as a fault-in approach does, or that we simply always fault > > > pages > > > > in. > > > > > > As far as I can see, the two approach are now identical. Am I > > > missing > > > something? > > > And yes, getting rid of the eager mapping works for me. > > > > > > > As far as I can tell, our user_mem_abort() uses gfn_to_pfn_prot() which > > goes doesn a long trail which ends up at hva_to_pfn_remapped(), which > > might result in doing the same offset calculation that we do in > > kvm_arch_prepare_memory_region(), but it also considers other scenarios. > > > > Even if we analyze all that and convince oursleves it's always all the > > same on arm64, the two code paths could change, leading to really hard > > to debug differing behavior, and nobody will actively keep the two paths > > in sync. I'd be fine with keeping the performance optimization if we > > have good grounds for that though, and using the same translation > > mechanism for VM_PFNMAP as user_mem_abort. > > > > Am I missing something? > > I'm not disputing any of the above. I'm only trying to keep this patch > minimal so that we can easily backport it (although it is arguable that > deleting code isn't that big a deal). Yes, sorry, I wasn't arguing we should change the patch, only what the direction for the future should be. Sorry for being unclear. > > [...] > > > > > I can't seem to decide for myself if I think there's a sematic > > > > difference between trying to execute from somewhere the VMM has > > > > explicitly told us is device memory and from somewhere which we > > > happen > > > > to have mapped with VM_PFNMAP from user space. But I also can't > > > seem to > > > > really fault it (pun intended). Thoughts? > > > > > > The issue is that the VMM never really tells us whether something is > > > a > > > device mapping or not (the only exception being the GICv2 cpuif). > > > Even > > > with PFNMAP, we guess it (it could well be memory that lives outside > > > of the linear mapping). I don't see a way to lift this ambiguity. > > > > > > Ideally, faulting on executing a non-mapping should be offloaded to > > > userspace for emulation, in line with your patches that offload > > > non-emulated data accesses. That'd be a new ABI, and I can't imagine > > > anyone willing to deal with it. > > > > So what I was asking was if it makes sense to report the Prefetch Abort > > in the case where the VMM has already told us that it doesn't want to > > register anything backing the IPA (no memslot), and instead return an > > error to user space, so that it can make a decision (for example inject > > an external abort, which may have been the right thing to do in the > > former case as well, but that could be considered ABI now, so let's not > > kick that hornet's nest). > > > > In any case, no strong feelings here, I just have a vague feeling that > > injecting more prefetch aborts on execute-from-some-device is not > > necessarily the right thing to do. > > The ARMv8 ARM has the following stuff in B2.7.2 (Device Memory): > > <quote> > Hardware does not prevent speculative instruction fetches from a memory > location with any of the Device > memory attributes unless the memory location is also marked as Execute-never > for all Exception levels. > > Note > > This means that to prevent speculative instruction fetches from memory > locations with Device memory > attributes, any location that is assigned any Device memory type must also > be marked as Execute-never for > all Exception levels. Failure to mark a memory location with any Device > memory attribute as Execute-never > for all Exception levels is a programming error. > </quote> > > and > > <quote> > For instruction fetches, if branches cause the program counter to point to > an area of memory with the Device > attribute which is not marked as Execute-never for the current Exception > level, an implementation can either: > > - Treat the instruction fetch as if it were to a memory location with the > Normal Non-cacheable attribute. > > - Take a Permission fault. > </quote> > > My reading here is that a prefetch abort is the right thing to do. > What we don't do correctly is that we qualify it as an external abort > instead of a permission fault (which is annoying as it requires us > to find out about the S1 translation level). I did not remember we have the permission fault option as a valid implementation option. In that case, never mind my ramblings. Thanks, Christoffer
diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c index a48994af70b8..0b32a904a1bb 100644 --- a/virt/kvm/arm/mmu.c +++ b/virt/kvm/arm/mmu.c @@ -38,6 +38,11 @@ static unsigned long io_map_base; #define KVM_S2PTE_FLAG_IS_IOMAP (1UL << 0) #define KVM_S2_FLAG_LOGGING_ACTIVE (1UL << 1) +static bool is_iomap(unsigned long flags) +{ + return flags & KVM_S2PTE_FLAG_IS_IOMAP; +} + static bool memslot_is_logging(struct kvm_memory_slot *memslot) { return memslot->dirty_bitmap && !(memslot->flags & KVM_MEM_READONLY); @@ -1698,6 +1703,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa, vma_pagesize = vma_kernel_pagesize(vma); if (logging_active || + (vma->vm_flags & VM_PFNMAP) || !fault_supports_stage2_huge_mapping(memslot, hva, vma_pagesize)) { force_pte = true; vma_pagesize = PAGE_SIZE; @@ -1760,6 +1766,9 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa, writable = false; } + if (exec_fault && is_iomap(flags)) + return -ENOEXEC; + spin_lock(&kvm->mmu_lock); if (mmu_notifier_retry(kvm, mmu_seq)) goto out_unlock; @@ -1781,7 +1790,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa, if (writable) kvm_set_pfn_dirty(pfn); - if (fault_status != FSC_PERM) + if (fault_status != FSC_PERM && !is_iomap(flags)) clean_dcache_guest_page(pfn, vma_pagesize); if (exec_fault) @@ -1948,9 +1957,8 @@ int kvm_handle_guest_abort(struct kvm_vcpu *vcpu, struct kvm_run *run) if (kvm_is_error_hva(hva) || (write_fault && !writable)) { if (is_iabt) { /* Prefetch Abort on I/O address */ - kvm_inject_pabt(vcpu, kvm_vcpu_get_hfar(vcpu)); - ret = 1; - goto out_unlock; + ret = -ENOEXEC; + goto out; } /* @@ -1992,6 +2000,11 @@ int kvm_handle_guest_abort(struct kvm_vcpu *vcpu, struct kvm_run *run) ret = user_mem_abort(vcpu, fault_ipa, memslot, hva, fault_status); if (ret == 0) ret = 1; +out: + if (ret == -ENOEXEC) { + kvm_inject_pabt(vcpu, kvm_vcpu_get_hfar(vcpu)); + ret = 1; + } out_unlock: srcu_read_unlock(&vcpu->kvm->srcu, idx); return ret;
A device mapping is normally always mapped at Stage-2, since there is very little gain in having it faulted in. Nonetheless, it is possible to end-up in a situation where the device mapping has been removed from Stage-2 (userspace munmaped the VFIO region, and the MMU notifier did its job), but present in a userspace mapping (userpace has mapped it back at the same address). In such a situation, the device mapping will be demand-paged as the guest performs memory accesses. This requires to be careful when dealing with mapping size, cache management, and to handle potential execution of a device mapping. Cc: stable@vger.kernel.org Reported-by: Alexandru Elisei <alexandru.elisei@arm.com> Signed-off-by: Marc Zyngier <maz@kernel.org> --- virt/kvm/arm/mmu.c | 21 +++++++++++++++++---- 1 file changed, 17 insertions(+), 4 deletions(-)