| Message ID | 20240726235234.228822-35-seanjc@google.com (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | KVM: Stop grabbing references to PFNMAP'd pages | expand |
On 7/27/24 01:51, Sean Christopherson wrote: > Add a kvm_follow_pfn() wrapper, kvm_lookup_pfn(), to allow looking up a > gfn=>pfn mapping without the caller getting a reference to any underlying > page. The API will be used in flows that want to know if a gfn points at > a valid pfn, but don't actually need to do anything with the pfn. Can you rename the function kvm_gfn_has_pfn(), or kvm_gfn_can_be_mapped(), and make it return a bool? (As an aside, I wonder if reexecute_instruction() could just use kvm_is_error_hva(kvm_vcpu_gfn_to_hva(vcpu, gpa_to_gfn(gpa)) instead of going all the way to a pfn. But it's ok to be more restrictive). Paolo
On Tue, Jul 30, 2024, Paolo Bonzini wrote: > On 7/27/24 01:51, Sean Christopherson wrote: > > Add a kvm_follow_pfn() wrapper, kvm_lookup_pfn(), to allow looking up a > > gfn=>pfn mapping without the caller getting a reference to any underlying > > page. The API will be used in flows that want to know if a gfn points at > > a valid pfn, but don't actually need to do anything with the pfn. > > Can you rename the function kvm_gfn_has_pfn(), or kvm_gfn_can_be_mapped(), > and make it return a bool? Heh, sure. I initially planned on having it return a bool, but I couldn't figure out a name, mainly because the kernel's pfn_valid() makes things like kvm_gfn_has_valid_pfn() confusing/misleading :-( > (As an aside, I wonder if reexecute_instruction() could just use > kvm_is_error_hva(kvm_vcpu_gfn_to_hva(vcpu, gpa_to_gfn(gpa)) instead of going > all the way to a pfn. But it's ok to be more restrictive). Heh #2, I wondered the same thing. I think it would work? Verifying that there's a usable pfn also protects against retrying an access that hit -EHWPOISON, but I'm prety sure that would require a rare race, and I don't think it could result in the guest being put into an infinite loop.
On 7/30/24 22:15, Sean Christopherson wrote: > On Tue, Jul 30, 2024, Paolo Bonzini wrote: >> On 7/27/24 01:51, Sean Christopherson wrote: >>> Add a kvm_follow_pfn() wrapper, kvm_lookup_pfn(), to allow looking up a >>> gfn=>pfn mapping without the caller getting a reference to any underlying >>> page. The API will be used in flows that want to know if a gfn points at >>> a valid pfn, but don't actually need to do anything with the pfn. >> >> Can you rename the function kvm_gfn_has_pfn(), or kvm_gfn_can_be_mapped(), >> and make it return a bool? > > Heh, sure. I initially planned on having it return a bool, but I couldn't figure > out a name, mainly because the kernel's pfn_valid() makes things like > kvm_gfn_has_valid_pfn() confusing/misleading :-( > >> (As an aside, I wonder if reexecute_instruction() could just use >> kvm_is_error_hva(kvm_vcpu_gfn_to_hva(vcpu, gpa_to_gfn(gpa)) instead of going >> all the way to a pfn. But it's ok to be more restrictive). > > Heh #2, I wondered the same thing. I think it would work? Verifying that there's > a usable pfn also protects against retrying an access that hit -EHWPOISON, but I'm > prety sure that would require a rare race, and I don't think it could result in > the guest being put into an infinite loop. Indeed, and even the check in kvm_alloc_apic_access_page() is totally useless. The page can go away at any time between the call and vmx_set_apic_access_page_addr() or, for AMD, the #NPF on APIC_DEFAULT_PHYS_BASE. Yes, it's verifying that the system isn't under extreme memory pressure, but in practice a 4K get_user_pages is never going to fail, it's just going to cause something else to be swapped. I'd just get rid of both of them, so there's no need for kvm_lookup_pfn(). Paolo
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h index 82ca0971c156..5a572cef4adc 100644 --- a/include/linux/kvm_host.h +++ b/include/linux/kvm_host.h @@ -1212,6 +1212,8 @@ static inline void kvm_release_page_unused(struct page *page) void kvm_release_page_clean(struct page *page); void kvm_release_page_dirty(struct page *page); +kvm_pfn_t kvm_lookup_pfn(struct kvm *kvm, gfn_t gfn); + kvm_pfn_t gfn_to_pfn(struct kvm *kvm, gfn_t gfn); kvm_pfn_t gfn_to_pfn_prot(struct kvm *kvm, gfn_t gfn, bool write_fault, bool *writable); diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index 0b3c0bddaa07..ad84dab8c5dc 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -3118,6 +3118,22 @@ kvm_pfn_t kvm_vcpu_gfn_to_pfn(struct kvm_vcpu *vcpu, gfn_t gfn) } EXPORT_SYMBOL_GPL(kvm_vcpu_gfn_to_pfn); +kvm_pfn_t kvm_lookup_pfn(struct kvm *kvm, gfn_t gfn) +{ + struct page *refcounted_page = NULL; + struct kvm_follow_pfn kfp = { + .slot = gfn_to_memslot(kvm, gfn), + .gfn = gfn, + .flags = FOLL_WRITE, + .refcounted_page = &refcounted_page, + }; + kvm_pfn_t pfn; + + pfn = kvm_follow_pfn(&kfp); + kvm_release_page_unused(refcounted_page); + return pfn; +} + int kvm_prefetch_pages(struct kvm_memory_slot *slot, gfn_t gfn, struct page **pages, int nr_pages) {
Add a kvm_follow_pfn() wrapper, kvm_lookup_pfn(), to allow looking up a gfn=>pfn mapping without the caller getting a reference to any underlying page. The API will be used in flows that want to know if a gfn points at a valid pfn, but don't actually need to do anything with the pfn. Signed-off-by: Sean Christopherson <seanjc@google.com> --- include/linux/kvm_host.h | 2 ++ virt/kvm/kvm_main.c | 16 ++++++++++++++++ 2 files changed, 18 insertions(+)