| Message ID | 20240129213918.3124494-1-coltonlewis@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | KVM: arm64: Add capability for unconditional WFx passthrough | expand |
Hi Colton, On Mon, Jan 29, 2024 at 09:39:17PM +0000, Colton Lewis wrote: > Add KVM_CAP_ARM_WFX_PASSTHROUGH capability to always allow WFE/WFI > instructions to run without trapping. Current behavior is to only > allow this if the vcpu is the only task running. This commit keeps the > old behavior when the capability is not set. > > This allows userspace to set deterministic behavior and increase > efficiency for platforms with direct interrupt injection support. Marc and I actually had an offlist conversation (shame on us!) about this very topic since there are users asking for the _opposite_ of this patch (unconditionally trap) [*]. I had originally wanted something like this, but Marc made the very good point that (1) the behavior of WFx traps is in no way user-visible and (2) it is entirely an IMP DEF behavior. The architecture only requires the traps be effective if the instruction does not complete in finite time. We need to think of an interface that doesn't depend on implementation-specific behavior, such as a control based on runqueue depth. [*] https://lore.kernel.org/kvmarm/a481ef04-ddd2-dfc1-41b1-d2ec45c6a3b5@huawei.com/
Hi Marc, Oliver, On 2024/1/30 7:17, Oliver Upton wrote: > Hi Colton, > > On Mon, Jan 29, 2024 at 09:39:17PM +0000, Colton Lewis wrote: >> Add KVM_CAP_ARM_WFX_PASSTHROUGH capability to always allow WFE/WFI >> instructions to run without trapping. Current behavior is to only >> allow this if the vcpu is the only task running. This commit keeps the >> old behavior when the capability is not set. >> >> This allows userspace to set deterministic behavior and increase >> efficiency for platforms with direct interrupt injection support. > > Marc and I actually had an offlist conversation (shame on us!) about > this very topic since there are users asking for the _opposite_ of this > patch (unconditionally trap) [*]. > > I had originally wanted something like this, but Marc made the very good > point that (1) the behavior of WFx traps is in no way user-visible and > (2) it is entirely an IMP DEF behavior. The architecture only requires > the traps be effective if the instruction does not complete in finite > time. > > We need to think of an interface that doesn't depend on > implementation-specific behavior, such as a control based on runqueue > depth. If I understand correctly, this run queue belongs to the scheduler, right? And I will be appreciated if you can share any more detail information about this. > > [*] https://lore.kernel.org/kvmarm/a481ef04-ddd2-dfc1-41b1-d2ec45c6a3b5@huawei.com/ > Thanks, Dongxu
diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h index 21c57b812569..e0d5ec2983fa 100644 --- a/arch/arm64/include/asm/kvm_host.h +++ b/arch/arm64/include/asm/kvm_host.h @@ -274,6 +274,8 @@ struct kvm_arch { #define KVM_ARCH_FLAG_TIMER_PPIS_IMMUTABLE 6 /* Initial ID reg values loaded */ #define KVM_ARCH_FLAG_ID_REGS_INITIALIZED 7 + /* Never trap WFE/WFI instructions */ +#define KVM_ARCH_FLAG_WFX_PASSTHROUGH_ENABLED 8 unsigned long flags; /* VM-wide vCPU feature set */ diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c index a25265aca432..6d993991bd7a 100644 --- a/arch/arm64/kvm/arm.c +++ b/arch/arm64/kvm/arm.c @@ -116,6 +116,10 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm, } mutex_unlock(&kvm->slots_lock); break; + case KVM_CAP_ARM_WFX_PASSTHROUGH: + r = 0; + set_bit(KVM_ARCH_FLAG_WFX_PASSTHROUGH_ENABLED, &kvm->arch.flags); + break; default: r = -EINVAL; break; @@ -456,7 +460,8 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu) if (kvm_arm_is_pvtime_enabled(&vcpu->arch)) kvm_make_request(KVM_REQ_RECORD_STEAL, vcpu); - if (single_task_running()) + if (single_task_running() || + test_bit(KVM_ARCH_FLAG_WFX_PASSTHROUGH_ENABLED, &vcpu->kvm->arch.flags)) vcpu_clear_wfx_traps(vcpu); else vcpu_set_wfx_traps(vcpu); diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h index c3308536482b..7635b5cd2b3b 100644 --- a/include/uapi/linux/kvm.h +++ b/include/uapi/linux/kvm.h @@ -1155,6 +1155,7 @@ struct kvm_ppc_resize_hpt { #define KVM_CAP_MEMORY_ATTRIBUTES 233 #define KVM_CAP_GUEST_MEMFD 234 #define KVM_CAP_VM_TYPES 235 +#define KVM_CAP_ARM_WFX_PASSTHROUGH 236 #ifdef KVM_CAP_IRQ_ROUTING
Add KVM_CAP_ARM_WFX_PASSTHROUGH capability to always allow WFE/WFI instructions to run without trapping. Current behavior is to only allow this if the vcpu is the only task running. This commit keeps the old behavior when the capability is not set. This allows userspace to set deterministic behavior and increase efficiency for platforms with direct interrupt injection support. The implementation adds a new flag KVM_ARCH_FLAG_WFX_PASSTHROUGH_ENABLED to kvm.arch.flags. Signed-off-by: Colton Lewis <coltonlewis@google.com> --- arch/arm64/include/asm/kvm_host.h | 2 ++ arch/arm64/kvm/arm.c | 7 ++++++- include/uapi/linux/kvm.h | 1 + 3 files changed, 9 insertions(+), 1 deletion(-) This patch is based on v6.8-rc1 -- 2.43.0.429.g432eaa2c6b-goog