On 2/1/2023 5:36 AM, Sean Christopherson wrote:
> On Tue, Jan 31, 2023, Sean Christopherson wrote:
>> On Tue, Nov 29, 2022, Maxim Levitsky wrote:
>>> @@ -10015,13 +10022,34 @@ static void process_nmi(struct kvm_vcpu *vcpu)
>>> * Otherwise, allow two (and we'll inject the first one immediately).
>>> */
>>> if (static_call(kvm_x86_get_nmi_mask)(vcpu) || vcpu->arch.nmi_injected)
>>> - limit = 1;
>>> + limit--;
>>> +
>>> + /* Also if there is already a NMI hardware queued to be injected,
>>> + * decrease the limit again
>>> + */
>>> + if (static_call(kvm_x86_get_hw_nmi_pending)(vcpu))
>>> + limit--;
>>
>> I don't think this is correct. If a vNMI is pending and NMIs are blocked, then
>> limit will end up '0' and KVM will fail to pend the additional NMI in software.
>
> Scratch that, dropping the second NMI in this case is correct. The "running" part
> of the existing "x86 is limited to one NMI running, and one NMI pending after it"
> confused me. The "running" thing is really just a variant on NMIs being blocked.
>
> I'd also like to avoid the double decrement logic. Accounting the virtual NMI is
> a very different thing than dealing with concurrent NMIs, I'd prefer to reflect
> that in the code.
>
> Any objection to folding in the below to end up with:
>
> unsigned limit;
>
> /*
> * x86 is limited to one NMI pending, but because KVM can't react to
> * incoming NMIs as quickly as bare metal, e.g. if the vCPU is
> * scheduled out, KVM needs to play nice with two queued NMIs showing
> * up at the same time. To handle this scenario, allow two NMIs to be
> * (temporarily) pending so long as NMIs are not blocked and KVM is not
> * waiting for a previous NMI injection to complete (which effectively
> * blocks NMIs). KVM will immediately inject one of the two NMIs, and
> * will request an NMI window to handle the second NMI.
> */
> if (static_call(kvm_x86_get_nmi_mask)(vcpu) || vcpu->arch.nmi_injected)
> limit = 1;
> else
> limit = 2;
>
> /*
> * Adjust the limit to account for pending virtual NMIs, which aren't
> * tracked in in vcpu->arch.nmi_pending.
> */
> if (static_call(kvm_x86_is_vnmi_pending)(vcpu))
> limit--;
>
> vcpu->arch.nmi_pending += atomic_xchg(&vcpu->arch.nmi_queued, 0);
> vcpu->arch.nmi_pending = min(vcpu->arch.nmi_pending, limit);
>
I believe, you missed the function below hunk -
if (vcpu->arch.nmi_pending &&
static_call(kvm_x86_set_vnmi_pending(vcpu)))
vcpu->arch.nmi_pending--;
Or am I missing something.. please suggest.
> if (vcpu->arch.nmi_pending)
> kvm_make_request(KVM_REQ_EVENT, vcpu);
>
> --
> From: Sean Christopherson <seanjc@xxxxxxxxxx>
> Date: Tue, 31 Jan 2023 16:02:21 -0800
> Subject: [PATCH] KVM: x86: Tweak the code and comment related to handling
> concurrent NMIs
>
> Tweak the code and comment that deals with concurrent NMIs to explicitly
> call out that x86 allows exactly one pending NMI, but that KVM needs to
> temporarily allow two pending NMIs in order to workaround the fact that
> the target vCPU cannot immediately recognize an incoming NMI, unlike bare
> metal.
>
> No functional change intended.
>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
> arch/x86/kvm/x86.c | 15 +++++++++++----
> 1 file changed, 11 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 030136b6ebbd..fda09ba48b6b 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10122,15 +10122,22 @@ static int kvm_check_and_inject_events(struct kvm_vcpu *vcpu,
>
> static void process_nmi(struct kvm_vcpu *vcpu)
> {
> - unsigned limit = 2;
> + unsigned limit;
>
> /*
> - * x86 is limited to one NMI running, and one NMI pending after it.
> - * If an NMI is already in progress, limit further NMIs to just one.
> - * Otherwise, allow two (and we'll inject the first one immediately).
> + * x86 is limited to one NMI pending, but because KVM can't react to
> + * incoming NMIs as quickly as bare metal, e.g. if the vCPU is
> + * scheduled out, KVM needs to play nice with two queued NMIs showing
> + * up at the same time. To handle this scenario, allow two NMIs to be
> + * (temporarily) pending so long as NMIs are not blocked and KVM is not
> + * waiting for a previous NMI injection to complete (which effectively
> + * blocks NMIs). KVM will immediately inject one of the two NMIs, and
> + * will request an NMI window to handle the second NMI.
> */
> if (static_call(kvm_x86_get_nmi_mask)(vcpu) || vcpu->arch.nmi_injected)
> limit = 1;
> + else
> + limit = 2;
>
> vcpu->arch.nmi_pending += atomic_xchg(&vcpu->arch.nmi_queued, 0);
> vcpu->arch.nmi_pending = min(vcpu->arch.nmi_pending, limit);
>
Looks good to me, will include in v3.
Thanks,
Santosh
