Thank you for your comments.
> From: Jan Kiszka
> Sent: Tuesday, March 22, 2016 3:42 PM
>
> On 2016-03-22 06:28, Yuki Shibuya wrote:
> > Non maskable interrupts (NMI) are preferred to interrupts in current
> > implementation. If a NMI is pending and NMI is blocked by the result
> > of nmi_allowed(), pending interrupt is not injected and
> > enable_irq_window() is not executed, even if interrupts injection is
> > allowed.
> >
> > In old kernel (e.g. 2.6.32), schedule() is often called in NMI context.
> > In this case, interrupts are needed to execute iret that intends end
> > of NMI. The flag of blocking new NMI is not cleared until the guest
> > execute the iret, and interrupts are blocked by pending NMI. Due to
> > this, iret can't be invoked in the guest, and the guest is starved
> > until block is cleared by some events (e.g. canceling injection).
> >
> > This patch injects pending interrupts, when it's allowed, even if NMI
> > is blocked. And, if NMI pending count == 2, NMI is not blocked and an
> > interrupt is pending, NMI pending count is decremented to execute
> > enable_irq_window().
>
> The first part I understand and agree with. But the part after "And"
> worries me still: if we can simply decrement that pending counter once again
> - why can't we just clear it to 0 in the first place?
We can clear pending counter in the first place. However, in current implementation,
there is a possibility of nmi pending counter == 2. In this case, one nmi is injected
and enable_nmi_window() is executed to inject another nmi in next emulation.
In my opinion, if injectable pending interrupt does not exist, it is not necessary to
prevent above logic because we don't skip injectable pending interrupt. Therefore, I added
the second part.
>
> BTW, some inline documentation of this tricky logic would probably be good.
>
> Jan
>
> >
> > Signed-off-by: Yuki Shibuya <shibuya.yk@xxxxxxxxxxxxxx>
> > ---
> > arch/x86/kvm/x86.c | 12 +++++++-----
> > 1 file changed, 7 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index
> > 7236bd3..1373627 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -6087,12 +6087,14 @@ static int inject_pending_event(struct kvm_vcpu
> *vcpu, bool req_int_win)
> > }
> >
> > /* try to inject new event if pending */
> > - if (vcpu->arch.nmi_pending) {
> > - if (kvm_x86_ops->nmi_allowed(vcpu)) {
> > + if (vcpu->arch.nmi_pending && kvm_x86_ops->nmi_allowed(vcpu)) {
> > + --vcpu->arch.nmi_pending;
> > + vcpu->arch.nmi_injected = true;
> > + kvm_x86_ops->set_nmi(vcpu);
> > +
> > + if (vcpu->arch.nmi_pending &&
> kvm_cpu_has_injectable_intr(vcpu)
> > + &&
> kvm_x86_ops->interrupt_allowed(vcpu))
> > --vcpu->arch.nmi_pending;
> > - vcpu->arch.nmi_injected = true;
> > - kvm_x86_ops->set_nmi(vcpu);
> > - }
> > } else if (kvm_cpu_has_injectable_intr(vcpu)) {
> > /*
> > * Because interrupts can be injected asynchronously, we
> are
> >
>
> --
> Siemens AG, Corporate Technology, CT RDA ITP SES-DE Corporate Competence
> Center Embedded Linux
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