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?
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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