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    SubjectRe: [PATCHv4 3/5] kvm: host side for eoi optimization
    On Wed, May 16, 2012 at 03:15:00PM -0300, Marcelo Tosatti wrote:
    > On Wed, May 16, 2012 at 08:58:57PM +0300, Michael S. Tsirkin wrote:
    > > On Wed, May 16, 2012 at 02:20:58PM -0300, Marcelo Tosatti wrote:
    > > > On Wed, May 16, 2012 at 07:22:47PM +0300, Michael S. Tsirkin wrote:
    > > > > On Wed, May 16, 2012 at 12:49:40PM -0300, Marcelo Tosatti wrote:
    > > > > > > @@ -1245,9 +1306,20 @@ int kvm_get_apic_interrupt(struct kvm_vcpu *vcpu)
    > > > > > > int vector = kvm_apic_has_interrupt(vcpu);
    > > > > > > struct kvm_lapic *apic = vcpu->arch.apic;
    > > > > > >
    > > > > > > - if (vector == -1)
    > > > > > > + /* Detect interrupt nesting and disable EOI optimization */
    > > > > > > + if (pv_eoi_enabled(vcpu) && vector == -2)
    > > > > > > + pv_eoi_clr_pending(vcpu);
    > > > > > > +
    > > > > > > + if (vector < 0)
    > > > > >
    > > > > > With interrupt window exiting, the guest will exit:
    > > > > >
    > > > > > - as soon as it sets RFLAGS.IF=1 and there is any
    > > > > > interrupt pending in IRR.
    > > > > > - any new interrupt is set in IRR will kick vcpu
    > > > > > out of guest mode and recalculate interrupt-window-exiting.
    > > > > >
    > > > > > Doesnt this make this bit unnecessary ?
    > > > >
    > > > > Looks like we could cut it out. But I'm not sure how architectural it is
    > > > > that we exit on interrupt window.
    > > > > I guess there are reasons to exit on interrupt window but
    > > > > isn't it better to make the feature independent of it?
    > > >
    > > > Hum... not sure. Is it helpful for the Hyper-V interface?
    > > >
    > > > > This almost never happens in my testing anyway, so
    > > > > however we handle it is unlikely to affect performance.
    > > >
    > > > It decreases the amount of state that must be maintained.
    > > >
    > > > BTW there is a bug covered by interrupt window exiting:
    > > >
    > > > vcpu0 host
    > > > - irr 5 set
    > > > - isr 5 set, irr 5 cleared
    > > > - eoi_skip bit not set,
    > > > no other bit set in irr.
    > > > - enter guest
    > > >
    > > > irr 4 set
    > > > kick vcpu0 out of guest mode
    > > >
    > > > - eoi pending bit not set
    > > > (previous interrupt injection
    > > > still pending)
    > > > - skip eoi
    > > >
    > > > If it were not for interrupt window exiting, this would
    > > > inject vector 4 on an unrelated exit who knows how long
    > > > in the future.
    > > >
    > > > Also note optimization depends on the fact that the host
    > > > kicks vcpu out unconditionally (so it is dependent on
    > > > certain kvm implementation details).
    > > >
    > >
    > > Look we can summarize as follows: irq windows exit is
    > > required both before and after this patch.
    > > But it does not make the check above redundant.
    >
    > Right, it is not redundant.
    >
    > The above is still a bug: a case where eoi pending bit is not updated
    > properly.
    >
    > How come is this compatible with hyper-v again? Enlight me.

    Set kvm
    to point pv_eoi msr data to va and it will just work.

    --
    MST


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