Re: [PATCH v9 2/3] x86, apicv: add virtual x2apic support

Gleb Natapov
2013-01-10T12:34:57Z

On Thu, Jan 10, 2013 at 12:22:51PM +0000, Zhang, Yang Z wrote:
> Gleb Natapov wrote on 2013-01-10:
> > On Thu, Jan 10, 2013 at 11:54:21AM +0000, Zhang, Yang Z wrote:
> >>>> The right logic should be:
> >>>> When register virtualization enabled, read all apic msr(except apic id reg and
> >>> tmcct which have the hook) not cause vmexit and only write TPR not cause
> >>> vmexit.
> >>>> When vid enabled, write to TPR, EOI and SELF IPI not cause vmexit.
> >>>> It's better to put the patch of envirtualize x2apic mode as first patch.
> >>>>
> >>> There is no point whatsoever to enable virtualize x2apic without
> >>> allowing at least one non intercepted access to x2apic MSR range and
> >>> this is what your patch is doing when run on cpu without vid capability.
> >> No, at least read/write TPR will not cause vmexit if virtualize x2apic mode is
> > enabled.
> > For that you need to disable 808H MSR intercept, which your patch does not do.
> I want to do this in next patch.
>
Then don't. There is no point in the patch that enables virtualize
x2apic and does not allow at least one non-intercepted MSR access.

> >> I am not sure whether I understand your comments right in previous
> >> discussion, here is my thinking: 1. enable virtualize x2apic mode if
> >> guest is really using x2apic and clear TPR in msr read and write
> >> bitmap. This will benefit reading TPR. 2. If APIC registers
> >> virtualization is enabled, clear all bit in rang
> > 0x800-0x8ff(except apic id reg and tmcct).
> > Clear all read bits in the range.
> >
> >> 3. If vid is enabled, clear EOI and SELF IPI in msr write map.
> >>
> > Looks OK.
> >
> >> One concern you mentioned is that vid enabled and virtualize x2apic is disabled
> > but guest still use x2apic. In this case, we still use MSR bitmap to intercept x2apic.
> > This means the vEOI update will never happen. But we still can benefit from
> > interrupt window.
> >>
> > What interrupt windows? Without virtualized x2apic TPR/EOI
> > virtualization will not happen and we rely on that in the code.
> Yes, but we can eliminate vmexit of interrupt window. Interrupt still can delivery to guest without vmexit when guest enables interrupt if vid is enabled. See SDM vol 3, 29.2.2.
>
What does it matter that you eliminated vmexit of interrupt window if
you broke everything else? The vid patch assumes that apic emulation
either entirely happens in a software when vid is disabled or in a CPU
if vid is enabled. Mixed mode will not work (apic->isr_count = 1 trick
will break things for instance). And it is not worth it to complicate
the code to make it work.

--
Gleb.