Re: [PATCH v2 11/12] KVM: SVM: Do not inhibit APICv when x2APIC is present

[Suravee Suthikulpanit]

Hi Maim,

On 4/19/22 8:29 PM, Maxim Levitsky wrote:
> On Tue, 2022-04-12 at 06:58 -0500, Suravee Suthikulpanit wrote:
>
> Hi!
>
>
> I just got an idea, while writing a kvm selftest that would use AVIC,
> and finding out that selftest code uploads the '-host' cpuid right away
> which has x2apic enabled and that inhibits AVIC, and later clearing x2apic
> in the cpuid doesn't un-inhibit it.
>   
> That can be fixed in few ways but that got me thinking:
>   
> Why do we inhibit AVIC when the guest uses x2apic, even without X2AVIC?
> I think that if we didn't it would just work, and even work faster than
> pure software x2apic.
>   
> My thinking is:
>   
> - when a vcpu itself uses its x2apic, even if its avic is not inhibited,
> the guest will write x2apic msrs which kvm intercepts and will correctly emulate a proper x2apic.
>   
> - vcpu peers will also use x2apic msrs and again it will work correctly
> (even when there are more than 256 vcpus).
>   
> - and the host + iommu will still be able to use AVIC's doorbell to send interrupts to the guest
> and that doesn't need apic ids or anything, it should work just fine.
>
> Also AVIC should have no issues scanning IRR and injecting interrupts on VM entry,
> x2apic mode doesn't matter for that.
>   
> AVIC mmio can still be though discovered by the guest which is technically against x86 spec
> (in x2apic mode, mmio supposed to not work) but that can be fixed easily by disabing
> the AVIC memslot if any of the vCPUs are in x2apic mode, or this can be ignored since
> it should not cause any issues.
> We seem to have a quirk for that KVM_X86_QUIRK_LAPIC_MMIO_HOLE.
>   
> On top of all this, removing this inhibit will also allow to test AVIC with guest
> which does have x2apic in the CPUID but doesn't use it (e.g kvm unit test, or
> linux booted with nox2apic, which is also nice IMHO)
>   
> What do you think?

This is actually a good idea!!! Let's call it hybrid-x2AVIC :)

I am working on prototype and test out the support for this, which will be introduced in V3.

Regards,
Suravee