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

From: Suravee Suthikulpanit
Date: Mon Apr 25 2022 - 22:25:21 EST

Hi Maim,

On 4/19/22 8:29 PM, Maxim Levitsky wrote:
On Tue, 2022-04-12 at 06:58 -0500, Suravee Suthikulpanit wrote:


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.