On Tue, 2022-04-26 at 09:25 +0700, Suravee Suthikulpanit wrote: > 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. Thanks! Best regards, Maxim Levitsky > > Regards, > Suravee >
