Re: [PATCH v3 0/4] KVM: x86: hyper-v: Conditionally allow SynIC with APICv/AVIC

[Maxim Levitsky]

On Wed, 2021-06-09 at 17:09 +0200, Vitaly Kuznetsov wrote:
> Changes since v2:
> - First two patches got merged, rebase.
> - Use 'enable_apicv = avic = ...' in PATCH1 [Paolo]
> - Collect R-b tags for PATCH2 [Sean, Max]
> - Use hv_apicv_update_work() to get out of SRCU lock [Max]
> - "KVM: x86: Check for pending interrupts when APICv is getting disabled"
>   added.
> 
> Original description:
> 
> APICV_INHIBIT_REASON_HYPERV is currently unconditionally forced upon
> SynIC activation as SynIC's AutoEOI is incompatible with APICv/AVIC. It is,
> however, possible to track whether the feature was actually used by the
> guest and only inhibit APICv/AVIC when needed.
> 
> The series can be tested with the followin hack:
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 9a48f138832d..65a9974f80d9 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -147,6 +147,13 @@ void kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu)
>                                            vcpu->arch.ia32_misc_enable_msr &
>                                            MSR_IA32_MISC_ENABLE_MWAIT);
>         }
> +
> +       /* Dirty hack: force HV_DEPRECATING_AEOI_RECOMMENDED. Not to be merged! */
> +       best = kvm_find_cpuid_entry(vcpu, HYPERV_CPUID_ENLIGHTMENT_INFO, 0);
> +       if (best) {
> +               best->eax &= ~HV_X64_APIC_ACCESS_RECOMMENDED;
> +               best->eax |= HV_DEPRECATING_AEOI_RECOMMENDED;
> +       }
>  }
>  EXPORT_SYMBOL_GPL(kvm_update_cpuid_runtime);
>  
> Vitaly Kuznetsov (4):
>   KVM: x86: Use common 'enable_apicv' variable for both APICv and AVIC
>   KVM: x86: Drop vendor specific functions for APICv/AVIC enablement
>   KVM: x86: Check for pending interrupts when APICv is getting disabled
>   KVM: x86: hyper-v: Deactivate APICv only when AutoEOI feature is in
>     use
> 
>  arch/x86/include/asm/kvm_host.h |  9 +++++-
>  arch/x86/kvm/hyperv.c           | 51 +++++++++++++++++++++++++++++----
>  arch/x86/kvm/svm/avic.c         | 14 ++++-----
>  arch/x86/kvm/svm/svm.c          | 22 ++++++++------
>  arch/x86/kvm/svm/svm.h          |  2 --
>  arch/x86/kvm/vmx/capabilities.h |  1 -
>  arch/x86/kvm/vmx/vmx.c          |  2 --
>  arch/x86/kvm/x86.c              | 18 ++++++++++--
>  8 files changed, 86 insertions(+), 33 deletions(-)
> 

Hi!

I hate to say it, but at least one of my VMs doesn't boot amymore
with avic=1, after the recent updates. I'll bisect this soon,
but this is likely related to this series.

I will also review this series very soon.

When the VM fails, it hangs on the OVMF screen and I see this
in qemu logs:

KVM: injection failed, MSI lost (Operation not permitted)
KVM: injection failed, MSI lost (Operation not permitted)
KVM: injection failed, MSI lost (Operation not permitted)
KVM: injection failed, MSI lost (Operation not permitted)
KVM: injection failed, MSI lost (Operation not permitted)
KVM: injection failed, MSI lost (Operation not permitted)

Best regards,
	Maxim Levitsky