linux-kernel - Re: [PATCH] KVM: x86: Give a hint when Win2016 might fail to boot due to XSAVES erratum

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Date: Fri, 26 Jan 2024 19:36:48 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
Cc: Sean Christopherson <seanjc@...gle.com>, Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: Give a hint when Win2016 might fail to boot due
 to XSAVES erratum

On Wed, Jan 24, 2024 at 9:18 PM Maciej S. Szmigiero
<mail@...iej.szmigiero.name> wrote:
> +static void kvm_hv_xsaves_xsavec_maybe_warn_unlocked(struct kvm_vcpu *vcpu)

Calling this function "unlocked" is confusing (others would say
"locked" is confusing instead). The double-underscore convention is
more common.

> +{
> +       struct kvm *kvm = vcpu->kvm;
> +       struct kvm_hv *hv = to_kvm_hv(kvm);
> +
> +       if (hv->xsaves_xsavec_warned)
> +               return;
> +
> +       if (!vcpu->arch.hyperv_enabled)
> +               return;

I think these two should be in kvm_hv_xsaves_xsavec_maybe_warn(),
though the former needs to be checked again under the lock.

> +       if ((hv->hv_guest_os_id & KVM_HV_WIN2016_GUEST_ID_MASK) !=
> +           KVM_HV_WIN2016_GUEST_ID)
> +               return;

At this point there is no need to return. You can set
xsaves_xsavec_warned and save the checks in the future.

> +       /* UP configurations aren't affected */
> +       if (atomic_read(&kvm->online_vcpus) < 2)
> +               return;
> +
> +       if (boot_cpu_has(X86_FEATURE_XSAVES) ||
> +           !guest_cpuid_has(vcpu, X86_FEATURE_XSAVEC))
> +               return;

boot_cpu_has can also be done first to cull the whole check.

> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 27e23714e960..db0a2c40d749 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1782,6 +1782,10 @@ static int set_efer
>        if ((efer ^ old_efer) & KVM_MMU_EFER_ROLE_BITS)
>                kvm_mmu_reset_context(vcpu);
>
> +       if (guest_cpuid_is_amd_or_hygon(vcpu) &&
> +           efer & EFER_SVME)
> +               kvm_hv_xsaves_xsavec_maybe_warn(vcpu);
> +
>        return 0;
> }

Checking guest_cpuid_is_amd_or_hygon() is relatively expensive, it
should be done after "efer & EFER_SVME" but really the bug can happen
just as well on Intel as far as I understand? It's just less likely
due to the AMD erratum.

I'll send a v2.

Paolo