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

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Message-ID: <592977b7-e4c9-43af-aa33-1ce3b6fa1275@maciej.szmigiero.name>
Date: Fri, 26 Jan 2024 19:48:52 +0100
From: "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
To: Paolo Bonzini <pbonzini@...hat.com>
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 26.01.2024 19:36, Paolo Bonzini wrote:
> 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.

Yes, I've checked this guest on an Intel host and it also fails to
boot in !XSAVES && XSAVEC configuration.

Only on Intel it's purely a theoretical problem as AFAIK there's
no corresponding Intel errata that disables just XSAVES.

> 
> I'll send a v2.
> 
> Paolo
> 

Thanks,
Maciej