linux-kernel - Re: [PATCH] KVM: x86: Always set kvm_run->if

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Message-ID: <0d540a6b-8838-bdf5-ddad-f3b9576ca9f2@redhat.com>
Date:   Thu, 9 Dec 2021 18:52:01 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Marc Orr <marcorr@...gle.com>,
        Tom Lendacky <Thomas.Lendacky@....com>
Cc:     seanjc@...gle.com, vkuznets@...hat.com, wanpengli@...cent.com,
        jmattson@...gle.com, joro@...tes.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, hpa@...or.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: Always set kvm_run->if_flag

On 12/7/21 18:28, Marc Orr wrote:
>>>>> +static bool svm_get_if_flag(struct kvm_vcpu *vcpu)
>>>>> +{
>>>>> +     struct vmcb *vmcb = to_svm(vcpu)->vmcb;
>>>>> +
>>>>> +     return !!(vmcb->control.int_state & SVM_GUEST_INTERRUPT_MASK);
>>>> I'm not sure if this is always valid to use for non SEV-ES guests. Maybe
>>>> the better thing would be:
>>>>
>>>>           return sev_es_guest(vcpu->kvm) ? vmcb->control.int_state & SVM_GUEST_INTERRUPT_MASK
>>>>                                          : kvm_get_rflags(vcpu) & X86_EFLAGS_IF;
>>>>
>>>> (Since this function returns a bool, I don't think you need the !!)
>>>
>>> I had the same reservations when writing the patch. (Why fix what's
>>> not broken.) The reason I wrote the patch this way is based on what I
>>> read in APM vol2: Appendix B Layout of VMCB: "GUEST_INTERRUPT_MASK -
>>> Value of the RFLAGS.IF bit for the guest."
>>
>> I just verified with the hardware team that this flag is indeed only set
>> for a guest with protected state (SEV-ES / SEV-SNP). An update to the APM
>> will be made.
>
> Got it now. Then the change you suggested is a must! Thanks, Tom.

Besides, the bit wouldn't have existed on old (pre-SEV-ES) processors.

Paolo