linux-kernel - Re: [PATCH v5 3/7] KVM: X86: Expose IA32

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Message-ID: <YYqT/cOm3Psf1gj1@google.com>
Date:   Tue, 9 Nov 2021 15:30:05 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Chenyi Qiang <chenyi.qiang@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Xiaoyao Li <xiaoyao.li@...el.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 3/7] KVM: X86: Expose IA32_PKRS MSR

On Tue, Nov 09, 2021, Chenyi Qiang wrote:
> 
> On 11/9/2021 1:44 AM, Sean Christopherson wrote:
> > Hrm.  Ideally this would be open coded in vmx_set_msr().  Long term, the RESET/INIT
> > paths should really treat MSR updates as "normal" host_initiated writes instead of
> > having to manually handle every MSR.
> > 
> > That would be a bit gross to handle in vmx_vcpu_reset() since it would have to
> > create a struct msr_data (because __kvm_set_msr() isn't exposed to vendor code),
> > but since vcpu->arch.pkrs is relevant to the MMU I think it makes sense to
> > initiate the write from common x86.
> > 
> > E.g. this way there's not out-of-band special code, vmx_vcpu_reset() is kept clean,
> > and if/when SVM gains support for PKRS this particular path Just Works.  And it would
> > be an easy conversion for my pipe dream plan of handling MSRs at RESET/INIT via a
> > list of MSRs+values.
> > 
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index ac83d873d65b..55881d13620f 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -11147,6 +11147,9 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
> >          kvm_set_rflags(vcpu, X86_EFLAGS_FIXED);
> >          kvm_rip_write(vcpu, 0xfff0);
> > 
> > +       if (kvm_cpu_cap_has(X86_FEATURE_PKS))
> > +               __kvm_set_msr(vcpu, MSR_IA32_PKRS, 0, true);
> > +
> 
> Got it. In addition, is it necessary to add on-INIT check? like:
> 
> if (kvm_cpu_cap_has(X86_FEATURE_PKS) && !init_event)
> 	__kvm_set_msr(vcpu, MSR_IA32_PKRS, 0, true);
> 
> PKRS should be preserved on INIT, not cleared. The SDM doesn't make this
> clear either.

Hmm, but your cover letter says:

  To help patches review, one missing info in SDM is that PKSR will be
  cleared on Powerup/INIT/RESET, which should be listed in Table 9.1
  "IA-32 and Intel 64 Processor States Following Power-up, Reset, or INIT"

Which honestly makes me a little happy because I thought I was making stuff up
for a minute :-)

So which is it?