linux-kernel - Re: [PATCH 02/43] KVM: VMX: Set EDX at INIT with CPUID.0x1, Family-Model-Stepping

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Date:   Fri, 21 May 2021 00:07:04 -0700
From:   Reiji Watanabe <reijiw@...gle.com>
To:     Sean Christopherson <seanjc@...gle.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>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 02/43] KVM: VMX: Set EDX at INIT with CPUID.0x1, Family-Model-Stepping

On Wed, May 19, 2021 at 11:47 AM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Tue, May 18, 2021, Reiji Watanabe wrote:
> > > @@ -4504,7 +4505,11 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
> > >
> > >         vmx->msr_ia32_umwait_control = 0;
> > >
> > > -       vmx->vcpu.arch.regs[VCPU_REGS_RDX] = get_rdx_init_val();
> > > +       eax = 1;
> > > +       if (!kvm_cpuid(vcpu, &eax, &dummy, &dummy, &dummy, true))
> > > +               eax = get_rdx_init_val();
> > > +       kvm_rdx_write(vcpu, eax);
> >
> > Reviewed-by: Reiji Watanabe <reijiw@...gle.com>
> >
> > For RESET, I assume that rdx should be set by userspace
> > when userspace changes CPUID.0x1.EAX.
>
> Ya, although the ideal solution is to add a proper RESET ioctl() so userspace can
> configure the vCPU model and then pull RESET#.

Thank you so much for the answer !
It sounds like a good idea in terms of userspace handling as well
as KVM implementation (I assume it would be something similar to
KVM_ARM_VCPU_INIT).


> > BTW, I would think having a default CPUID for CPUID.(EAX=0x1) would be better
> > for consistency of a vCPU state for RESET.  I would think it doesn't matter
> > practically anyway though.
>
> Probably, but that would require defining default values for all of CPUID.0x0 and
> CPUID.0x1, which is a can of worms I'd rather not open.  E.g. vendor info, basic
> feature set, APIC ID, etc... would all need default values.  On the other hand,
> the EDX value stuffing predates CPUID, so using 0x600 isn't provably wrong, just
> a bit anachronistic. :-)

I see... Then I don't think it's worth doing...
Just out of curiosity, can't we simply use a vcpu_id for the APIC ID ?
Also, can't we simply use the same values that KVM_GET_SUPPORTED_CPUID
provides for other CPUID fields ?

Thanks,
Reiji