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

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Message-ID: <CAAeT=FxuBEY1C7MMQ5f34-AbQjtDRAgVeR88LkS9=763dCjb=A@mail.gmail.com>
Date:   Sun, 23 May 2021 21:29:46 -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 Tue, May 18, 2021, Reiji Watanabe wrote:
> > > > 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 ?
>
> That would mostly work, but theoretically we could overflow the 8 bit field
> because max vCPUs is 288.  Thanks Larrabee.
>
>   commit 682f732ecf7396e9d6fe24d44738966699fae6c0
>   Author: Radim Krčmář <rkrcmar@...hat.com>
>   Date:   Tue Jul 12 22:09:29 2016 +0200
>
>     KVM: x86: bump MAX_VCPUS to 288
>
>     288 is in high demand because of Knights Landing CPU.
>     We cannot set the limit to 640k, because that would be wasting space.
>
> > Also, can't we simply use the same values that KVM_GET_SUPPORTED_CPUID
> > provides for other CPUID fields ?
>
> Yes, that would mostly work.  It's certainly possible to have a moderately sane
> default, but there's essentially zero benefit in doing so since practically
> speaking all userspace VMMs will override CPUID anyways.  KVM could completely
> default to the host CPUID, but again, it wouldn't provide any meaningful benefit,
> while doing so would step on userspace's toes since KVM's approach is that KVM is
> "just" an accelerator, while userspace defines the CPU model, devices, etc...
> And it would also mean KVM has to start worrying about silly corner cases like
> the max vCPUs thing.  That's why I say it's a can of worms :-)

Ah, I see.  Thank you for the answer and the helpful information !

Regards,
Reiji