linux-kernel - Re: [PATCH v2 4/7] KVM: x86: allow kvm_x86_ops.set

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Date:   Thu, 20 Aug 2020 17:43:50 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Maxim Levitsky <mlevitsk@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@...r.kernel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Joerg Roedel <joro@...tes.org>,
        Wanpeng Li <wanpengli@...cent.com>,
        Borislav Petkov <bp@...en8.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v2 4/7] KVM: x86: allow kvm_x86_ops.set_efer to return a
 value

On Thu, Aug 20, 2020 at 02:43:56PM -0700, Jim Mattson wrote:
> On Thu, Aug 20, 2020 at 6:34 AM Maxim Levitsky <mlevitsk@...hat.com> wrote:
> >
> > This will be used later to return an error when setting this msr fails.
> >
> > For VMX, it already has an error condition when EFER is
> > not in the shared MSR list, so return an error in this case.
> >
> > Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>
> > ---
> 
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -1471,7 +1471,8 @@ static int set_efer(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> >         efer &= ~EFER_LMA;
> >         efer |= vcpu->arch.efer & EFER_LMA;
> >
> > -       kvm_x86_ops.set_efer(vcpu, efer);
> > +       if (kvm_x86_ops.set_efer(vcpu, efer))
> > +               return 1;
> 
> This seems like a userspace ABI change to me. Previously, it looks
> like userspace could always use KVM_SET_MSRS to set MSR_EFER to 0 or
> EFER_SCE, and it would always succeed. Now, it looks like it will fail
> on CPUs that don't support EFER in hardware. (Perhaps it should fail,
> but it didn't before, AFAICT.)

KVM emulates SYSCALL, presumably that also works when EFER doesn't exist in
hardware.

The above also adds weirdness to nested VMX as vmx_set_efer() simply can't
fail.