linux-kernel - Re: [PATCH 1/3] KVM: emulate: #GP when emulating rdpmc if CR0.PE is 1

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Date:   Sat, 9 Oct 2021 17:09:40 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        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>
Subject: Re: [PATCH 1/3] KVM: emulate: #GP when emulating rdpmc if CR0.PE is 1

On Fri, 8 Oct 2021 at 23:20, Sean Christopherson <seanjc@...gle.com> wrote:
>
> The shortlog makes it sound like "inject a #GP if CR0.PE=1", i.e. unconditionally
> inject #GP for RDMPC in protected mode.  Maybe "Don't inject #GP when emulating
> RDMPC if CR0.PE=0"?
>

Agreed.

> On Fri, Oct 08, 2021, Wanpeng Li wrote:
> > From: Wanpeng Li <wanpengli@...cent.com>
> >
> > SDM mentioned that, RDPMC:
> >
> >   IF (((CR4.PCE = 1) or (CPL = 0) or (CR0.PE = 0)) and (ECX indicates a supported counter))
> >       THEN
> >           EAX := counter[31:0];
> >           EDX := ZeroExtend(counter[MSCB:32]);
> >       ELSE (* ECX is not valid or CR4.PCE is 0 and CPL is 1, 2, or 3 and CR0.PE is 1 *)
> >           #GP(0);
> >   FI;
> >
> > Let's add the CR0.PE is 1 checking to rdpmc emulate.
> >
> > Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> > ---
> >  arch/x86/kvm/emulate.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> > index 9a144ca8e146..ab7ec569e8c9 100644
> > --- a/arch/x86/kvm/emulate.c
> > +++ b/arch/x86/kvm/emulate.c
> > @@ -4213,6 +4213,7 @@ static int check_rdtsc(struct x86_emulate_ctxt *ctxt)
> >  static int check_rdpmc(struct x86_emulate_ctxt *ctxt)
> >  {
> >       u64 cr4 = ctxt->ops->get_cr(ctxt, 4);
> > +     u64 cr0 = ctxt->ops->get_cr(ctxt, 0);
> >       u64 rcx = reg_read(ctxt, VCPU_REGS_RCX);
> >
> >       /*
> > @@ -4222,7 +4223,7 @@ static int check_rdpmc(struct x86_emulate_ctxt *ctxt)
> >       if (enable_vmware_backdoor && is_vmware_backdoor_pmc(rcx))
> >               return X86EMUL_CONTINUE;
> >
> > -     if ((!(cr4 & X86_CR4_PCE) && ctxt->ops->cpl(ctxt)) ||
> > +     if ((!(cr4 & X86_CR4_PCE) && ctxt->ops->cpl(ctxt) && (cr0 & X86_CR0_PE)) ||
>
> I don't think it's possible for CPL to be >0 if CR0.PE=0, e.g. we could probably
> WARN in the #GP path.  Realistically it doesn't add value though, so maybe just
> add a blurb in the changelog saying this isn't strictly necessary?

Do it in v2.

    Wanpeng