Re: [PATCH 3/3] KVM: x86: correct mwait and monitor emulation

[Nadav Amit]

On 6/18/14, 8:59 PM, Eric Northup wrote:
> On Wed, Jun 18, 2014 at 7:19 AM, Nadav Amit <namit@xxxxxxxxxxxxxxxxx> wrote:
> > mwait and monitor are currently handled as nop. Considering this behavior, they
> > should still be handled correctly, i.e., check execution conditions and generate
> > exceptions when required. mwait and monitor may also be executed in real-mode
> > and are not handled in that case.  This patch performs the emulation of
> > monitor-mwait according to Intel SDM (other than checking whether interrupt can
> > be used as a break event).
> >
> > Signed-off-by: Nadav Amit <namit@xxxxxxxxxxxxxxxxx>
> > ---
> >   arch/x86/kvm/emulate.c | 41 +++++++++++++++++++++++++++++++++++++++--
> >   arch/x86/kvm/svm.c     | 22 ++--------------------
> >   arch/x86/kvm/vmx.c     | 27 +++++++++++----------------
> >   3 files changed, 52 insertions(+), 38 deletions(-)
> >
> > diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> > index ef7a5a0..424b58d 100644
> > --- a/arch/x86/kvm/emulate.c
> > +++ b/arch/x86/kvm/emulate.c
> > @@ -3344,6 +3344,43 @@ static int em_bswap(struct x86_emulate_ctxt *ctxt)
> >          return X86EMUL_CONTINUE;
> >   }
> >
> > +static int em_monitor(struct x86_emulate_ctxt *ctxt)
> > +{
> > +       int rc;
> > +       struct segmented_address addr;
> > +       u64 rcx = reg_read(ctxt, VCPU_REGS_RCX);
> > +       u64 rax = reg_read(ctxt, VCPU_REGS_RAX);
> > +       u8 byte;
>
> I'd request:
>
> u32 ebx, ecx, edx, eax = 1;
> ctxt->opt->get_cpuid(ctxt, &eax, &ebx, &ecx, &edx);
> if (!(ecx & FFL(MWAIT)))
>          return emulate_ud(ctxt);
>
> and also in em_mwait.

I had similar implementation on previous version, which also checked on 
mwait whether "interrupt as break event" matches ECX value. However, I 
was under the impression that it was decided that MWAIT will always be 
emulated as NOP to avoid misbehaving VMs that ignore CPUID (see the 
discussion at http://www.spinics.net/lists/kvm/msg102766.html ).

Nadav
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