Re: [Xen-devel] [PATCH v2] x86/hvm: fix interaction between internal and external emulation

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: 05 December 2017 14:32
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
> Subject: RE: [PATCH v2] x86/hvm: fix interaction between internal and
> external emulation
> 
> >>> On 05.12.17 at 15:00, <Paul.Durrant@xxxxxxxxxx> wrote:
> >> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> >> Sent: 05 December 2017 13:53
> >> >>> On 28.11.17 at 15:05, <paul.durrant@xxxxxxxxxx> wrote:
> >>     rc = x86_emulate(&hvmemul_ctxt->ctxt, ops);
> >>
> >>     if ( rc != X86EMUL_RETRY )
> >>     {
> >>         vio->mmio_cache_count = 0;
> >>         vio->mmio_insn_bytes = 0;
> >>     }
> >>     else
> >>     {
> >>         ...
> >>     }
> >>     if ( rc == X86EMUL_OKAY && vio->mmio_retry )
> >>         rc = X86EMUL_RETRY;
> >>
> >
> > But that's not safe is it? If we've only completed some of the reps of an
> > instruction then we can't flush the instruction cache and we can't allow the
> > guest to take interrupts, can we?
> 
> Of course we can, just like a repeated string insn may be
> interrupted on bare hardware between any two iterations (with
> RIP still pointing at that insn). In fact with EFLAGS.TF set it is a
> requirement to deliver #DB after every iteration.

Ok. In that case your re-ordering would seem sound.

  Paul

> 
> Jan


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