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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan


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