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Re: [Xen-devel] [PATCH] xen: Disable RAM-to-RAM copy in hvmemul_rep_movs() when mem_access_emulate_enabled

  • To: Jan Beulich <JBeulich@xxxxxxxx>
  • From: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>
  • Date: Tue, 05 May 2015 14:27:04 +0300
  • Cc: andrew.cooper3@xxxxxxxxxx, keir@xxxxxxx, xen-devel@xxxxxxxxxxxxx
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  • Delivery-date: Tue, 05 May 2015 11:26:45 +0000
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On 05/05/2015 01:23 PM, Jan Beulich wrote:
>>>> On 05.05.15 at 12:01, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
>> The mem_access client might want to use hvm_emulate_one_no_write(),
>> in which case the RAM-to-RAM copy code in hvmemul_rep_movs() would
>> lead to an unwanted (and unexpected) write operation.
> I don't follow: hvm_emulate_one_no_write() uses
> hvm_emulate_ops_no_write, which in turn uses
> hvmemul_rep_movs_discard(). What unwanted writes are you
> talking about? And if it was needed, why would
> hvmemul_rep_stos() not require a similar tweak?

You're right, my mistake. I'm testing a few patches meant for the a new
series, and one of them introduces a third kind of emulation, and the
problem is only there - the nowrite case is fine, as you rightly pointed
out. Sorry for the false alarm.

On the bright side, this just saved a bit of back-and-forth when the
series is submitted.


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