Re: [Xen-devel] [PATCH v3 5/8] x86emul: support MOVDIR{I, 64B} insns

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 03/09/2019 13:25, Jan Beulich wrote:
> On 03.09.2019 12:28, Andrew Cooper wrote:
>> On 03/09/2019 10:39, Jan Beulich wrote:
>>> Note that SDM revision 070 doesn't specify exception behavior for
>>> ModRM.mod != 0b11; assuming #UD here.
>>>
>>> Signed-off-by: Jan Beulich <jbeulich@suse.com>
>> What are we going to do about the ->write() hook atomicity?  I'm happy
>> to put it on the TODO list, but we can't simply ignore the problem.
> So do you not agree with my assessment that our memcpy()
> implementation satisfies the need, and it's just not very
> nice that the ->write() hook is dependent upon this?

We use __builtin_memcpy(), not necessarily our own local implementation.

Our own copy uses rep movsq followed by rep movsb of up to 7 bytes,
which doesn't handle 2 and 4 byte copies atomically.  More generally,
rep movs doesn't provide guarantees about the external visibility of
component writes, owing to the many different ways that such writes may
be implemented, and optimised.

Even if the above were fixed (and I'm not sure could make it correct
even for misaligned writes), we cannot depend on our own memcpy never
changing in the future.  For one, it should be a straight rep movsb on
most Intel hardware these days, for performance.

~Andrew

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