Re: [Xen-devel] [PATCH 1/3] x86emul: use DstEax also for {, I}{MUL, DIV}

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 16/08/16 15:57, Jan Beulich wrote:
>>>> On 16.08.16 at 16:08, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 16/08/16 10:32, Jan Beulich wrote:
>>> Just like said in commit c0bc0adf24 ("x86emul: use DstEax where
>>> possible"): While it avoids just a few instructions, we should
>>> nevertheless make use of generic code as much as possible.
>>>
>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> This does reduce the amount of code, but it isn't strictly true.  The
>> mul and div instructions are DstEaxEdx, as are a number of other
>> instructions.
>>
>> We shouldn't end up with special casing the eax part because we have an
>> easy literal for it, but leaving the edx hard coded because that is
>> easier to express in the current code.
> I think the code reduction is nevertheless worth it, and reduction
> here can only help readability imo. Would you be okay if I added
> a comment to the place where the DstEax gets set here? (Note
> that DstEdxEax wouldn't be true for 8-bit operations, so I'd rather
> not use this as another alias or even a completely new operand
> kind description. And please also remember that the tables don't
> express all operands in all cases anyway - just consider
> SHLD/SHRD.)

The other option would be to use DstNone and explicitly fill in
_regs.eax, which avoids all the code to play with dst, and matches how
rdtsc/rdmsr/wrmsr currently work.

~Andrew

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