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Re: [PATCH] x86: use alternative_input() in cache_flush()
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Mon, 30 Sep 2024 16:47:16 +0100
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Mon, 30 Sep 2024 15:47:26 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 30/09/2024 4:39 pm, Jan Beulich wrote:
> On 30.09.2024 17:27, Andrew Cooper wrote:
>> On 30/09/2024 4:09 pm, Jan Beulich wrote:
>>> There's no point using alternative_io() when there are no outputs.
>>>
>>> No functional change.
>>>
>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>>
>>> --- a/xen/arch/x86/flushtlb.c
>>> +++ b/xen/arch/x86/flushtlb.c
>>> @@ -286,11 +286,10 @@ void cache_flush(const void *addr, unsig
>>> * + prefix than a clflush + nop, and hence the prefix is added
>>> instead
>>> * of letting the alternative framework fill the gap by appending
>>> nops.
>>> */
>>> - alternative_io("ds; clflush %[p]",
>>> - "data16 clflush %[p]", /* clflushopt */
>>> - X86_FEATURE_CLFLUSHOPT,
>>> - /* no outputs */,
>>> - [p] "m" (*(const char *)(addr)));
>>> + alternative_input("ds; clflush %[p]",
>> Drop the ; as you're altering the line anyway?
> Oh, sure.
>
>> Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Thanks.
>
>> However, switching to ds ds will avoid a trailing nop, so will be
>> marginally better.
> I don't think I understand, which may or may not be due to me not being
> sure whether "ds ds" is a typo. What trailing NOP are you seeing? The
> "ds" that's there already covers for the sole trailing NOP that would
> otherwise result due to the "data16" prefix.
Oh of course. I can't count. Sorry for the noise.
~Andrew
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