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Re: [PATCH] x86/entry: Use 32bit XOR in handle_ist_exception


  • To: Jan Beulich <jbeulich@xxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Tue, 21 Nov 2023 12:20:51 +0000
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  • Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Tue, 21 Nov 2023 12:21:02 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On 21/11/2023 8:56 am, Jan Beulich wrote:
> On 20.11.2023 20:57, Andrew Cooper wrote:
>> This is a micro-optimsiation for Silvermont microarchitectures, which don't
>> recognise the 64bit form as a zeroing idiom.
>>
>> No functional change.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

Thanks.

>
> For my education, ...
>
>> --- a/xen/arch/x86/x86_64/entry.S
>> +++ b/xen/arch/x86/x86_64/entry.S
>> @@ -1027,7 +1027,7 @@ handle_ist_exception:
>>           * Interrupted guest context. Clear the restore value for xen_cr3
>>           * and copy the context to stack bottom.
>>           */
>> -        xor   %r15, %r15
>> +        xor   %r15d, %r15d
>>          xor   %ebx, %ebx
>>          GET_CPUINFO_FIELD(guest_cpu_user_regs,di)
>>          movq  %rsp,%rsi
>>
>> base-commit: fa2da5bce90b3777aa7a323e1cf201c97b56d278
> ... while I understand this, what ...
>
>> prerequisite-patch-id: a9e4e1e34d08e876d1fcb3299c6d563086768722
>> prerequisite-patch-id: 703590f2c99382f6509c94bb5955f47ab2d7c57d
> ... are these about? They look like they would be naming prereq changes
> which aren't committed yet, but (a) base-commit names the tip of the
> staging branch and (b) the hashes would be meaningless without a repo
> reference if they pointed at something that wasn't committed yet.

It's a slightly irritating side-effect of the functionality that
generates base-commit.

What it means was that this patch was 2 commits deep into a branch and I
haven't sent out the other two patches.

In this case, misc debugging that's not relevant to the change at all.

I try to strip it when I remember, but I forgot in this case.

~Andrew



 


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