Re: [Xen-devel] [PATCH v3 4/6] x86/XPTI: use %r12 to write zero into xen_cr3

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 15.03.18 at 17:02, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 13/03/18 13:49, Jan Beulich wrote:
>> Now that we zero all registers early on all entry paths, use that to
>> avoid a couple of immediates here.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> ---
>> We may want to consider eliminating a few more $0 this way. But
>> especially for byte ones I'm not sure it's worth it, due to the REX
>> prefix the use of %r12 would incur.
>>
>> --- a/xen/arch/x86/x86_64/compat/entry.S
>> +++ b/xen/arch/x86/x86_64/compat/entry.S
>> @@ -216,7 +216,7 @@ ENTRY(cstar_enter)
>>          mov   %rcx, STACK_CPUINFO_FIELD(xen_cr3)(%rbx)
>>          neg   %rcx
>>          mov   %rcx, %cr3
>> -        movq  $0, STACK_CPUINFO_FIELD(xen_cr3)(%rbx)
>> +        mov   %r12, STACK_CPUINFO_FIELD(xen_cr3)(%rbx)
> 
> It is unreasonable to expect people to realise that this use of %r12 is
> for a zero, because there is no write to %r12 visible.  These need some
> kind of comment.

Well, okay, I'll add the same comment in all 7 places.

Jan


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