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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 15/03/18 16:39, Jan Beulich wrote:
>>>> 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.

With that change, Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

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