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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.  Perhaps:

diff --git a/xen/arch/x86/x86_64/compat/entry.S 
b/xen/arch/x86/x86_64/compat/entry.S

index f52bffc..14c87a0 100644

--- a/xen/arch/x86/x86_64/compat/entry.S

+++ b/xen/arch/x86/x86_64/compat/entry.S

@@ -215,7 +215,7 @@ ENTRY(cstar_enter)

         mov   %rcx, STACK_CPUINFO_FIELD(xen_cr3)(%rbx)

         neg   %rcx

         mov   %rcx, %cr3

-        movq  $0, STACK_CPUINFO_FIELD(xen_cr3)(%rbx)

+        movq  %r12, STACK_CPUINFO_FIELD(xen_cr3)(%rbx) /* Arbitrary zero reg. 
*/

 .Lcstar_cr3_okay:

         sti

 
~Andrew

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