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 <jbeul...@suse.com> > --- > 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 _______________________________________________ Xen-devel mailing list Xen-devel@lists.xenproject.org https://lists.xenproject.org/mailman/listinfo/xen-devel
