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Re: [Xen-devel] [PATCH 4/4] SVM: streamline entry.S code

On 09/04/2013 11:20 AM, Jan Beulich wrote:
On 04.09.13 at 17:09, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx> wrote:
I meant something like this:

diff --git a/xen/arch/x86/hvm/svm/entry.S b/xen/arch/x86/hvm/svm/entry.S
index 1969629..b362637 100644
--- a/xen/arch/x86/hvm/svm/entry.S
+++ b/xen/arch/x86/hvm/svm/entry.S
@@ -92,10 +92,12 @@ UNLIKELY_END(svm_trace)


+       GET_CURRENT(%rax)
           push %rdi
           push %rsi
           push %rdx
           push %rcx
+       mov  VCPU_svm_vmcb(%rax),%rcx
           push %rax
           push %r8
           push %r9
@@ -108,17 +110,15 @@ UNLIKELY_END(svm_trace)
           push %r14
           push %r15

-        GET_CURRENT(%rbx)
-        movb $0,VCPU_svm_vmcb_in_sync(%rbx)
-        mov  VCPU_svm_vmcb(%rbx),%rcx
-        mov  VMCB_rax(%rcx),%rax
-        mov  %rax,UREGS_rax(%rsp)
-        mov  VMCB_rip(%rcx),%rax
-        mov  %rax,UREGS_rip(%rsp)
-        mov  VMCB_rsp(%rcx),%rax
-        mov  %rax,UREGS_rsp(%rsp)
-        mov  VMCB_rflags(%rcx),%rax
-        mov  %rax,UREGS_eflags(%rsp)
+        movb $0,VCPU_svm_vmcb_in_sync(%rax)
+        mov  VMCB_rax(%rcx),%rdi
+        mov  %rdi,UREGS_rax(%rsp)
+        mov  VMCB_rip(%rcx),%rdi
+        mov  %rdi,UREGS_rip(%rsp)
+        mov  VMCB_rsp(%rcx),%rdi
+        mov  %rdi,UREGS_rsp(%rsp)
+        mov  VMCB_rflags(%rcx),%rdi
+        mov  %rdi,UREGS_eflags(%rsp)

   #ifndef NDEBUG
           mov  $0xbeef,%ax

%rax is clobbered anyway by ' mov  VMCB_rax(%rcx),%rax'
But the "current" pointer also isn't needed anymore after the clearing
of VCPU_svm_vmcb_in_sync and before doing the first function call
(it's needed after the function call, but for that it's can't be in %rax).

So yes, that clearing of VCPU_svm_vmcb_in_sync could certainly be
done using %rax instead of %rbx, but no other code change would
seem necessary/desirable. If you agree, I'd be willing to do this one
line adjustment.

Oh, yes, that's right, no reason to change other lines.

My only reason for suggesting this change though was to eliminate
'mov  %rax,%rbx' instruction (in your patch, not in the code above).
If you want to keep it in though then I don't think the line adjustment
that you are talking about is needed.


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