diff mbox

[3/3] x86: avoid FS/GS base reads when possible

Message ID 59EA23420200007800188C1C@prv-mh.provo.novell.com (mailing list archive)
State New, archived
Headers show

Commit Message

Jan Beulich Oct. 20, 2017, 2:24 p.m. UTC 
They're being zeroed a few lines down when non-null selectors are being
found in the respective registers.

Signed-off-by: Jan Beulich <jbeulich@suse.com>

Comments

Andrew Cooper Oct. 20, 2017, 3:17 p.m. UTC | #1 
On 20/10/17 15:24, Jan Beulich wrote:
> They're being zeroed a few lines down when non-null selectors are being
> found in the respective registers.
>
> Signed-off-by: Jan Beulich <jbeulich@suse.com>

Given that this is within an cpu_has_fsgsbase conditional, there is a
moderate chance that the conditional is more overhead than the
rd{fs,gs}base instruction alone.

ISTR Andy Lutomirsky finding that they were actually very efficient
instructions.

I'm not sure which is the better option here, but I'm not aversed to the
change, so Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

>
> --- a/xen/arch/x86/domain.c
> +++ b/xen/arch/x86/domain.c
> @@ -1449,10 +1449,11 @@ static void save_segments(struct vcpu *v
>  
>      if ( cpu_has_fsgsbase && !is_pv_32bit_vcpu(v) )
>      {
> -        v->arch.pv_vcpu.fs_base = __rdfsbase();
> +        if ( !(regs->fs & ~3) )
> +            v->arch.pv_vcpu.fs_base = __rdfsbase();
>          if ( v->arch.flags & TF_kernel_mode )
>              v->arch.pv_vcpu.gs_base_kernel = __rdgsbase();
> -        else
> +        else if ( !(regs->gs & ~3) )
>              v->arch.pv_vcpu.gs_base_user = __rdgsbase();
>      }
>  
>
>
>
diff mbox

Patch

--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -1449,10 +1449,11 @@  static void save_segments(struct vcpu *v
 
     if ( cpu_has_fsgsbase && !is_pv_32bit_vcpu(v) )
     {
-        v->arch.pv_vcpu.fs_base = __rdfsbase();
+        if ( !(regs->fs & ~3) )
+            v->arch.pv_vcpu.fs_base = __rdfsbase();
         if ( v->arch.flags & TF_kernel_mode )
             v->arch.pv_vcpu.gs_base_kernel = __rdgsbase();
-        else
+        else if ( !(regs->gs & ~3) )
             v->arch.pv_vcpu.gs_base_user = __rdgsbase();
     }