Re: [Xen-devel] [V9 1/3] x86/xsaves: enable xsaves/xrstors/xsavec in xen

[Shuai Ruan <shuai.ruan@xxxxxxxxxxxxxxx>]

On Thu, Nov 05, 2015 at 02:06:25AM -0700, Jan Beulich wrote:
> >>> On 05.11.15 at 02:34, <shuai.ruan@xxxxxxxxxxxxxxx> wrote:
> > On Wed, Nov 04, 2015 at 10:04:33AM -0700, Jan Beulich wrote:
> >> >>> On 03.11.15 at 07:27, <shuai.ruan@xxxxxxxxxxxxxxx> wrote:
> >> > @@ -158,6 +334,20 @@ void xsave(struct vcpu *v, uint64_t mask)
> >> >      case 4: case 2:
> >> > -        asm volatile ( "1: .byte 0x0f,0xae,0x2f\n"
> >> > -                       ".section .fixup,\"ax\" \n"
> >> > -                       "2: mov %5,%%ecx        \n"
> >> > -                       "   xor %1,%1           \n"
> >> > -                       "   rep stosb           \n"
> >> > -                       "   lea %2,%0           \n"
> >> > -                       "   mov %3,%1           \n"
> >> > -                       "   jmp 1b              \n"
> >> > -                       ".previous              \n"
> >> > -                       _ASM_EXTABLE(1b, 2b)
> >> > -                       : "+&D" (ptr), "+&a" (lmask)
> >> > -                       : "m" (*ptr), "g" (lmask), "d" (hmask),
> >> > -                         "m" (xsave_cntxt_size)
> >> > -                       : "ecx" );
> >> > +        alternative_input("1: "".byte 0x0f,0xae,0x2f",
> >> > +                          ".byte 0x0f,0xc7,0x1f",
> >> > +                          X86_FEATURE_XSAVES,
> >> > +                          "D" (ptr), "m" (*ptr), "a" (lmask), "d" 
> > (hmask));
> >> > +        asm volatile (XSTATE_FIXUP);
> >> >          break;
> >> >      }
> >> >  }
> >> > +#undef XSTATE_FIXUP
> >> 
> >> Repeating my comment on v8: "I wonder whether at least for the
> >> restore side alternative asm wouldn't result in better readable code
> >> and at the same time in a smaller patch." Did you at least look into
> >> that option?
> >> 
> > I may misunderstand your meaning. I have adressed the comment by changing 
> > the restor side using alternative_input. Does "alternative_input" not what 
> > you want ? 
> > if it is not what you want, please give me some suggestions how to
> > address this ?  
> 
> Oh, I'm sorry, I should have looked more closely. The fact that
> XSTATE_FIXUP survived made me draw wrong conclusions without
> looking more closely. Now the bad news is - you can't split things
> like this, as the compiler doesn't make any guarantees as to
> register values between two asm()-s. The whole construct needs
> to and up as a single asm(), which is why XSTATE_FIXUP and is
> unlikely to be of much use here (at least in the context of this
> patch; a separate cleanup patch might eliminate the redundancy).
> 
Ok. So alternative_input will not used here (means use the way
xrstor in Patch 8)? Or put the XSTATE_FIXUP into alternative_input ?
Which one is ok to you ?

Thanks
> 
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> 

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