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

[Shuai Ruan <shuai.ruan@xxxxxxxxxxxxxxx>]

On Wed, Nov 04, 2015 at 10:04:33AM -0700, Jan Beulich wrote:
> >>> On 03.11.15 at 07:27, <shuai.ruan@xxxxxxxxxxxxxxx> wrote:
> > diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
> > index fe3be30..108d4f8 100644
> > --- a/xen/arch/x86/domain.c
> > +++ b/xen/arch/x86/domain.c
> > @@ -883,7 +883,12 @@ int arch_set_info_guest(
> >      {
> >          memcpy(v->arch.fpu_ctxt, &c.nat->fpu_ctxt, 
> > sizeof(c.nat->fpu_ctxt));
> >          if ( v->arch.xsave_area )
> > +        {
> >               v->arch.xsave_area->xsave_hdr.xstate_bv = XSTATE_FP_SSE;
> > +             if ( cpu_has_xsaves || cpu_has_xsavec )
> > +                  v->arch.xsave_area->xsave_hdr.xcomp_bv = XSTATE_FP_SSE |
> > +                                                           
> > XSTATE_COMPACTION_ENABLED;
> > +        }
> 
> So here you nicely extend the existing conditional.
> 
> > @@ -1568,6 +1573,8 @@ static void __context_switch(void)
> >              if ( xcr0 != get_xcr0() && !set_xcr0(xcr0) )
> >                  BUG();
> >          }
> > +        if ( cpu_has_xsaves && has_hvm_container_vcpu(n) )
> > +            set_msr_xss(n->arch.hvm_vcpu.msr_xss);
> 
> Why not also here (the previous if() uses cpu_has_xsave, which
> you surely depend on)? Agreed the difference is minor for modern
> CPUs, but I wanted to ask anyway. I.e. an explanation will do,
> no need to re-submit just because of this.
> 
Yes. It is better to put the cpu_has_xsaves into the previous if().
> > @@ -158,6 +334,20 @@ void xsave(struct vcpu *v, uint64_t mask)
> >          ptr->fpu_sse.x[FPU_WORD_SIZE_OFFSET] = word_size;
> >  }
> > +#define XSTATE_FIXUP ".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"
> > +
> >  void xrstor(struct vcpu *v, uint64_t mask)
> >  {
> >      uint32_t hmask = mask >> 32;
> > @@ -187,39 +377,22 @@ void xrstor(struct vcpu *v, uint64_t mask)
> >      switch ( __builtin_expect(ptr->fpu_sse.x[FPU_WORD_SIZE_OFFSET], 8) )
> >      {
> >      default:
> > -        asm volatile ( "1: .byte 0x48,0x0f,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 0x48,0x0f,0xae,0x2f",
> > +                          ".byte 0x48,0x0f,0xc7,0x1f",
> > +                          X86_FEATURE_XSAVES,
> > +                          "D" (ptr), "m" (*ptr), "a" (lmask), "d" (hmask));
> > +        asm volatile (XSTATE_FIXUP);
> >          break;
> >      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 ?  
Thanks 
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> 

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