Re: [PATCH] x86emul: recognize CLDEMOTE

[Jan Beulich <jbeulich@xxxxxxxx>]

On 25.01.2022 15:22, Jan Beulich wrote:
> We claim to support the insn, but so far the emulator has been handling
> it as a NOP.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

I'm sorry, I should have Cc-ed Paul here as well.

Jan

> ---
> While handling x86emul_cldemote separately in hvmemul_cache_op() means
> to carry some redundant code, folding it with CLFLUSH{,OPT} / CLWB
> didn't seem very attractive either.
> 
> --- a/xen/arch/x86/arch.mk
> +++ b/xen/arch/x86/arch.mk
> @@ -23,6 +23,7 @@ $(call as-option-add,CFLAGS,CC,"xsaveopt
>  $(call as-option-add,CFLAGS,CC,"rdseed %eax",-DHAVE_AS_RDSEED)
>  $(call as-option-add,CFLAGS,CC,"clac",-DHAVE_AS_CLAC_STAC)
>  $(call as-option-add,CFLAGS,CC,"clwb (%rax)",-DHAVE_AS_CLWB)
> +$(call as-option-add,CFLAGS,CC,"cldemote (%rax)",-DHAVE_AS_CLDEMOTE)
>  $(call as-option-add,CFLAGS,CC,".equ \"x\"$$(comma)1",-DHAVE_AS_QUOTED_SYM)
>  $(call as-option-add,CFLAGS,CC,"invpcid 
> (%rax)$$(comma)%rax",-DHAVE_AS_INVPCID)
>  $(call as-option-add,CFLAGS,CC,"movdiri 
> %rax$$(comma)(%rax)",-DHAVE_AS_MOVDIR)
> --- a/xen/arch/x86/hvm/emulate.c
> +++ b/xen/arch/x86/hvm/emulate.c
> @@ -2351,6 +2351,28 @@ static int hvmemul_cache_op(
>           * to be sensibly used is in (virtualization unaware) firmware.
>           */
>          break;
> +
> +    case x86emul_cldemote:
> +        ASSERT(!is_x86_system_segment(seg));
> +
> +        if ( !boot_cpu_has(X86_FEATURE_CLDEMOTE) ||
> +             hvmemul_virtual_to_linear(seg, offset, 0, NULL, hvm_access_none,
> +                                       hvmemul_ctxt, &addr) != X86EMUL_OKAY )
> +            break;
> +
> +        if ( hvmemul_ctxt->seg_reg[x86_seg_ss].dpl == 3 )
> +            pfec |= PFEC_user_mode;
> +
> +        mapping = hvmemul_map_linear_addr(addr, 0, pfec, hvmemul_ctxt);
> +        if ( mapping == ERR_PTR(~X86EMUL_EXCEPTION) )
> +            x86_emul_reset_event(&hvmemul_ctxt->ctxt);
> +        if ( IS_ERR_OR_NULL(mapping) )
> +            break;
> +
> +        cldemote(mapping);
> +
> +        hvmemul_unmap_linear_addr(mapping, addr, 0, hvmemul_ctxt);
> +        break;
>      }
>  
>      return X86EMUL_OKAY;
> --- a/xen/arch/x86/include/asm/system.h
> +++ b/xen/arch/x86/include/asm/system.h
> @@ -37,6 +37,16 @@ static inline void clwb(const void *p)
>  #endif
>  }
>  
> +static inline void cldemote(const void *p)
> +{
> +#if defined(HAVE_AS_CLDEMOTE)
> +    asm volatile ( "cldemote %0" :: "m" (*(const char *)p) );
> +#else
> +    asm volatile ( ".byte 0x0f, 0x1c, 0x02"
> +                   :: "d" (p), "m" (*(const char *)p) );
> +#endif
> +}
> +
>  #define xchg(ptr,v) \
>      ((__typeof__(*(ptr)))__xchg((unsigned long)(v),(ptr),sizeof(*(ptr))))
>  
> --- a/xen/arch/x86/x86_emulate/x86_emulate.c
> +++ b/xen/arch/x86/x86_emulate/x86_emulate.c
> @@ -6290,7 +6290,8 @@ x86_emulate(
>  
>      case X86EMUL_OPC(0x0f, 0x0d): /* GrpP (prefetch) */
>      case X86EMUL_OPC(0x0f, 0x18): /* Grp16 (prefetch/nop) */
> -    case X86EMUL_OPC(0x0f, 0x19) ... X86EMUL_OPC(0x0f, 0x1f): /* nop */
> +    case X86EMUL_OPC(0x0f, 0x19) ... X86EMUL_OPC(0x0f, 0x1b): /* nop */
> +    case X86EMUL_OPC(0x0f, 0x1d) ... X86EMUL_OPC(0x0f, 0x1f): /* nop */
>          break;
>  
>  #ifndef X86EMUL_NO_MMX
> @@ -6627,6 +6628,12 @@ x86_emulate(
>  
>  #endif /* !X86EMUL_NO_SIMD */
>  
> +    case X86EMUL_OPC(0x0f, 0x1c): /* cldemote / nop */
> +        if ( ctxt->cpuid->feat.cldemote && !vex.pfx && !modrm_reg &&
> +             ops->cache_op )
> +            ops->cache_op(x86emul_cldemote, ea.mem.seg, ea.mem.off, ctxt);
> +        break;
> +
>      case X86EMUL_OPC(0x0f, 0x20): /* mov cr,reg */
>      case X86EMUL_OPC(0x0f, 0x21): /* mov dr,reg */
>      case X86EMUL_OPC(0x0f, 0x22): /* mov reg,cr */
> --- a/xen/arch/x86/x86_emulate/x86_emulate.h
> +++ b/xen/arch/x86/x86_emulate/x86_emulate.h
> @@ -177,6 +177,7 @@ enum x86_emulate_fpu_type {
>  };
>  
>  enum x86emul_cache_op {
> +    x86emul_cldemote,
>      x86emul_clflush,
>      x86emul_clflushopt,
>      x86emul_clwb,
> 
>