Re: [Xen-devel] [PATCH] x86emul: honor MXCSR.MM

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 13/10/16 13:57, Jan Beulich wrote:
> Commit 6dc9ac9f52 ("x86emul: check alignment of SSE and AVX memory
> operands") didn't consider a specific AMD mode: Mis-alignment #GP
> faults can be masked on some of their hardware.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

This highlights that the following CPUID dependency change is also required

diff --git a/xen/tools/gen-cpuid.py b/xen/tools/gen-cpuid.py
index 33e68eb..e803654 100755
--- a/xen/tools/gen-cpuid.py
+++ b/xen/tools/gen-cpuid.py
@@ -185,8 +185,9 @@ def crunch_numbers(state):
         # the first place.
         APIC: [X2APIC],
 
-        # AMD built MMXExtentions and 3DNow as extentions to MMX.
-        MMX: [MMXEXT, _3DNOW],
+        # AMD built MMXExtentions, 3DNow and SSE Misalignment as
extensions to
+        # MMX.
+        MMX: [MMXEXT, _3DNOW, MISALIGNSSE],
 
         # The FXSAVE/FXRSTOR instructions were introduced into hardware
before
         # SSE, which is why they behave differently based on
%CR4.OSFXSAVE and



>
> --- a/xen/arch/x86/x86_emulate/x86_emulate.c
> +++ b/xen/arch/x86/x86_emulate/x86_emulate.c
> @@ -446,6 +446,9 @@ typedef union {
>  #define EFLG_PF   (1<<2)
>  #define EFLG_CF   (1<<0)
>  
> +/* MXCSR bit definitions. */
> +#define MXCSR_MM  (1U << 17)
> +
>  /* Exception definitions. */
>  #define EXC_DE  0
>  #define EXC_DB  1
> @@ -1253,6 +1256,7 @@ static bool_t vcpu_has(
>  
>  #define vcpu_has_clflush() vcpu_has(       1, EDX, 19, ctxt, ops)
>  #define vcpu_has_lzcnt() vcpu_has(0x80000001, ECX,  5, ctxt, ops)
> +#define vcpu_has_misalignsse() vcpu_has(0x80000001, ECX, 7, ctxt, ops)
>  #define vcpu_has_bmi1()  vcpu_has(0x00000007, EBX,  3, ctxt, ops)
>  #define vcpu_has_hle()   vcpu_has(0x00000007, EBX,  4, ctxt, ops)
>  #define vcpu_has_rtm()   vcpu_has(0x00000007, EBX, 11, ctxt, ops)
> @@ -4675,7 +4679,13 @@ x86_emulate(
>              ea.bytes = vex.pfx & VEX_PREFIX_DOUBLE_MASK ? 8 : 4;
>          if ( ea.type == OP_MEM )
>          {
> -            generate_exception_if((b >= 0x28) &&
> +            uint32_t mxcsr = 0;
> +
> +            if ( b < 0x28 )
> +                mxcsr = MXCSR_MM;
> +            else if ( vcpu_has_misalignsse() )
> +                asm ( "stmxcsr %0" : "=m" (mxcsr) );
> +            generate_exception_if(!(mxcsr & MXCSR_MM) &&
>                                    !is_aligned(ea.mem.seg, ea.mem.off, 
> ea.bytes,
>                                                ctxt, ops),
>                                    EXC_GP, 0);
> @@ -4955,7 +4965,13 @@ x86_emulate(
>          }
>          if ( ea.type == OP_MEM )
>          {
> -            generate_exception_if((vex.pfx == vex_66) &&
> +            uint32_t mxcsr = 0;
> +
> +            if ( vex.pfx != vex_66 )
> +                mxcsr = MXCSR_MM;
> +            else if ( vcpu_has_misalignsse() )
> +                asm ( "stmxcsr %0" : "=m" (mxcsr) );
> +            generate_exception_if(!(mxcsr & MXCSR_MM) &&
>                                    !is_aligned(ea.mem.seg, ea.mem.off, 
> ea.bytes,
>                                                ctxt, ops),
>                                    EXC_GP, 0);

According to the docs, we should also be possibly raising #AC here.

~Andrew

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