Re: [Xen-devel] [PATCH] x86/emulate: Don't assume that addr_size == 32 implies protected mode

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 16.12.16 at 10:55, <george.dunlap@xxxxxxxxxx> wrote:
> Callers of x86_emulate() generally define addr_size based on the code
> segment.  In vm86 mode, the code segment is set by the hardware to be
> 16-bits; but it is entirely possible to enable protected mode, set the
> CS to 32-bits, and then disable protected mode.  (This is commonly
> called "unreal mode".)

To better match this description I think it would be preferable ...

> --- a/xen/arch/x86/x86_emulate/x86_emulate.c
> +++ b/xen/arch/x86/x86_emulate/x86_emulate.c
> @@ -2149,11 +2149,8 @@ x86_decode(
>              default:
>                  BUG(); /* Shouldn't be possible. */
>              case 2:
> -                if ( in_realmode(ctxt, ops) || (state->regs->eflags & 
> EFLG_VM) )
> -                    break;
> -                /* fall through */
>              case 4:
> -                if ( modrm_mod != 3 )
> +                if ( modrm_mod != 3 || !in_protmode(ctxt, ops) )
>                      break;

... to keep the EFLAGS.VM in case 2, and check in_realmode()
in case 4. Otoh what you have now is the more compact form,
resulting in fewer branches ...

Jan


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