Re: [Xen-devel] [PATCH] x86/emul: Corrections to cmpxchg{8, 16}b emulation (to fix 32bit PV guests)

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 20.01.17 at 09:52, <andrew.cooper3@xxxxxxxxxx> wrote:
> @@ -2852,6 +2852,11 @@ x86_emulate(
>          else if ( !(d & Mov) ) /* optimisation - avoid slow emulated read */
>          {
>              fail_if(lock_prefix ? !ops->cmpxchg : !ops->write);
> +
> +            /* cmpxchg{8,16}b handles its own operand read. */
> +            if ( ext == ext_0f && b == 0xc7 )
> +                break;

This part in particular is why I don't like this variant of the fix
(and if at all it would need placing ahead of the fail_if()).
I'm sorry for having talked you into that direction yesterday.

Jan


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