Re: [Xen-devel] [PATCH] x86/VMX: Implement vmptrst

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 06/01/17 14:37, Jan Beulich wrote:
>>>> On 05.01.17 at 19:42, <anshul.makkar@xxxxxxxxxx> wrote:
>> +static always_inline u64 __vmptrst(void)
>> +{
>> +    u64 paddr;
>> +
>> +    asm volatile (
>> +#ifdef HAVE_GAS_VMX
>> +                   "vmptrst %0\n"
>> +#else
>> +                   VMPTRST_OPCODE MODRM_EAX_07
>> +#endif
>> +
>> +#ifdef HAVE_GAS_VMX
>> +                   : "=m" (paddr)
>> +                   :
>> +#else
>> +                   :
>> +                   : "a" (&paddr),
>> +#endif
>> +                   : "memory");
> I don't see the point of the memory clobber here in the
> HAVE_GAS_VMX case (and in the other case it could be easily
> avoided by making the output common).

Currently it is the only thing covering the fact that paddr actually
gets written to.

> In fact some time ago I
> did raise the question already as to whether some of the other
> inline functions shouldn't also be relaxed.

Didn't we agree that removing the memory clobbers was a good thing to
do?  I recall that you asked, but I don't recall what the outcome was.

~Andrew

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