Re: [Xen-devel] [PATCH v5 3/4] VMX: use proper instruction mnemonics if assembler supports them

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 29.08.13 at 13:47, Tim Deegan <tim@xxxxxxx> wrote:
> At 16:31 +0100 on 26 Aug (1377534696), Jan Beulich wrote:
>> Additionally I was quite puzzled to find that all the asm()-s involved
>> here have memory clobbers - what are they needed for? Or can they be
>> dropped at least in some cases?
> 
> The vmread/write ones are, I think, red herrings.  We're not allowed to
> make assumptions about the memory state of a loaded VMCS anyway.
> 
> invept I think does, to make sure all EPT changes have been written.
> invvpid too, for similar reasons.

Considering that these instructions aren't documented to be
ordered in any way with earlier ones (at least the instruction
pages don't say anything to that effect), wouldn't we rather
need an MFENCE to enforce this?

> vmptrld/clear I'm not sure about: if we were to (say) copy a VMCS or
> move it we'd need that barrier.  (AFAIK we don't do that but we might be
> very surprised if we started).

Similarly here.

Jan


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