Re: [PATCH] xen: fix logical error in tlb flushing

Jan Beulich
2012-09-05T08:03:38Z

>>> On 05.09.12 at 07:34, Alex Shi <alex.shi@intel.com> wrote:
> On 08/25/2012 03:45 AM, Jan Beulich wrote:
>>>>> On 24.08.12 at 20:17, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> wrote:
>>> How can I reproduce this
>>
>> I don't know, I spotted this while looking at the code.
>
> Again, since the old buggy code doesn't cause trouble in PV guest, guess
> the hypercall for MMUEXT_INVLPG_MULTI was translated or treated as
> MMUEXT_TLB_FLUSH_MULTI. If so, believe correct this will bring a big
> performance benefit.

It's not clear to me what was buggy with the code prior to your
change. And no, there's no magic widening of the scope of these
MMU operations - if you ask the hypervisor for a single page
invalidation, that's what it's going to do. But of course, there
are cases where extra (full) invalidations need to be done
without a guest asking for them. But that's nothing a guest can
validly make itself dependent upon.

Jan