Re: [Xen-devel] [PATCH 0/7] Fixes to pagetable handling

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 01/03/17 16:24, Jan Beulich wrote:
>>>> On 27.02.17 at 15:03, <andrew.cooper3@xxxxxxxxxx> wrote:
>> Outstanding hardware issues discovered include:
>>   1) There is an observable delay in AMD Fam 10h processors between loading a
>>      segment selector, and the results of the LDT/GDT memory access being
>>      visible in the pagetables (via the Access bits being set).
> Are you saying the processor continues executing instructions
> while the accessed bits are still clear? Or just that it takes very
> long to complete the instruction doing the descriptor table access?

The processor does continue to execute instructions before the access
bit gets set.  I discovered this because my XTF test which checks for
the correct behaviour of the A/D bits tripped over it.  (On that note, I
really need to clean up that test and post it.)

Even a 1000-nop loop isn't always enough of a delay to observe the
access bit becoming set.  OTOH, a serialising instruction, or forcing a
memory access through the newly-loaded segment reliably cause the
effects of the load to become visible.

The memory access definitely occurs at the point of the implicit load,
because #GP are raised properly for bad segment descriptor settings...

~Andrew

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