[Lguest] [PATCH] x86/cpu: Fix SMAP check in PVOPS environments

[Rusty Russell]

Andrew Cooper <andrew.cooper3 at citrix.com> writes:
> On 04/06/15 07:38, H. Peter Anvin wrote:
>> On 06/03/2015 02:31 AM, Andrew Cooper wrote:
>>> There appears to be no formal statement of what pv_irq_ops.save_fl() is
>>> supposed to return precisely.  Native returns the full flags, while lguest and
>>> Xen only return the Interrupt Flag, and both have comments by the
>>> implementations stating that only the Interrupt Flag is looked at.  This may
>>> have been true when initially implemented, but no longer is.
>>>
>>> To make matters worse, the Xen PVOP leaves the upper bits undefined, making
>>> the BUG_ON() undefined behaviour.  Experimentally, this now trips for 32bit PV
>>> guests on Broadwell hardware.  The BUG_ON() is consistent for an individual
>>> build, but not consistent for all builds.  It has also been a sitting timebomb
>>> since SMAP support was introduced.
>>>
>>> Use native_save_fl() instead, which will obtain an accurate view of the AC
>>> flag.
>> Could we fix the Xen pvops wrapper instead to not do things like this?
>>
>> 	-hpa
>>
>>
>
> We could, and I have a patch for that, but the check would still then be
> broken in lguest, and it makes a hotpath rather longer.
>
> Either pv_irq_ops.save_fl() gets defined to handle all flags, and Xen &
> lguest need correcting in this regard, or save_fl() gets defined to
> handle the interrupt flag only, and this becomes the single problematic
> caller in the codebase.
>
> The problem with expanding save_fl() to handle all flags is that
> restore_fl() should follow suit, and there are a number of system flags
> are inapplicable in such a situation.

Yeah, hard cases make bad law.

I'm not too unhappy with this fix; ideally we'd rename save_fl and
restore_fl to save_eflags_if and restore_eflags_if too.

Cheers,
Rusty.