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

[Rusty Russell]

Andy Lutomirski <luto at kernel.org> writes:
> On 04/20/2015 10:09 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.

That should work for lguest.  Indeed, it does (in practice those bits
are 0).

Tested-by: Rusty Russell <rusty at rustcorp.com.au> (lguest)

Thanks,
Rusty.

>> Signed-off-by: Andrew Cooper <andrew.cooper3 at citrix.com>
>> CC: Thomas Gleixner <tglx at linutronix.de>
>> CC: Ingo Molnar <mingo at redhat.com>
>> CC: H. Peter Anvin <hpa at zytor.com>
>> CC: x86 at kernel.org
>> CC: linux-kernel at vger.kernel.org
>> CC: Konrad Rzeszutek Wilk <konrad.wilk at oracle.com>
>> CC: Boris Ostrovsky <boris.ostrovsky at oracle.com>
>> CC: David Vrabel <david.vrabel at citrix.com>
>> CC: xen-devel <xen-devel at lists.xen.org>
>> CC: Rusty Russell <rusty at rustcorp.com.au>
>> CC: lguest at lists.ozlabs.org
>>
>> ---
>> This patch is RFC because I am not certain that native_save_fl() is
>> necessarily the correct solution on lguest, but it does seem that setup_smap()
>> wants to check the actual AC bit, rather than an idealised value.
>>
>> A different approach, given the dual nature of the AC flag now is to gate
>> setup_smap() on a kernel rpl of 0.  SMAP necessarily can't be used in a
>> paravirtual situation where the kernel runs in cpl > 0.
>>
>> Another different approach would be to formally state that
>> pv_irq_ops.save_fl() needs to return all the flags, which would make
>> local_irq_save() safe to use in this circumstance, but that makes a hotpath
>> longer for the sake of a single boot time check.
>
> ...which reminds me:
>
> Why does native_restore_fl restore anything other than IF?  A branch and 
> sti should be considerably faster than popf.
>
> Also, if we did this, could Xen use PVI and then use native_restore_fl 
> and avoid lots of pvops?
>
> --Andy