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SubjectRe: [PATCH] [RFC] x86/cpu: Fix SMAP check in PVOPS environments
Andy Lutomirski <> 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 <> (lguest)


>> Signed-off-by: Andrew Cooper <>
>> CC: Thomas Gleixner <>
>> CC: Ingo Molnar <>
>> CC: H. Peter Anvin <>
>> CC:
>> CC:
>> CC: Konrad Rzeszutek Wilk <>
>> CC: Boris Ostrovsky <>
>> CC: David Vrabel <>
>> CC: xen-devel <>
>> CC: Rusty Russell <>
>> CC:
>> ---
>> 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

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