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

[Andy Lutomirski <luto@kernel.org>]

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.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
> CC: Thomas Gleixner <tglx@linutronix.de>
> CC: Ingo Molnar <mingo@redhat.com>
> CC: H. Peter Anvin <hpa@zytor.com>
> CC: x86@kernel.org
> CC: linux-kernel@vger.kernel.org
> CC: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> CC: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> CC: David Vrabel <david.vrabel@citrix.com>
> CC: xen-devel <xen-devel@lists.xen.org>
> CC: Rusty Russell <rusty@rustcorp.com.au>
> CC: lguest@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