Messages in this thread | | | Date | Mon, 20 Apr 2015 17:35:35 -0700 | From | Andy Lutomirski <> | Subject | Re: [PATCH] [RFC] x86/cpu: Fix SMAP check in PVOPS environments |
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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
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