Messages in this thread | | | From | Rusty Russell <> | Subject | Re: [PATCH] [RFC] x86/cpu: Fix SMAP check in PVOPS environments | Date | Tue, 21 Apr 2015 14:37:33 +0930 |
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Andy Lutomirski <luto@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@rustcorp.com.au> (lguest)
Thanks, Rusty.
>> 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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