Messages in this thread | | | Date | Thu, 4 Jun 2015 09:58:04 +0100 | From | Andrew Cooper <> | Subject | Re: [PATCH] x86/cpu: Fix SMAP check in PVOPS environments |
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On 04/06/15 07:38, H. Peter Anvin wrote: > On 06/03/2015 02:31 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. > Could we fix the Xen pvops wrapper instead to not do things like this? > > -hpa > >
We could, and I have a patch for that, but the check would still then be broken in lguest, and it makes a hotpath rather longer.
Either pv_irq_ops.save_fl() gets defined to handle all flags, and Xen & lguest need correcting in this regard, or save_fl() gets defined to handle the interrupt flag only, and this becomes the single problematic caller in the codebase.
The problem with expanding save_fl() to handle all flags is that restore_fl() should follow suit, and there are a number of system flags are inapplicable in such a situation.
~Andrew
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