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Message-ID: <5570131C.1000704@citrix.com>
Date: Thu, 4 Jun 2015 09:58:04 +0100
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: "H. Peter Anvin" <hpa@...or.com>,
Xen-devel <xen-devel@...ts.xen.org>
CC: David Vrabel <david.vrabel@...rix.com>,
Rusty Russell <rusty@...tcorp.com.au>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, <x86@...nel.org>,
<linux-kernel@...r.kernel.org>,
"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>,
Boris Ostrovsky <boris.ostrovsky@...cle.com>,
<lguest@...ts.ozlabs.org>, <stable@...r.kernel.org>
Subject: Re: [PATCH] x86/cpu: Fix SMAP check in PVOPS environments
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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