linux-kernel - Re: [PATCH] x86/cpu: Fix SMAP check in PVOPS environments

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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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