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

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Date:	Mon, 20 Apr 2015 18:11:51 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Andrew Cooper <andrew.cooper3@...rix.com>,
	Xen-devel <xen-devel@...ts.xen.org>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, <x86@...nel.org>,
	<linux-kernel@...r.kernel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	"Rusty Russell" <rusty@...tcorp.com.au>, <lguest@...ts.ozlabs.org>
Subject: Re: [PATCH] [RFC] x86/cpu: Fix SMAP check in PVOPS environments

On 20/04/15 18:09, 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.

Reviewed-by: David Vrabel <david.vrabel@...rix.com>

David

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