linux-kernel - Re: [Xen-devel] [PATCH] hypervisor/x86/xen: Unset X86_BUG_SYSRET_SS

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Message-ID: <943161960.20150502141040@eikelenboom.it>
Date:	Sat, 2 May 2015 14:10:40 +0200
From:	Sander Eikelenboom <linux@...elenboom.it>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
CC:	David Vrabel <david.vrabel@...rix.com>, konrad.wilk@...cle.com,
	<hpa@...or.com>, <mingo@...hat.com>, <tglx@...utronix.de>,
	<xen-devel@...ts.xenproject.org>, <luto@...capital.net>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH] hypervisor/x86/xen: Unset X86_BUG_SYSRET_SS_ATTRS on Xen PV guests


Friday, May 1, 2015, 12:37:54 PM, you wrote:

> On 30/04/15 20:08, Boris Ostrovsky wrote:
>> Commit 61f01dd941ba ("x86_64, asm: Work around AMD SYSRET SS descriptor
>> attribute issue") makes AMD processors set SS to __KERNEL_DS in
>> __switch_to() to deal with cases when SS is NULL.
>> 
>> This breaks Xen PV guests who do not want to load SS with__KERNEL_DS.
>> 
>> Since the problem that the commit is trying to address would have to be
>> fixed in the hypervisor (if it in fact exists under Xen) there is no
>> reason to set X86_BUG_SYSRET_SS_ATTRS flag for PV VPCUs here.
>> 
>> This can be easily achieved by adding x86_hyper_xen_hvm.set_cpu_features
>> op which will clear this flag. (And since this structure is no longer
>> HVM-specific we should do some renaming).

> Applied to for-linus-4.1b, thanks.

> David
                    
Also works fine here.
Thanks Boris !

--
Sander

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