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

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Message-ID: <55435782.3040506@citrix.com>
Date:	Fri, 1 May 2015 11:37:54 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	<konrad.wilk@...cle.com>, <david.vrabel@...rix.com>,
	<hpa@...or.com>, <mingo@...hat.com>, <tglx@...utronix.de>
CC:	<xen-devel@...ts.xenproject.org>, <luto@...capital.net>,
	<linux-kernel@...r.kernel.org>, <linux@...elenboom.it>
Subject: Re: [Xen-devel] [PATCH] hypervisor/x86/xen: Unset X86_BUG_SYSRET_SS_ATTRS
 on Xen PV guests

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
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