linux-kernel - Re: [PATCH v3] x86/amd: don't set X86_BUG_SYSRET_SS

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Date:   Thu, 11 May 2017 14:46:58 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Juergen Gross <jgross@...e.com>
cc:     linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org,
        x86@...nel.org, boris.ostrovsky@...cle.com, hpa@...or.com,
        mingo@...hat.com
Subject: Re: [PATCH v3] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS when
 running under Xen

On Thu, 11 May 2017, Juergen Gross wrote:

> On 27/04/17 07:01, Juergen Gross wrote:
> > When running as Xen pv guest X86_BUG_SYSRET_SS_ATTRS must not be set
> > on AMD cpus.
> > 
> > This bug/feature bit is kind of special as it will be used very early
> > when switching threads. Setting the bit and clearing it a little bit
> > later leaves a critical window where things can go wrong. This time
> > window has enlarged a little bit by using setup_clear_cpu_cap() instead
> > of the hypervisor's set_cpu_features callback. It seems this larger
> > window now makes it rather easy to hit the problem.
> > 
> > The proper solution is to never set the bit in case of Xen.
> > 
> > Signed-off-by: Juergen Gross <jgross@...e.com>
> 
> Any objections for carrying this through the Xen tree?


Acked-by: Thomas Gleixner <tglx@...utronix.de>