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

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Date:   Thu, 27 Apr 2017 06:44:02 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org,
        x86@...nel.org, boris.ostrovsky@...cle.com, hpa@...or.com,
        tglx@...utronix.de, mingo@...hat.com
Subject: Re: [PATCH] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS if forced to
 zero

On 27/04/17 00:04, Borislav Petkov wrote:
> On Wed, Apr 26, 2017 at 08:24:12PM +0200, Juergen Gross wrote:
>> I'm not feeling strong about it. So if you want to test for
>> X86_FEATURE_XENPV to avoid setting X86_BUG_SYSRET_SS_ATTRS I'm fine
>> with it.
>>
>> Will send V2 with that change.
> 
> And remove the corresponding
> 
> 	clear_cpu_bug(c, X86_BUG_SYSRET_SS_ATTRS);
> 
> in xen_set_cpu_features().

Okay, you are right, we can omit this one now.

> So that we can set it once, only on !XENPV feature set.
> 
> /me looks again at the code...
> 
> Gah, except that we do
> 
> 	set_cpu_cap(c, X86_FEATURE_XENPV);
> 
> and that runs as part of init_hypervisor() and it runs *after* c_init().

No, this is called by xen_start_kernel(), long before c_init().

> So, back to square one. :-\
> 
> So lemme try to explain again what I mean:
> 
> I'd like to have a generic way of detecting whether I'm running as a xen
> guest at ->c_init() time and depending on the result of that detection,
> to set X86_BUG_SYSRET_SS_ATTRS or not set it.
> 
> Does that make more sense?

This does make sense and it is working, as Sander could confirm.


Juergen