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

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Message-ID: <20170426220448.sjsg4f7odacb2uz3@pd.tnic>
Date:   Thu, 27 Apr 2017 00:04:48 +0200
From:   Borislav Petkov <bp@...en8.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,
        tglx@...utronix.de, mingo@...hat.com
Subject: Re: [PATCH] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS if forced to
 zero

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().

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().

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?

Thanks.

-- 
Regards/Gruss,
    Boris.

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