linux-kernel - Re: [Qemu-devel] [Patch V2 1/2] x86, mce: Basic support to add LMCE support to QEMU

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Message-ID: <20151214163738.GJ11630@pd.tnic>
Date:	Mon, 14 Dec 2015 17:37:38 +0100
From:	Borislav Petkov <bp@...e.de>
To:	Eduardo Habkost <ehabkost@...hat.com>
Cc:	Ashok Raj <ashok.raj@...el.com>, kvm@...r.kernel.org,
	Tony Luck <tony.luck@...el.com>,
	Gong Chen <gong.chen@...el.com>,
	Gleb Natapov <gleb@...nel.org>, linux-kernel@...r.kernel.org,
	qemu-devel@...gnu.org, Andi Kleen <andi.kleen@...el.com>,
	Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [Qemu-devel] [Patch V2 1/2] x86, mce: Basic support to add LMCE
 support to QEMU

On Mon, Dec 14, 2015 at 02:23:56PM -0200, Eduardo Habkost wrote:
> > -#define MCE_CAP_DEF     (MCG_CTL_P|MCG_SER_P)
> > +#define MCE_CAP_DEF     (MCG_CTL_P|MCG_SER_P|MCG_LMCE_P)
> 
> This makes mcg_cap change when upgrading QEMU.
> 
> VMs with MCG_LMCE_P enabled shouldn't be migratable to hosts
> running older kernels, or the guest may try to read or write
> MSR_IA32_MCG_EXT_CTL after miration and get a #GP. That means:
> 
> 1) Older machine-types (pc-2.5 and older) should keep the
>    old (MCG_CTL_P|MCG_SER_P) default.
> 2) We can't make pc-2.6 enable LMCE by default, either, because
>    QEMU guarantees that just changing the machine-type shouldn't
>    introduce new host requirements (see:
>    http://article.gmane.org/gmane.comp.emulators.qemu/346651)
> 
> It looks like we need a new -cpu option to enable the feature,
> then. At least until we raise the minimum kernel version
> requirements of QEMU.

... and obviously LMCE is vendor-specific so it cannot be enabled on
!Intel guests with a define like that. mce_init() in qemu should check
vendor too.

The same mistake was done with SER_P but that's much harder to change,
as we discussed previously.

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
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