linux-kernel - Re: [PATCH 1/6] kvm: Add KVM_GET_EMULATED

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Date:	Sat, 28 Sep 2013 12:49:04 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Eduardo Habkost <ehabkost@...hat.com>
Cc:	Gleb Natapov <gleb@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...e.de>, "H. Peter Anvin" <hpa@...or.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Andre Przywara <andre@...rep.de>,
	Joerg Roedel <joro@...tes.org>, X86 ML <x86@...nel.org>,
	KVM <kvm@...r.kernel.org>, qemu-devel@...gnu.org,
	libvir-list@...hat.com, Jiri Denemark <jdenemar@...hat.com>
Subject: Re: [PATCH 1/6] kvm: Add KVM_GET_EMULATED_CPUID

On Fri, Sep 27, 2013 at 11:21:34AM -0300, Eduardo Habkost wrote:
> The problem here is that "requested_features" doesn't include just
> the explicit "+flag" flags, but any flag included in the CPU model
> definition. See the "-cpu n270" example below.

Oh, you mean if requested_features would contain a flag included from
the CPU model definition - a flag which we haven't requested explicitly
- and if kvm emulates that flag, then it will get enabled?

Hmm.

> It should, but your patch will make it stop failing because of MOVBE, as
> now it can be emulated[1].

Right.

> "enforce" makes sure all features are really being enabled. It makes
> QEMU abort if there's any feature that can't be enabled on that host.

Ok.

> [1] Maybe one source of confusion is that the existing code have two
> feature-filtering functions doing basically the same thing:
> filter_features_for_kvm() and kvm_check_features_against_host().  That's

Yes, and the first gets executed unconditionally and does the feature
filtering,  right after the second has run in the kvm_enabled() branch.

> something we must clean up, and they should be unified. "enforce" should
> become synonymous to "make sure filtered_features is all zeroes".  This
> way, libvirt can emulate what 'enforce" does while being able to collect
> detailed error information (which is not easy to do if QEMU simply
> aborts).

Ok, maybe someone who's more knowledgeable with this code should do it -
not me :)

Also, there's another aspect, while we're here: now that QEMU emulates
MOVBE with TCG too, how do we specify on the command line, which
emulation should be used - kvm.ko or QEMU?

Thanks.

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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