linux-kernel - Re: [PATCH] KVM: SVM: fix trashing of MSR_TSC

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Message-ID: <20160708110900.GF3808@pd.tnic>
Date:	Fri, 8 Jul 2016 13:09:00 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Eduardo Habkost <ehabkost@...hat.com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, stable@...r.kernel.org,
	Yazen Ghannam <yazen.ghannam@....com>,
	Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH] KVM: SVM: fix trashing of MSR_TSC_AUX

On Thu, Jul 07, 2016 at 02:43:49PM -0300, Eduardo Habkost wrote:
> I see. If you have an useful use case for it, we may consider
> that. But first I would like to see an actual case where a
> feature was not added to GET_SUPPORTED_CPUID yet, but would not
> crash and burn if forcibly enabled by QEMU.

Ok.

> I mean live migration to a different host (that normally has the
> same CPU vendor). When you live-migrate or use savevm, you need
> to send the machine state to the other host. This is implemented
> using VMStateDescription structs describing the data to be
> migrated. See vmstate_x86_cpu in target-i386/machine.c, for
> example.
> 
> You need additional migration sections if the feature introduces
> additional state (e.g. CPU registers) that need to be migrated
> too, to keep the feature working. If there's new state but no
> migration support is implemented yet, you need to add the feature
> to unmigratable_flags.
> 
> For an example where no additional state is introduced by new
> features, see:

Thanks for the examples and the explanation - I see the deal now.

Ok, I'll go through the features and see what kind of state the kernel
programs in there and add them to a VMStateDescription thing. Hohumm,
makes sense to me.

Thanks.

-- 
Regards/Gruss,
    Boris.

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