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Re: [Xen-devel] [PATCH 2/2] hvm: allow pass-through of new CPUID feature

To: Andre Przywara <andre.przywara@xxxxxxx>
Subject: Re: [Xen-devel] [PATCH 2/2] hvm: allow pass-through of new CPUID features
From: Keir Fraser <keir@xxxxxxx>
Date: Thu, 27 Jan 2011 16:07:32 +0000
Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Stefano Stabellini <Stefano.Stabellini@xxxxxxxxxxxxx>
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Thread-topic: [Xen-devel] [PATCH 2/2] hvm: allow pass-through of new CPUID features
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On 27/01/2011 15:43, "Andre Przywara" <andre.przywara@xxxxxxx> wrote:

> Ian Jackson wrote:
>> Stefano Stabellini writes ("Re: [Xen-devel] [PATCH 2/2] hvm: allow
>> pass-through of new CPUID features"):
>>> I think Ian was just looking for a well written explanation of why we
>>> need this now :)
> OK, I feel challenged, what about this:
> The new CPU features are pure "compute" features (ALU/FPU only), they
> don't need any support or awareness from any system level software.
> Following the definition of virtualization they should be available to
> guests unless explicitly disabled.
> The AES feature was introduced at the same time as the PCLMULQDQ
> feature, AES is passed through, PCLMULQDQ has obviously been forgotten(?).
> The Linux kernel uses both instructions in newer versions for optimized
> cryptographic algorithms implementation
> (arch/x86/crypto/ghash-clmulni-intel_asm.S)
> The current version of the Xen hypervisor would deny this feature to any
> PV domains, including Dom0, leaving systems running under Xen with an
> inferior cryptographic performance.
> This patch allows PV domains to use the feature to get on par with
> systems running with other virtualization software.

You've lost your mind a bit there. The patch affects only HVM guests. PV
guests have a feature blacklist policy, and so these features are already
passed through to PV guests (inc. dom0) just fine.

 -- Keir

> Regards,
> Andre.

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