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

Subject: Re: [Xen-devel] [PATCH 2/2] hvm: allow pass-through of new CPUID features
From: Andre Przywara <andre.przywara@xxxxxxx>
Date: Thu, 27 Jan 2011 16:43:16 +0100
Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Keir Fraser <keir@xxxxxxx>, Stefano Stabellini <Stefano.Stabellini@xxxxxxxxxxxxx>
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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.


Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany

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