Re: [PATCH 2/2] hvm: allow pass-through of new CPUID features

[Andre Przywara <andre.przywara@amd.com>]

Andre Przywara 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.

Seems like I was a bit to eager with my rationale, this patch of course 
affects only HVM guests.

Regards,
Andre.

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