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Re: [Qemu-devel] [PATCH 1/2] i386: define the AMD 'amd-ssbd' CPUID featu

From: Tom Lendacky
Subject: Re: [Qemu-devel] [PATCH 1/2] i386: define the AMD 'amd-ssbd' CPUID feature bit
Date: Fri, 8 Jun 2018 16:22:32 -0500
User-agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.8.0

On 6/6/2018 9:20 AM, Daniel P. Berrangé wrote:
> On Tue, Jun 05, 2018 at 08:31:41AM -0500, Tom Lendacky wrote:
>> On 6/4/2018 3:07 PM, Eduardo Habkost wrote:
>>> On Fri, Jun 01, 2018 at 11:38:08AM -0400, Konrad Rzeszutek Wilk wrote:
>>>> AMD future CPUs expose _two_ ways to utilize the Intel equivalant
>>>> of the Speculative Store Bypass Disable. The first is via
>>>> the virtualized VIRT_SPEC CTRL MSR (0xC001_011f) and the second
>>>> is via the SPEC_CTRL MSR (0x48). The document titled:
>>>> 124441_AMD64_SpeculativeStoreBypassDisable_Whitepaper_final.pdf
>>>> gives priority of SPEC CTRL MSR over the VIRT SPEC CTRL MSR.
>>>> A copy of this document is available at
>>>>       https://bugzilla.kernel.org/show_bug.cgi?id=199889
>>>> Anyhow, this means that on future AMD CPUs there will be  _two_ ways to
>>>> deal with SSBD.
>>> Does anybody know if there are AMD CPUs where virt-ssbd won't
>>> work and would require amd-ssbd to mitigate vulnerabilities?
>> The idea behind virt-ssbd was to provide an architectural method for
>> a guest to do SSBD when amd-ssbd isn't present.  The amd-ssbd feature
>> will use SPEC_CTRL which is intended to not be intercepted and
>> will be fast.  The use of virt-ssbd will always be intercepted and
>> therefore will not be as fast.  So a guest should be presented with
>> amd-ssbd, if available, in preference to virt-ssbd.
> Can you clarify whether 'amd-ssbd' is also an architectural method

Yes, amd-ssbd is architectural - it is a defined CPUID bit.


> or not ?  ie is it safe to use 'amd-ssbd' in a guest which can be
> live migrated between different generations/families of AMD CPU,
> or must be use virt-ssbd in that case ?
> Regards,
> Daniel

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