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

[Daniel P . Berrangé]

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
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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