Re: [PATCH for-4.2 1/2] i386: Add new versions of Skylake/Cascadelake/Icelake without TSX

[Eduardo Habkost <ehabkost@redhat.com>]

On Wed, Nov 20, 2019 at 06:40:06PM +0100, Paolo Bonzini wrote:
> On 20/11/19 17:49, Eduardo Habkost wrote:
> > One of the mitigation methods for TAA[1] is to disable TSX
> > support on the host system.  Linux added a mechanism to disable
> > TSX globally through the kernel command line, and many Linux
> > distributions now default to tsx=off.  This makes existing CPU
> > models that have HLE and RTM enabled not usable anymore.
> > 
> > Add new versions of all CPU models that have the HLE and RTM
> > features enabled, that can be used when TSX is disabled in the
> > host system.
> 
> What is the effect of this when using "-cpu CascadeLake-Server" and
> upgrading QEMU?  Would it automatically switch to the new version?  If
> so, would it be better to include a duplicate of the models (and if so,
> that would conflict with my VMX features patch, which is also for 4.2).

It won't, because PCMachineClass::default_cpu_version==1 for all
versioned PC machine-types, currently.

The plan is to set default_cpu_version=CPU_VERSION_LATEST on
pc-*-5.0 (or, more likely, 5.1).  But this will happen only after
libvirt starts resolving CPU model versions.  See the
"Runnability guarantee of CPU models" section at
qemu-deprecated.texi.

-- 
Eduardo