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Re: [PATCH for-4.2 1/2] i386: Add new versions of Skylake/Cascadelake/Ic
From: |
Eduardo Habkost |
Subject: |
Re: [PATCH for-4.2 1/2] i386: Add new versions of Skylake/Cascadelake/Icelake without TSX |
Date: |
Wed, 20 Nov 2019 15:42:58 -0300 |
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