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Re: [Qemu-devel] how to pass pcid to guest?

From: Daniel P. Berrange
Subject: Re: [Qemu-devel] how to pass pcid to guest?
Date: Wed, 10 Jan 2018 13:02:05 +0000
User-agent: Mutt/1.9.1 (2017-09-22)

On Mon, Jan 08, 2018 at 04:07:53PM -0600, Eric Blake wrote:
> On 01/08/2018 02:03 PM, Stefan Priebe - Profihost AG wrote:
> > Hello,
> > 
> > for meltdown mitigation and performance it's important to have the pcid
> > flag passed down to the guest (f.e.
> > https://groups.google.com/forum/m/#!topic/mechanical-sympathy/L9mHTbeQLNU).
> Indeed; you are still waiting on the qemu patch mentioned here:
> https://www.qemu.org/2018/01/04/spectre/
> which is still undergoing the review process, but should be up (in the
> form of 2.11.1) "in the next few days".

There's two different things here. The Spectre patch you mention is about
exposing a new CPUID feature 'ibrs' that is added to fix Spectre. The 'pcid'
feature Stefan mentions is a pre-existing feature that is not a security fix,
but rather a way to improve the performance when the KAISER/KPTI patches are
enabled for Meltdown.

'pcid' is available in QEMU's  Haswell, Broadwell & Skylake based CPU
models by default. For other named CPU models, you can turn it on using
',+pcid' after the model name. eg  '-cpu  Westmere,+pcid' (as long as your
host supports it of course)

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