Re: [Qemu-devel] [PATCH] target-i386: add pcid to both Sandy Bridge and Ivy Bridge

[Paolo Bonzini]

----- Original Message -----
> From: "Eduardo Habkost" <address@hidden>
> To: "Vincent Bernat" <address@hidden>
> Cc: "Paolo Bonzini" <address@hidden>, "Richard Henderson" <address@hidden>, 
> address@hidden
> Sent: Monday, January 8, 2018 10:16:23 PM
> Subject: Re: [PATCH] target-i386: add pcid to both Sandy Bridge and Ivy Bridge
> 
> On Mon, Jan 08, 2018 at 09:50:52PM +0100, Vincent Bernat wrote:
> > PCID has been introduced in Sandy Bridge and, currently, KVM doesn't
> > object exposing it to VM as long as it is present on the host. Update
> > CPU model for both Sandy Bridge and Ivy Bridge accordingly.
> > 
> > Signed-off-by: Vincent Bernat <address@hidden>
> 
> Thanks for your patch.
> 
> We need two things, though:
> 
> First, confirming that all hosts where the SandyBridge and
> IvyBridge CPU models are runnable will support exposing PCID to
> guests (otherwise updating QEMU can make a runnable VM
> configuration suddenly stop being runnable).  This can happen if
> the host kernel is too old.

I've been reading it's also Westmere.  I'll check more carefully tomorrow.
The difference between consumer and server SKUs is important too.

> One possible way to work around this problem is to declare that
> QEMU 2.12 with KVM will require Linux v3.6 and newer (because we
> need Linux kernel commit ad756a1603c5 "KVM: VMX: Implement
> PCID/INVPCID for guests with EPT").

Note that PCID is still not supported for guests without EPT, so
this would break ept=0 with recent "-cpu" models.  I'm not sure of
a way to fix it; probably it just has to be documented.

Paolo