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

[Eduardo Habkost]

On Mon, Jan 08, 2018 at 05:37:16PM -0500, Paolo Bonzini wrote:
> 
> 
> ----- 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.

GET_SUPPORTED_CPUID seems to still return PCID as supported
without EPT, doesn't it?

(BTW, is PCID useful for KPTI performance without INVPCID?)

-- 
Eduardo