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Re: [Qemu-devel] [PATCH] target-i386: add pcid to both Sandy Bridge and
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [PATCH] target-i386: add pcid to both Sandy Bridge and Ivy Bridge |
Date: |
Mon, 8 Jan 2018 18:09:30 -0500 (EST) |
----- Original Message -----
> From: "Eduardo Habkost" <address@hidden>
> To: "Paolo Bonzini" <address@hidden>
> Cc: "Vincent Bernat" <address@hidden>, "Richard Henderson" <address@hidden>,
> address@hidden
> Sent: Monday, January 8, 2018 11:56:25 PM
> Subject: Re: [PATCH] target-i386: add pcid to both Sandy Bridge and Ivy Bridge
>
> 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?
Indeed it is! It will also be useful for KPTI performance without
INVPCID, but it won't be useful without EPT.
Paolo
> (BTW, is PCID useful for KPTI performance without INVPCID?)