Re: [PATCH] KVM: x86: Implement PCID/INVPCID for guests with EPT

[Marcelo Tosatti <mtosatti@redhat.com>]

On Tue, May 15, 2012 at 03:28:13AM +0000, Mao, Junjie wrote:
> > -----Original Message-----
> > From: Marcelo Tosatti [mailto:mtosatti@redhat.com]
> > Sent: Tuesday, May 15, 2012 10:20 AM
> > To: Mao, Junjie
> > Cc: Eric Northup; kvm@vger.kernel.org
> > Subject: Re: [PATCH] KVM: x86: Implement PCID/INVPCID for guests with EPT
> > 
> > On Mon, May 14, 2012 at 07:15:18AM +0000, Mao, Junjie wrote:
> > > > On Wed, May 9, 2012 at 5:32 PM, Mao, Junjie <junjie.mao@intel.com>
> > wrote:
> > > > > This patch handles PCID/INVPCID for guests.
> > > > >
> > > > > Process-context identifiers (PCIDs) are a facility by which a logical
> > processor may cache information for multiple linear-address spaces so that the
> > processor may retain cached information when software switches to a
> > different linear-address space. Refer to section 4.10.1 in IA32 Intel Software
> > Developer's Manual Volume 3A for details.
> > > > >
> > > > > For guests with EPT, the PCID feature is enabled and INVPCID behaves as
> > running natively.
> > > > > For guests without EPT, the PCID feature is disabled and INVPCID triggers
> > #UD.
> > > > >
> > > > Do I understand correctly that this means it is impossible to migrate a
> > guest which is using PCID from a host with EPT to a host without EPT (by
> > passing enable_ept=0 to the module, for example) ?
> > >
> > > I think you are right. Guests using PCID/INVPCID cannot migrate to a host
> > without it, and EPT is a precondition of this feature.
> > 
> > Are there processors that support PCI/INVPCID for the host but lack support
> > for SECONDARY_EXEC_ENABLE_INVPCID?
> 
> There're some with PCID but without INVPCID. For those processors support INVPCID, SECONDARY_EXEC_ENABLE_INVPCID support should be present. As PCID has little benefits without INVPCID, these two features are exposed/hidden as a whole.

That means migration control should check INVPCID host support before deciding
whether to migrate.

Thanks