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

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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@xxxxxxxxx> 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?

> > Does the emulated CR3 load need to learn about the new function of the low bits?
> 
> I have not found any restrictions on writing CR3 brings about by PCID.

It would be good for completeness.

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