Re: About two-dimensional page translation (e.g., Intel EPT) and shadow page table in Linux QEMU/KVM

[Sean Christopherson <seanjc@google.com>]

On Mon, Jul 12, 2021, Maxim Levitsky wrote:
> On Mon, 2021-07-12 at 08:02 -0500, harry harry wrote:
> > Dear Maxim,
> > 
> > Thanks for your reply. I knew, in our current design/implementation,
> > EPT/NPT is enabled by a module param. I think it is possible to modify
> > the QEMU/KVM code to let it support EPT/NPT and show page table (SPT)
> > simultaneously (e.g., for an 80-core server, 40 cores use EPT/NPT and
> > the other 40 cores use SPT). What do you think? Thanks!
> > 
> > Best regards,
> > Harry
> > 
> > On Mon, Jul 12, 2021 at 4:49 AM Maxim Levitsky <mlevitsk@redhat.com> wrote:
> > > On Sun, 2021-07-11 at 15:13 -0500, harry harry wrote:
> > > > Hi all,
> > > > 
> > > > I hope you are very well! May I know whether it is possible to enable
> > > > two-dimensional page translation (e.g., Intel EPT) mechanisms and
> > > > shadow page table mechanisms in Linux QEMU/KVM at the same time on a
> > > > physical server? For example, if the physical server has 80 cores, is
> > > > it possible to let 40 cores use Intel EPT mechanisms for page
> > > > translation and the other 40 cores use shadow page table mechanisms?
> > > > Thanks!
> > > 
> > > Nope sadly. EPT/NPT is enabled by a module param.
> > >
> For same VM, I don't think it is feasable.

Heh, because the MMUs are all per-vCPU, it actually wouldn't be that much effort
beyond supporting !TDP and TDP for different VMs...

> For multiple VMs make some use NPT/EPT and some don't,
> this should be possible to implement.

...but supporting !TDP and TDP in a single KVM instance isn't going to happen.
It's certainly possible, but comes with a very high complexity cost, and likely
even performance costs.

The more sane way to support !TDP and TDP on a single host would be to support
multiple instances of KVM, e.g. /dev/kvm0, /dev/kvm1, etc...  Being able to use
!TDP and TDP isn't strong justification for the work required, but supporting
multiple KVM instances would allow upgrading KVM without having to migrate VMs
off the host, which is very desirable.  If multiple KVM instances are supported,
running !TDP and TDP KVM instances should Just Work.