Re: [RFC KVM 18/27] kvm/isolation: function to copy page table entries for percpu buffer

[Jonathan Adams <jwadams@google.com>]

On Tue, May 14, 2019 at 3:38 PM Sean Christopherson
<sean.j.christopherson@intel.com> wrote:
> On Tue, May 14, 2019 at 02:55:18PM -0700, Andy Lutomirski wrote:
> > > On May 14, 2019, at 2:06 PM, Sean Christopherson <sean.j.christopherson@intel.com> wrote:
> > >> On Tue, May 14, 2019 at 01:33:21PM -0700, Andy Lutomirski wrote:
> > >> I suspect that the context switch is a bit of a red herring.  A
> > >> PCID-don't-flush CR3 write is IIRC under 300 cycles.  Sure, it's slow,
> > >> but it's probably minor compared to the full cost of the vm exit.  The
> > >> pain point is kicking the sibling thread.
> > >
> > > Speaking of PCIDs, a separate mm for KVM would mean consuming another
> > > ASID, which isn't good.
> >
> > I’m not sure we care. We have many logical address spaces (two per mm plus a
> > few more).  We have 4096 PCIDs, but we only use ten or so.  And we have some
> > undocumented number of *physical* ASIDs with some undocumented mechanism by
> > which PCID maps to a physical ASID.
>
> Yeah, I was referring to physical ASIDs.
>
> > I don’t suppose you know how many physical ASIDs we have?
>
> Limited number of physical ASIDs.  I'll leave it at that so as not to
> disclose something I shouldn't.
>
> > And how it interacts with the VPID stuff?
>
> VPID and PCID get factored into the final ASID, i.e. changing either one
> results in a new ASID.  The SDM's oblique way of saying that:
>
>   VPIDs and PCIDs (see Section 4.10.1) can be used concurrently. When this
>   is done, the processor associates cached information with both a VPID and
>   a PCID. Such information is used only if the current VPID and PCID both
>   match those associated with the cached information.
>
> E.g. enabling PTI in both the host and guest consumes four ASIDs just to
> run a single task in the guest:
>
>   - VPID=0, PCID=kernel
>   - VPID=0, PCID=user
>   - VPID=1, PCID=kernel
>   - VPID=1, PCID=user
>
> The impact of consuming another ASID for KVM would likely depend on both
> the guest and host configurations/worloads, e.g. if the guest is using a
> lot of PCIDs then it's probably a moot point.  It's something to keep in
> mind though if we go down this path.

One answer to that would be to have the KVM page tables use the same
PCID as the normal user-mode PTI page tables.  It's not ideal (since
the qemu/whatever process can see some kernel data via meltdown it
wouldn't be able to normally see), but might be an option to
investigate.

Cheers,
- jonathan