Re: [PATCH v10 0/9] KVM: mm: fd-based approach for supporting KVM

[Sean Christopherson]

On Thu, Jan 19, 2023, Isaku Yamahata wrote:
> On Sat, Jan 14, 2023 at 12:37:59AM +0000,
> Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> 
> > On Fri, Dec 02, 2022, Chao Peng wrote:
> > > This patch series implements KVM guest private memory for confidential
> > > computing scenarios like Intel TDX[1]. If a TDX host accesses
> > > TDX-protected guest memory, machine check can happen which can further
> > > crash the running host system, this is terrible for multi-tenant
> > > configurations. The host accesses include those from KVM userspace like
> > > QEMU. This series addresses KVM userspace induced crash by introducing
> > > new mm and KVM interfaces so KVM userspace can still manage guest memory
> > > via a fd-based approach, but it can never access the guest memory
> > > content.
> > > 
> > > The patch series touches both core mm and KVM code. I appreciate
> > > Andrew/Hugh and Paolo/Sean can review and pick these patches. Any other
> > > reviews are always welcome.
> > >   - 01: mm change, target for mm tree
> > >   - 02-09: KVM change, target for KVM tree
> > 
> > A version with all of my feedback, plus reworked versions of Vishal's selftest,
> > is available here:
> > 
> >   git@xxxxxxxxxx:sean-jc/linux.git x86/upm_base_support
> > 
> > It compiles and passes the selftest, but it's otherwise barely tested.  There are
> > a few todos (2 I think?) and many of the commits need changelogs, i.e. it's still
> > a WIP.
> > 
> > As for next steps, can you (handwaving all of the TDX folks) take a look at what
> > I pushed and see if there's anything horrifically broken, and that it still works
> > for TDX?
> > 
> > Fuad (and pKVM folks) same ask for you with respect to pKVM.  Absolutely no rush
> > (and I mean that).
> > 
> > On my side, the two things on my mind are (a) tests and (b) downstream dependencies
> > (SEV and TDX).  For tests, I want to build a lists of tests that are required for
> > merging so that the criteria for merging are clear, and so that if the list is large
> > (haven't thought much yet), the work of writing and running tests can be distributed.
> > 
> > Regarding downstream dependencies, before this lands, I want to pull in all the
> > TDX and SNP series and see how everything fits together.  Specifically, I want to
> > make sure that we don't end up with a uAPI that necessitates ugly code, and that we
> > don't miss an opportunity to make things simpler.  The patches in the SNP series to
> > add "legacy" SEV support for UPM in particular made me slightly rethink some minor
> > details.  Nothing remotely major, but something that needs attention since it'll
> > be uAPI.
> 
> Although I'm still debuging with TDX KVM, I needed the following.
> kvm_faultin_pfn() is called without mmu_lock held.  the race to change
> private/shared is handled by mmu_seq.  Maybe dedicated function only for
> kvm_faultin_pfn().

Gah, you're not on the other thread where this was discussed[*].  Simply deleting
the lockdep assertion is safe, for guest types that rely on the attributes to
define shared vs. private, KVM rechecks the attributes under the protection of
mmu_seq.

I'll get a fixed version pushed out today.

[*] https://lore.kernel.org/all/Y8gpl+LwSuSgBFks@xxxxxxxxxx