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

[Sean Christopherson]

On Thu, Jan 19, 2023, Isaku Yamahata wrote:
> On Thu, Jan 19, 2023 at 03:25:08PM +0000,
> Sean Christopherson <seanjc@google.com> wrote:
> 
> > On Thu, Jan 19, 2023, Isaku Yamahata wrote:
> > > On Sat, Jan 14, 2023 at 12:37:59AM +0000,
> > > Sean Christopherson <seanjc@google.com> 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@github.com: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@google.com
> 
> Now I have tdx kvm working. I've uploaded at the followings.
> It's rebased to v6.2-rc3.
>         git@github.com:yamahata/linux.git tdx/upm
>         git@github.com:yamahata/qemu.git tdx/upm

And I finally got a working, building version updated and pushed out (again to):

  git@github.com:sean-jc/linux.git x86/upm_base_support

Took longer than expected to get the memslot restrictions sussed out.  I'm done
working on the code for now, my plan is to come back to it+TDX+SNP in 2-3 weeks
to resolves any remaining todos (that no one else tackles) and to do the whole
"merge the world" excersise.

> kvm_mmu_do_page_fault() needs the following change.
> kvm_mem_is_private() queries mem_attr_array.  kvm_faultin_pfn() also uses
> kvm_mem_is_private(). So the shared-private check in kvm_faultin_pfn() doesn't
> make sense. This change would belong to TDX KVM patches, though.

Yeah, SNP needs similar treatment.  Sorting that out is high up on the todo 
list.