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SubjectRe: [PATCH v10 0/9] KVM: mm: fd-based approach for supporting KVM
On Sat, Jan 14, 2023 at 12:37:59AM +0000,
Sean Christopherson <> 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:
> 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

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 02be5e1cba1e..38699ca75ab8 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -2322,7 +2322,7 @@ static inline void kvm_account_pgtable_pages(void *virt, int nr)
static inline unsigned long kvm_get_memory_attributes(struct kvm *kvm, gfn_t gfn)
- lockdep_assert_held(&kvm->mmu_lock);
+ // lockdep_assert_held(&kvm->mmu_lock);

return xa_to_value(xa_load(&kvm->mem_attr_array, gfn));

Isaku Yamahata <>

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