Re: [PATCH v10 052/108] KVM: x86/tdp_mmu: Ignore unsupported mmu operation on private GFNs

Isaku Yamahata
2022-12-15T23:21:28Z

On Mon, Dec 05, 2022 at 02:23:35PM +0000,
"Wang, Wei W" <wei.w.wang@intel.com> wrote:

> On Sunday, October 30, 2022 2:23 PM, Yamahata, Isaku wrote:
> > From: Isaku Yamahata <isaku.yamahata@intel.com>
> >
> > Some KVM MMU operations (dirty page logging, page migration, aging page)
> > aren't supported for private GFNs (yet) with the first generation of TDX.
> > Silently return on unsupported TDX KVM MMU operations.
> >
> > Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
> > ---
> > arch/x86/kvm/mmu/mmu.c | 3 ++
> > arch/x86/kvm/mmu/tdp_mmu.c | 73
> > +++++++++++++++++++++++++++++++++++---
> > arch/x86/kvm/x86.c | 3 ++
> > 3 files changed, 74 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index
> > 02e7b5cf3231..efc3b3f2dd12 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -6588,6 +6588,9 @@ static bool kvm_mmu_zap_collapsible_spte(struct
> > kvm *kvm,
> > for_each_rmap_spte(rmap_head, &iter, sptep) {
> > sp = sptep_to_sp(sptep);
> >
> > + /* Private page dirty logging is not supported yet. */
> > + KVM_BUG_ON(is_private_sptep(sptep), kvm);
> > +
> > /*
> > * We cannot do huge page mapping for indirect shadow pages,
> > * which are found on the last rmap (level = 1) when not using diff --git
> > a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c index
> > 0e053b96444a..4b207ce83ffe 100644
> > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > @@ -1469,7 +1469,8 @@ typedef bool (*tdp_handler_t)(struct kvm *kvm,
> > struct tdp_iter *iter,
> >
> > static __always_inline bool kvm_tdp_mmu_handle_gfn(struct kvm *kvm,
> > struct kvm_gfn_range *range,
> > - tdp_handler_t handler)
> > + tdp_handler_t handler,
> > + bool only_shared)
> > {
> > struct kvm_mmu_page *root;
> > struct tdp_iter iter;
> > @@ -1480,9 +1481,23 @@ static __always_inline bool
> > kvm_tdp_mmu_handle_gfn(struct kvm *kvm,
> > * into this helper allow blocking; it'd be dead, wasteful code.
> > */
> > for_each_tdp_mmu_root(kvm, root, range->slot->as_id) {
> > + gfn_t start;
> > + gfn_t end;
> > +
> > + if (only_shared && is_private_sp(root))
> > + continue;
> > +
> > rcu_read_lock();
> >
> > - tdp_root_for_each_leaf_pte(iter, root, range->start, range->end)
> > + /*
> > + * For TDX shared mapping, set GFN shared bit to the range,
> > + * so the handler() doesn't need to set it, to avoid duplicated
> > + * code in multiple handler()s.
> > + */
> > + start = kvm_gfn_for_root(kvm, root, range->start);
> > + end = kvm_gfn_for_root(kvm, root, range->end);
> > +
> > + tdp_root_for_each_leaf_pte(iter, root, start, end)
> > ret |= handler(kvm, &iter, range);
> >
> > rcu_read_unlock();
> > @@ -1526,7 +1541,12 @@ static bool age_gfn_range(struct kvm *kvm, struct
> > tdp_iter *iter,
> >
> > bool kvm_tdp_mmu_age_gfn_range(struct kvm *kvm, struct kvm_gfn_range
> > *range) {
> > - return kvm_tdp_mmu_handle_gfn(kvm, range, age_gfn_range);
> > + /*
> > + * First TDX generation doesn't support clearing A bit for private
> > + * mapping, since there's no secure EPT API to support it. However
> > + * it's a legitimate request for TDX guest.
> > + */
> > + return kvm_tdp_mmu_handle_gfn(kvm, range, age_gfn_range, true);
> > }
> >
> > static bool test_age_gfn(struct kvm *kvm, struct tdp_iter *iter, @@ -1537,7
> > +1557,8 @@ static bool test_age_gfn(struct kvm *kvm, struct tdp_iter *iter,
> >
> > bool kvm_tdp_mmu_test_age_gfn(struct kvm *kvm, struct kvm_gfn_range
> > *range) {
> > - return kvm_tdp_mmu_handle_gfn(kvm, range, test_age_gfn);
> > + /* The first TDX generation doesn't support A bit. */
> > + return kvm_tdp_mmu_handle_gfn(kvm, range, test_age_gfn, true);
> > }
> >
> > static bool set_spte_gfn(struct kvm *kvm, struct tdp_iter *iter, @@ -1582,8
> > +1603,11 @@ bool kvm_tdp_mmu_set_spte_gfn(struct kvm *kvm, struct
> > kvm_gfn_range *range)
> > * No need to handle the remote TLB flush under RCU protection, the
> > * target SPTE _must_ be a leaf SPTE, i.e. cannot result in freeing a
> > * shadow page. See the WARN on pfn_changed in
> > __handle_changed_spte().
> > + *
> > + * .change_pte() callback should not happen for private page, because
> > + * for now TDX private pages are pinned during VM's life time.
> > */
> > - return kvm_tdp_mmu_handle_gfn(kvm, range, set_spte_gfn);
> > + return kvm_tdp_mmu_handle_gfn(kvm, range, set_spte_gfn, true);
> > }
> >
> > /*
> > @@ -1637,6 +1661,14 @@ bool kvm_tdp_mmu_wrprot_slot(struct kvm *kvm,
> >
> > lockdep_assert_held_read(&kvm->mmu_lock);
> >
> > + /*
> > + * Because first TDX generation doesn't support write protecting private
> > + * mappings and kvm_arch_dirty_log_supported(kvm) = false, it's a bug
> > + * to reach here for guest TD.
> > + */
> > + if (WARN_ON_ONCE(!kvm_arch_dirty_log_supported(kvm)))
> > + return false;
> > +
> > for_each_valid_tdp_mmu_root_yield_safe(kvm, root, slot->as_id, true)
> > spte_set |= wrprot_gfn_range(kvm, root, slot->base_gfn,
> > slot->base_gfn + slot->npages, min_level); @@ -1902,6
> > +1934,14 @@ bool kvm_tdp_mmu_clear_dirty_slot(struct kvm *kvm,
> >
> > lockdep_assert_held_read(&kvm->mmu_lock);
> >
> > + /*
> > + * First TDX generation doesn't support clearing dirty bit,
> > + * since there's no secure EPT API to support it. It is a
> > + * bug to reach here for TDX guest.
> > + */
> > + if (WARN_ON_ONCE(!kvm_arch_dirty_log_supported(kvm)))
> > + return false;
> > +
>
> It might not be a good choice to intercept everywhere in kvm_mmu just as tdx
> doesn't support it. I'm thinking maybe we could do the check in tdx.c, which is
> much simpler. For example:
>
> @@ -2592,6 +2605,12 @@ static void tdx_handle_changed_private_spte(struct kvm *kvm,
> lockdep_assert_held(&kvm->mmu_lock);
>
> if (change->new.is_present) {
> + /* Only flags change. This isn't supported currently. */
> + KVM_BUG_ON(change->old.is_present, kvm);
>
> Then we can have kvm_arch_dirty_log_supported completely removed.

Do you mean WARN_ON_ONCE()? If so, they can be removed from this patches because
the code should be blocked by "if (!kvm_arch_dirty_log_supported(kvm))" at the
caller.

--
Isaku Yamahata <isaku.yamahata@gmail.com>