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Subject[PATCH 2/4] KVM: x86/mmu: Document the "rules" for using host_pfn_mapping_level()
From
Add a comment to document how host_pfn_mapping_level() can be used safely,
as the line between safe and dangerous is quite thin. E.g. if KVM were
to ever support in-place promotion to create huge pages, consuming the
level is safe if the caller holds mmu_lock and checks that there's an
existing _leaf_ SPTE, but unsafe if the caller only checks that there's a
non-leaf SPTE.

Opportunistically tweak the existing comments to explicitly document why
KVM needs to use READ_ONCE().

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/mmu/mmu.c | 42 +++++++++++++++++++++++++++++++++++-------
1 file changed, 35 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index bebff1d5acd4..d5b644f3e003 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -2919,6 +2919,31 @@ static void direct_pte_prefetch(struct kvm_vcpu *vcpu, u64 *sptep)
__direct_pte_prefetch(vcpu, sp, sptep);
}

+/*
+ * Lookup the mapping level for @gfn in the current mm.
+ *
+ * WARNING! Use of host_pfn_mapping_level() requires the caller and the end
+ * consumer to be tied into KVM's handlers for MMU notifier events!
+ *
+ * There are several ways to safely use this helper:
+ *
+ * - Check mmu_notifier_retry_hva() after grabbing the mapping level, before
+ * consuming it. In this case, mmu_lock doesn't need to be held during the
+ * lookup, but it does need to be held while checking the MMU notifier.
+ *
+ * - Hold mmu_lock AND ensure there is no in-progress MMU notifier invalidation
+ * event for the hva. This can be done by explicit checking the MMU notifier
+ * or by ensuring that KVM already has a valid mapping that covers the hva.
+ *
+ * - Do not use the result to install new mappings, e.g. use the host mapping
+ * level only to decide whether or not to zap an entry. In this case, it's
+ * not required to hold mmu_lock (though it's highly likely the caller will
+ * want to hold mmu_lock anyways, e.g. to modify SPTEs).
+ *
+ * Note! The lookup can still race with modifications to host page tables, but
+ * the above "rules" ensure KVM will not _consume_ the result of the walk if a
+ * race with the primary MMU occurs.
+ */
static int host_pfn_mapping_level(struct kvm *kvm, gfn_t gfn,
const struct kvm_memory_slot *slot)
{
@@ -2941,16 +2966,19 @@ static int host_pfn_mapping_level(struct kvm *kvm, gfn_t gfn,
hva = __gfn_to_hva_memslot(slot, gfn);

/*
- * Lookup the mapping level in the current mm. The information
- * may become stale soon, but it is safe to use as long as
- * 1) mmu_notifier_retry was checked after taking mmu_lock, and
- * 2) mmu_lock is taken now.
- *
- * We still need to disable IRQs to prevent concurrent tear down
- * of page tables.
+ * Disable IRQs to prevent concurrent tear down of host page tables,
+ * e.g. if the primary MMU promotes a P*D to a huge page and then frees
+ * the original page table.
*/
local_irq_save(flags);

+ /*
+ * Read each entry once. As above, a non-leaf entry can be promoted to
+ * a huge page _during_ this walk. Re-reading the entry could send the
+ * walk into the weeks, e.g. p*d_large() returns false (sees the old
+ * value) and then p*d_offset() walks into the target huge page instead
+ * of the old page table (sees the new value).
+ */
pgd = READ_ONCE(*pgd_offset(kvm->mm, hva));
if (pgd_none(pgd))
goto out;
--
2.37.0.170.g444d1eabd0-goog
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