[PATCH v13 10/35] KVM: Add a dedicated mmu_notifier flag for reclaiming freed memory
Fuad Tabba
tabba at google.com
Thu Nov 2 06:55:03 PDT 2023
On Fri, Oct 27, 2023 at 7:22 PM Sean Christopherson <seanjc at google.com> wrote:
>
> Handle AMD SEV's kvm_arch_guest_memory_reclaimed() hook by having
> __kvm_handle_hva_range() return whether or not an overlapping memslot
> was found, i.e. mmu_lock was acquired. Using the .on_unlock() hook
> works, but kvm_arch_guest_memory_reclaimed() needs to run after dropping
> mmu_lock, which makes .on_lock() and .on_unlock() asymmetrical.
>
> Use a small struct to return the tuple of the notifier-specific return,
> plus whether or not overlap was found. Because the iteration helpers are
> __always_inlined, practically speaking, the struct will never actually be
> returned from a function call (not to mention the size of the struct will
> be two bytes in practice).
>
> Signed-off-by: Sean Christopherson <seanjc at google.com>
> ---
Reviewed-by: Fuad Tabba <tabba at google.com>
Tested-by: Fuad Tabba <tabba at google.com>
Cheers,
/fuad
> virt/kvm/kvm_main.c | 53 +++++++++++++++++++++++++++++++--------------
> 1 file changed, 37 insertions(+), 16 deletions(-)
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 3f5b7c2c5327..2bc04c8ae1f4 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -561,6 +561,19 @@ struct kvm_mmu_notifier_range {
> bool may_block;
> };
>
> +/*
> + * The inner-most helper returns a tuple containing the return value from the
> + * arch- and action-specific handler, plus a flag indicating whether or not at
> + * least one memslot was found, i.e. if the handler found guest memory.
> + *
> + * Note, most notifiers are averse to booleans, so even though KVM tracks the
> + * return from arch code as a bool, outer helpers will cast it to an int. :-(
> + */
> +typedef struct kvm_mmu_notifier_return {
> + bool ret;
> + bool found_memslot;
> +} kvm_mn_ret_t;
> +
> /*
> * Use a dedicated stub instead of NULL to indicate that there is no callback
> * function/handler. The compiler technically can't guarantee that a real
> @@ -582,22 +595,25 @@ static const union kvm_mmu_notifier_arg KVM_MMU_NOTIFIER_NO_ARG;
> node; \
> node = interval_tree_iter_next(node, start, last)) \
>
> -static __always_inline int __kvm_handle_hva_range(struct kvm *kvm,
> - const struct kvm_mmu_notifier_range *range)
> +static __always_inline kvm_mn_ret_t __kvm_handle_hva_range(struct kvm *kvm,
> + const struct kvm_mmu_notifier_range *range)
> {
> - bool ret = false, locked = false;
> + struct kvm_mmu_notifier_return r = {
> + .ret = false,
> + .found_memslot = false,
> + };
> struct kvm_gfn_range gfn_range;
> struct kvm_memory_slot *slot;
> struct kvm_memslots *slots;
> int i, idx;
>
> if (WARN_ON_ONCE(range->end <= range->start))
> - return 0;
> + return r;
>
> /* A null handler is allowed if and only if on_lock() is provided. */
> if (WARN_ON_ONCE(IS_KVM_NULL_FN(range->on_lock) &&
> IS_KVM_NULL_FN(range->handler)))
> - return 0;
> + return r;
>
> idx = srcu_read_lock(&kvm->srcu);
>
> @@ -631,8 +647,8 @@ static __always_inline int __kvm_handle_hva_range(struct kvm *kvm,
> gfn_range.end = hva_to_gfn_memslot(hva_end + PAGE_SIZE - 1, slot);
> gfn_range.slot = slot;
>
> - if (!locked) {
> - locked = true;
> + if (!r.found_memslot) {
> + r.found_memslot = true;
> KVM_MMU_LOCK(kvm);
> if (!IS_KVM_NULL_FN(range->on_lock))
> range->on_lock(kvm);
> @@ -640,14 +656,14 @@ static __always_inline int __kvm_handle_hva_range(struct kvm *kvm,
> if (IS_KVM_NULL_FN(range->handler))
> break;
> }
> - ret |= range->handler(kvm, &gfn_range);
> + r.ret |= range->handler(kvm, &gfn_range);
> }
> }
>
> - if (range->flush_on_ret && ret)
> + if (range->flush_on_ret && r.ret)
> kvm_flush_remote_tlbs(kvm);
>
> - if (locked) {
> + if (r.found_memslot) {
> KVM_MMU_UNLOCK(kvm);
> if (!IS_KVM_NULL_FN(range->on_unlock))
> range->on_unlock(kvm);
> @@ -655,8 +671,7 @@ static __always_inline int __kvm_handle_hva_range(struct kvm *kvm,
>
> srcu_read_unlock(&kvm->srcu, idx);
>
> - /* The notifiers are averse to booleans. :-( */
> - return (int)ret;
> + return r;
> }
>
> static __always_inline int kvm_handle_hva_range(struct mmu_notifier *mn,
> @@ -677,7 +692,7 @@ static __always_inline int kvm_handle_hva_range(struct mmu_notifier *mn,
> .may_block = false,
> };
>
> - return __kvm_handle_hva_range(kvm, &range);
> + return __kvm_handle_hva_range(kvm, &range).ret;
> }
>
> static __always_inline int kvm_handle_hva_range_no_flush(struct mmu_notifier *mn,
> @@ -696,7 +711,7 @@ static __always_inline int kvm_handle_hva_range_no_flush(struct mmu_notifier *mn
> .may_block = false,
> };
>
> - return __kvm_handle_hva_range(kvm, &range);
> + return __kvm_handle_hva_range(kvm, &range).ret;
> }
>
> static bool kvm_change_spte_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
> @@ -798,7 +813,7 @@ static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn,
> .end = range->end,
> .handler = kvm_mmu_unmap_gfn_range,
> .on_lock = kvm_mmu_invalidate_begin,
> - .on_unlock = kvm_arch_guest_memory_reclaimed,
> + .on_unlock = (void *)kvm_null_fn,
> .flush_on_ret = true,
> .may_block = mmu_notifier_range_blockable(range),
> };
> @@ -830,7 +845,13 @@ static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn,
> gfn_to_pfn_cache_invalidate_start(kvm, range->start, range->end,
> hva_range.may_block);
>
> - __kvm_handle_hva_range(kvm, &hva_range);
> + /*
> + * If one or more memslots were found and thus zapped, notify arch code
> + * that guest memory has been reclaimed. This needs to be done *after*
> + * dropping mmu_lock, as x86's reclaim path is slooooow.
> + */
> + if (__kvm_handle_hva_range(kvm, &hva_range).found_memslot)
> + kvm_arch_guest_memory_reclaimed(kvm);
>
> return 0;
> }
> --
> 2.42.0.820.g83a721a137-goog
>
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