On Fri, Aug 16, 2024 at 6:05 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Wed, Jul 24, 2024, James Houghton wrote:
> > Walk the TDP MMU in an RCU read-side critical section.
>
> ...without holding mmu_lock, while doing xxx. There are a lot of TDP MMU walks,
> pand they all need RCU protection.
Added "without holding mmu_lock when harvesting and potentially
updating age information on sptes".
> > This requires a way to do RCU-safe walking of the tdp_mmu_roots; do this with
> > a new macro. The PTE modifications are now done atomically, and
> > kvm_tdp_mmu_spte_need_atomic_write() has been updated to account for the fact
> > that kvm_age_gfn can now lockless update the accessed bit and the R/X bits).
> >
> > If the cmpxchg for marking the spte for access tracking fails, we simply
> > retry if the spte is still a leaf PTE. If it isn't, we return false
> > to continue the walk.
>
> Please avoid pronouns. E.g. s/we/KVM (and adjust grammar as needed), so that
> it's clear what actor in particular is doing the retry.
Fixed. Though, I have also changed this to reflect the change in the
retry logic I've made, given your other comment.
> > Harvesting age information from the shadow MMU is still done while
> > holding the MMU write lock.
> >
> > Suggested-by: Yu Zhao <yuzhao@xxxxxxxxxx>
> > Signed-off-by: James Houghton <jthoughton@xxxxxxxxxx>
> > ---
> > arch/x86/include/asm/kvm_host.h | 1 +
> > arch/x86/kvm/Kconfig | 1 +
> > arch/x86/kvm/mmu/mmu.c | 10 ++++-
> > arch/x86/kvm/mmu/tdp_iter.h | 27 +++++++------
> > arch/x86/kvm/mmu/tdp_mmu.c | 67 +++++++++++++++++++++++++--------
> > 5 files changed, 77 insertions(+), 29 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index 950a03e0181e..096988262005 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -1456,6 +1456,7 @@ struct kvm_arch {
> > * tdp_mmu_page set.
> > *
> > * For reads, this list is protected by:
> > + * RCU alone or
> > * the MMU lock in read mode + RCU or
> > * the MMU lock in write mode
> > *
> > diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
> > index 4287a8071a3a..6ac43074c5e9 100644
> > --- a/arch/x86/kvm/Kconfig
> > +++ b/arch/x86/kvm/Kconfig
> > @@ -23,6 +23,7 @@ config KVM
> > depends on X86_LOCAL_APIC
> > select KVM_COMMON
> > select KVM_GENERIC_MMU_NOTIFIER
> > + select KVM_MMU_NOTIFIER_YOUNG_LOCKLESS
> > select HAVE_KVM_IRQCHIP
> > select HAVE_KVM_PFNCACHE
> > select HAVE_KVM_DIRTY_RING_TSO
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 901be9e420a4..7b93ce8f0680 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -1633,8 +1633,11 @@ bool kvm_age_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
> > {
> > bool young = false;
> >
> > - if (kvm_memslots_have_rmaps(kvm))
> > + if (kvm_memslots_have_rmaps(kvm)) {
> > + write_lock(&kvm->mmu_lock);
> > young = kvm_handle_gfn_range(kvm, range, kvm_age_rmap);
> > + write_unlock(&kvm->mmu_lock);
> > + }
> >
> > if (tdp_mmu_enabled)
> > young |= kvm_tdp_mmu_age_gfn_range(kvm, range);
> > @@ -1646,8 +1649,11 @@ bool kvm_test_age_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
> > {
> > bool young = false;
> >
> > - if (kvm_memslots_have_rmaps(kvm))
> > + if (kvm_memslots_have_rmaps(kvm)) {
> > + write_lock(&kvm->mmu_lock);
> > young = kvm_handle_gfn_range(kvm, range, kvm_test_age_rmap);
> > + write_unlock(&kvm->mmu_lock);
> > + }
> >
> > if (tdp_mmu_enabled)
> > young |= kvm_tdp_mmu_test_age_gfn(kvm, range);
> > diff --git a/arch/x86/kvm/mmu/tdp_iter.h b/arch/x86/kvm/mmu/tdp_iter.h
> > index 2880fd392e0c..510936a8455a 100644
> > --- a/arch/x86/kvm/mmu/tdp_iter.h
> > +++ b/arch/x86/kvm/mmu/tdp_iter.h
> > @@ -25,6 +25,13 @@ static inline u64 kvm_tdp_mmu_write_spte_atomic(tdp_ptep_t sptep, u64 new_spte)
> > return xchg(rcu_dereference(sptep), new_spte);
> > }
> >
> > +static inline u64 tdp_mmu_clear_spte_bits_atomic(tdp_ptep_t sptep, u64 mask)
> > +{
> > + atomic64_t *sptep_atomic = (atomic64_t *)rcu_dereference(sptep);
> > +
> > + return (u64)atomic64_fetch_and(~mask, sptep_atomic);
> > +}
> > +
> > static inline void __kvm_tdp_mmu_write_spte(tdp_ptep_t sptep, u64 new_spte)
> > {
> > KVM_MMU_WARN_ON(is_ept_ve_possible(new_spte));
> > @@ -32,10 +39,11 @@ static inline void __kvm_tdp_mmu_write_spte(tdp_ptep_t sptep, u64 new_spte)
> > }
> >
> > /*
> > - * SPTEs must be modified atomically if they are shadow-present, leaf
> > - * SPTEs, and have volatile bits, i.e. has bits that can be set outside
> > - * of mmu_lock. The Writable bit can be set by KVM's fast page fault
> > - * handler, and Accessed and Dirty bits can be set by the CPU.
> > + * SPTEs must be modified atomically if they have bits that can be set outside
> > + * of the mmu_lock. This can happen for any shadow-present leaf SPTEs, as the
> > + * Writable bit can be set by KVM's fast page fault handler, the Accessed and
> > + * Dirty bits can be set by the CPU, and the Accessed and R/X bits can be
> > + * cleared by age_gfn_range.
> > *
> > * Note, non-leaf SPTEs do have Accessed bits and those bits are
> > * technically volatile, but KVM doesn't consume the Accessed bit of
> > @@ -46,8 +54,7 @@ static inline void __kvm_tdp_mmu_write_spte(tdp_ptep_t sptep, u64 new_spte)
> > static inline bool kvm_tdp_mmu_spte_need_atomic_write(u64 old_spte, int level)
> > {
> > return is_shadow_present_pte(old_spte) &&
> > - is_last_spte(old_spte, level) &&
> > - spte_has_volatile_bits(old_spte);
> > + is_last_spte(old_spte, level);
> > }
> >
> > static inline u64 kvm_tdp_mmu_write_spte(tdp_ptep_t sptep, u64 old_spte,
> > @@ -63,12 +70,8 @@ static inline u64 kvm_tdp_mmu_write_spte(tdp_ptep_t sptep, u64 old_spte,
> > static inline u64 tdp_mmu_clear_spte_bits(tdp_ptep_t sptep, u64 old_spte,
> > u64 mask, int level)
> > {
> > - atomic64_t *sptep_atomic;
> > -
> > - if (kvm_tdp_mmu_spte_need_atomic_write(old_spte, level)) {
> > - sptep_atomic = (atomic64_t *)rcu_dereference(sptep);
> > - return (u64)atomic64_fetch_and(~mask, sptep_atomic);
> > - }
> > + if (kvm_tdp_mmu_spte_need_atomic_write(old_spte, level))
> > + return tdp_mmu_clear_spte_bits_atomic(sptep, mask);
> >
> > __kvm_tdp_mmu_write_spte(sptep, old_spte & ~mask);
> > return old_spte;
> > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> > index c7dc49ee7388..3f13b2db53de 100644
> > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > @@ -29,6 +29,11 @@ static __always_inline bool kvm_lockdep_assert_mmu_lock_held(struct kvm *kvm,
> >
> > return true;
> > }
> > +static __always_inline bool kvm_lockdep_assert_rcu_read_lock_held(void)
> > +{
> > + WARN_ON_ONCE(!rcu_read_lock_held());
> > + return true;
> > +}
>
> I doubt KVM needs a manual WARN, the RCU deference stuff should yell loudly if
> something is missing an rcu_read_lock().
You're right -- removed.
> > void kvm_mmu_uninit_tdp_mmu(struct kvm *kvm)
> > {
> > @@ -178,6 +183,15 @@ static struct kvm_mmu_page *tdp_mmu_next_root(struct kvm *kvm,
> > ((_only_valid) && (_root)->role.invalid))) { \
> > } else
> >
> > +/*
> > + * Iterate over all TDP MMU roots in an RCU read-side critical section.
> > + */
> > +#define for_each_tdp_mmu_root_rcu(_kvm, _root, _as_id) \
> > + list_for_each_entry_rcu(_root, &_kvm->arch.tdp_mmu_roots, link) \
>
> This should just process valid roots:
>
> https://lore.kernel.org/all/20240801183453.57199-7-seanjc@xxxxxxxxxx
Thanks! I've added `|| (_root)->role.invalid)` to the below
conditional expression, and I've renamed the macro to
for_each_valid_tdp_mmu_root_rcu.
> > + if (kvm_lockdep_assert_rcu_read_lock_held() && \
> > + (_as_id >= 0 && kvm_mmu_page_as_id(_root) != _as_id)) { \
> > + } else
> > +
> > #define for_each_tdp_mmu_root(_kvm, _root, _as_id) \
> > __for_each_tdp_mmu_root(_kvm, _root, _as_id, false)
> >
> > @@ -1224,6 +1238,27 @@ static __always_inline bool kvm_tdp_mmu_handle_gfn(struct kvm *kvm,
> > return ret;
> > }
> >
> > +static __always_inline bool kvm_tdp_mmu_handle_gfn_lockless(
> > + struct kvm *kvm,
> > + struct kvm_gfn_range *range,
> > + tdp_handler_t handler)
>
> Please burn all the Google3 from your brain, and code ;-)
I indented this way to avoid going past the 80 character limit. I've
adjusted it to be more like the other functions in this file.
Perhaps I should put `static __always_inline bool` on its own line?
>
> > + struct kvm_mmu_page *root;
> > + struct tdp_iter iter;
> > + bool ret = false;
> > +
> > + rcu_read_lock();
> > +
> > + for_each_tdp_mmu_root_rcu(kvm, root, range->slot->as_id) {
> > + tdp_root_for_each_leaf_pte(iter, root, range->start, range->end)
> > + ret |= handler(kvm, &iter, range);
> > + }
> > +
> > + rcu_read_unlock();
> > +
> > + return ret;
> > +}
> > +
> > /*
> > * Mark the SPTEs range of GFNs [start, end) unaccessed and return non-zero
> > * if any of the GFNs in the range have been accessed.
> > @@ -1237,28 +1272,30 @@ static bool age_gfn_range(struct kvm *kvm, struct tdp_iter *iter,
> > {
> > u64 new_spte;
> >
> > +retry:
> > /* If we have a non-accessed entry we don't need to change the pte. */
> > if (!is_accessed_spte(iter->old_spte))
> > return false;
> >
> > if (spte_ad_enabled(iter->old_spte)) {
> > - iter->old_spte = tdp_mmu_clear_spte_bits(iter->sptep,
> > - iter->old_spte,
> > - shadow_accessed_mask,
> > - iter->level);
> > + iter->old_spte = tdp_mmu_clear_spte_bits_atomic(iter->sptep,
> > + shadow_accessed_mask);
> > new_spte = iter->old_spte & ~shadow_accessed_mask;
> > } else {
> > - /*
> > - * Capture the dirty status of the page, so that it doesn't get
> > - * lost when the SPTE is marked for access tracking.
> > - */
> > + new_spte = mark_spte_for_access_track(iter->old_spte);
> > + if (__tdp_mmu_set_spte_atomic(iter, new_spte)) {
> > + /*
> > + * The cmpxchg failed. If the spte is still a
> > + * last-level spte, we can safely retry.
> > + */
> > + if (is_shadow_present_pte(iter->old_spte) &&
> > + is_last_spte(iter->old_spte, iter->level))
> > + goto retry;
>
> Do we have a feel for how often conflicts actually happen? I.e. is it worth
> retrying and having to worry about infinite loops, however improbable they may
> be?
I'm not sure how common this is. I think it's probably better not to
retry actually. If the cmpxchg fails, this spte is probably young
anyway, so I can just `return true` instead of potentially retrying.
This is all best-effort anyway.
