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KVM: x86/mmu: Add helpers to read/write TDP MMU SPTEs and document RCU

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Add helpers to read and write TDP MMU SPTEs instead of open coding
rcu_dereference() all over the place, and to provide a convenient
location to document why KVM doesn't exempt holding mmu_lock for write
from having to hold RCU (and any future changes to the rules).

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Ben Gardon <bgardon@google.com>
Message-Id: <20220226001546.360188-11-seanjc@google.com>
Reviewed-by: Mingwei Zhang <mizhang@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Sean Christopherson 2022-02-26 00:15:28 +00:00 committed by Paolo Bonzini
parent a151aceca1
commit 0e587aa733
3 changed files with 22 additions and 6 deletions
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6
arch/x86/kvm/mmu/tdp_iter.c
View File

@ -12,7 +12,7 @@ static void tdp_iter_refresh_sptep(struct tdp_iter *iter)
{
iter->sptep = iter->pt_path[iter->level - 1] +
SHADOW_PT_INDEX(iter->gfn << PAGE_SHIFT, iter->level);
iter->old_spte = READ_ONCE(*rcu_dereference(iter->sptep));
iter->old_spte = kvm_tdp_mmu_read_spte(iter->sptep);
}
static gfn_t round_gfn_for_level(gfn_t gfn, int level)
@ -89,7 +89,7 @@ static bool try_step_down(struct tdp_iter *iter)
* Reread the SPTE before stepping down to avoid traversing into page
* tables that are no longer linked from this entry.
*/
iter->old_spte = READ_ONCE(*rcu_dereference(iter->sptep));
iter->old_spte = kvm_tdp_mmu_read_spte(iter->sptep);
child_pt = spte_to_child_pt(iter->old_spte, iter->level);
if (!child_pt)
@ -123,7 +123,7 @@ static bool try_step_side(struct tdp_iter *iter)
iter->gfn += KVM_PAGES_PER_HPAGE(iter->level);
iter->next_last_level_gfn = iter->gfn;
iter->sptep++;
iter->old_spte = READ_ONCE(*rcu_dereference(iter->sptep));
iter->old_spte = kvm_tdp_mmu_read_spte(iter->sptep);
return true;
}

16
arch/x86/kvm/mmu/tdp_iter.h
View File

@ -9,6 +9,22 @@
typedef u64 __rcu *tdp_ptep_t;
/*
* TDP MMU SPTEs are RCU protected to allow paging structures (non-leaf SPTEs)
* to be zapped while holding mmu_lock for read. Holding RCU isn't required for
* correctness if mmu_lock is held for write, but plumbing "struct kvm" down to
* the lower depths of the TDP MMU just to make lockdep happy is a nightmare, so
* all accesses to SPTEs are done under RCU protection.
*/
static inline u64 kvm_tdp_mmu_read_spte(tdp_ptep_t sptep)
{
return READ_ONCE(*rcu_dereference(sptep));
}
static inline void kvm_tdp_mmu_write_spte(tdp_ptep_t sptep, u64 val)
{
WRITE_ONCE(*rcu_dereference(sptep), val);
}
/*
* A TDP iterator performs a pre-order walk over a TDP paging structure.
*/

6
arch/x86/kvm/mmu/tdp_mmu.c
View File

@ -595,7 +595,7 @@ static inline int tdp_mmu_zap_spte_atomic(struct kvm *kvm,
* here since the SPTE is going from non-present
* to non-present.
*/
WRITE_ONCE(*rcu_dereference(iter->sptep), 0);
kvm_tdp_mmu_write_spte(iter->sptep, 0);
return 0;
}
@ -634,7 +634,7 @@ static inline void __tdp_mmu_set_spte(struct kvm *kvm, struct tdp_iter *iter,
*/
WARN_ON(is_removed_spte(iter->old_spte));
WRITE_ONCE(*rcu_dereference(iter->sptep), new_spte);
kvm_tdp_mmu_write_spte(iter->sptep, new_spte);
__handle_changed_spte(kvm, iter->as_id, iter->gfn, iter->old_spte,
new_spte, iter->level, false);
@ -1069,7 +1069,7 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
* because the new value informs the !present
* path below.
*/
iter.old_spte = READ_ONCE(*rcu_dereference(iter.sptep));
iter.old_spte = kvm_tdp_mmu_read_spte(iter.sptep);
}
if (!is_shadow_present_pte(iter.old_spte)) {