| Message ID | 20230425113932.3148-1-ubizjak@gmail.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | KVM: x86/mmu: Add comment on try_cmpxchg64 usage in tdp_mmu_set_spte_atomic | expand |
On Tue, Apr 25, 2023, Uros Bizjak wrote: > Commit aee98a6838d5 ("KVM: x86/mmu: Use try_cmpxchg64 in > tdp_mmu_set_spte_atomic") removed the comment that iter->old_spte is > updated when different logical CPU modifies the page table entry. > Although this is what try_cmpxchg does implicitly, it won't hurt > if this fact is explicitly mentioned in a restored comment. > > Cc: Paolo Bonzini <pbonzini@redhat.com> > Cc: Sean Christopherson <seanjc@google.com> > Cc: David Matlack <dmatlack@google.com> > Signed-off-by: Uros Bizjak <ubizjak@gmail.com> > --- > arch/x86/kvm/mmu/tdp_mmu.c | 10 +++++++++- > 1 file changed, 9 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c > index 7c25dbf32ecc..5d126b015086 100644 > --- a/arch/x86/kvm/mmu/tdp_mmu.c > +++ b/arch/x86/kvm/mmu/tdp_mmu.c > @@ -655,8 +655,16 @@ static inline int tdp_mmu_set_spte_atomic(struct kvm *kvm, > * Note, fast_pf_fix_direct_spte() can also modify TDP MMU SPTEs and > * does not hold the mmu_lock. > */ > - if (!try_cmpxchg64(sptep, &iter->old_spte, new_spte)) > + if (!try_cmpxchg64(sptep, &iter->old_spte, new_spte)) { > + /* > + * The page table entry was modified by a different logical > + * CPU. In this case the above try_cmpxchg updates > + * iter->old_spte with the current value, so the caller > + * operates on fresh data, e.g. if it retries > + * tdp_mmu_set_spte_atomic(). > + */ If there's no objection, when applying I'll massage this to extend the comment above the try_cmpxchg64(), e.g. /* * Note, fast_pf_fix_direct_spte() can also modify TDP MMU SPTEs and * does not hold the mmu_lock. On failure, i.e. if a different logical * CPU modified the SPTE, try_cmpxchg64() updates iter->old_spte with * the current value, so the caller operates on fresh data, e.g. if it * retries tdp_mmu_set_spte_atomic() */ if (!try_cmpxchg64(sptep, &iter->old_spte, new_spte)) return -EBUSY;
On Tue, 25 Apr 2023 13:39:32 +0200, Uros Bizjak wrote: > Commit aee98a6838d5 ("KVM: x86/mmu: Use try_cmpxchg64 in > tdp_mmu_set_spte_atomic") removed the comment that iter->old_spte is > updated when different logical CPU modifies the page table entry. > Although this is what try_cmpxchg does implicitly, it won't hurt > if this fact is explicitly mentioned in a restored comment. > > > [...] Applied to kvm-x86 mmu, thanks! [1/1] KVM: x86/mmu: Add comment on try_cmpxchg64 usage in tdp_mmu_set_spte_atomic https://github.com/kvm-x86/linux/commit/12ced095956a -- https://github.com/kvm-x86/linux/tree/next https://github.com/kvm-x86/linux/tree/fixes
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c index 7c25dbf32ecc..5d126b015086 100644 --- a/arch/x86/kvm/mmu/tdp_mmu.c +++ b/arch/x86/kvm/mmu/tdp_mmu.c @@ -655,8 +655,16 @@ static inline int tdp_mmu_set_spte_atomic(struct kvm *kvm, * Note, fast_pf_fix_direct_spte() can also modify TDP MMU SPTEs and * does not hold the mmu_lock. */ - if (!try_cmpxchg64(sptep, &iter->old_spte, new_spte)) + if (!try_cmpxchg64(sptep, &iter->old_spte, new_spte)) { + /* + * The page table entry was modified by a different logical + * CPU. In this case the above try_cmpxchg updates + * iter->old_spte with the current value, so the caller + * operates on fresh data, e.g. if it retries + * tdp_mmu_set_spte_atomic(). + */ return -EBUSY; + } __handle_changed_spte(kvm, iter->as_id, iter->gfn, iter->old_spte, new_spte, iter->level, true);
Commit aee98a6838d5 ("KVM: x86/mmu: Use try_cmpxchg64 in tdp_mmu_set_spte_atomic") removed the comment that iter->old_spte is updated when different logical CPU modifies the page table entry. Although this is what try_cmpxchg does implicitly, it won't hurt if this fact is explicitly mentioned in a restored comment. Cc: Paolo Bonzini <pbonzini@redhat.com> Cc: Sean Christopherson <seanjc@google.com> Cc: David Matlack <dmatlack@google.com> Signed-off-by: Uros Bizjak <ubizjak@gmail.com> --- arch/x86/kvm/mmu/tdp_mmu.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-)