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Peter Anvin" , kvm@vger.kernel.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="us-ascii" With some assistance from an AI review bot (well, more than "some"). On Wed, Mar 11, 2026, David Woodhouse wrote: > @@ -3806,39 +3788,32 @@ static void record_steal_time(struct kvm_vcpu *vcpu) > st_preempted & KVM_VCPU_FLUSH_TLB); > if (st_preempted & KVM_VCPU_FLUSH_TLB) > kvm_vcpu_flush_tlb_guest(vcpu); > - > - if (!user_access_begin(st, sizeof(*st))) > - goto dirty; > } else { > - if (!user_access_begin(st, sizeof(*st))) > - return; > - > - unsafe_put_user(0, &st->preempted, out); > + st->preempted = 0; These should all be WRITE_ONCE(), correct? > vcpu->arch.st.preempted = 0; > } > > - unsafe_get_user(version, &st->version, out); > + version = st->version; And then READ_ONCE()? > if (version & 1) > version += 1; /* first time write, random junk */ > > version += 1; > - unsafe_put_user(version, &st->version, out); > + st->version = version; > > smp_wmb(); > > - unsafe_get_user(steal, &st->steal, out); > + steal = st->steal; > steal += current->sched_info.run_delay - > vcpu->arch.st.last_steal; > vcpu->arch.st.last_steal = current->sched_info.run_delay; > - unsafe_put_user(steal, &st->steal, out); > + st->steal = steal; > > version += 1; > - unsafe_put_user(version, &st->version, out); > + st->version = version; And then here, doesn't there need to be an smp_wmb() before incrementing the version again? Because I believe making this all vanilla C means the compiler can reorder those two. Per the friendly bot: The previous code used unsafe_put_user(), which inherently acts as a compiler barrier due to its internal inline assembly > + > + kvm_gpc_mark_dirty_in_slot(gpc); > > - out: > - user_access_end(); > - dirty: > - mark_page_dirty_in_slot(vcpu->kvm, ghc->memslot, gpa_to_gfn(ghc->gpa)); > + read_unlock(&gpc->lock); > } > > /* > @@ -4173,8 +4148,12 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info) > > vcpu->arch.st.msr_val = data; > > - if (!(data & KVM_MSR_ENABLED)) > - break; > + if (data & KVM_MSR_ENABLED) { Curly braces aren't required. > + kvm_gpc_activate(&vcpu->arch.st.cache, data & ~KVM_MSR_ENABLED, > + sizeof(struct kvm_steal_time)); > + } else { > + kvm_gpc_deactivate(&vcpu->arch.st.cache); > + } ... > @@ -5266,20 +5244,28 @@ static void kvm_steal_time_set_preempted(struct kvm_vcpu *vcpu) > if (unlikely(current->mm != vcpu->kvm->mm)) > return; > > - slots = kvm_memslots(vcpu->kvm); > - > - if (unlikely(slots->generation != ghc->generation || > - gpa != ghc->gpa || > - kvm_is_error_hva(ghc->hva) || !ghc->memslot)) > - return; > + read_lock_irqsave(&gpc->lock, flags); I'm pretty sure this is going to make PROVE_LOCKING unhappy due to PREEMPT_RT making rwlock_t sleepable (when called from kvm_sched_out()). I've been content to ignore the kvm_xen_set_evtchn_fast() warning[*] because I can't imagine anyone is crazy enough to emulate Xen with an RT kernel, but I do know there are RT users that run VMs, and so this path would be more than just a PROVE_LOCKING issue. If we want to push the gpc stuff broadly, we need a solution to that (though I'm still not 100% convinced using a gpc here is a net positive). [*] https://lore.kernel.org/all/673f4bbc.050a0220.3c9d61.0174.GAE@google.com