Re: [PATCH v2] KVM: x86: Use gfn_to_pfn_cache for record_steal_time

[Sean Christopherson <seanjc@google.com>]

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