Re: [PATCH 4/4] KVM: x86/mmu: Free obsolete roots when pre-faulting SPTEs

[Yan Zhao]

On Fri, Feb 07, 2025 at 07:12:04AM -0800, Sean Christopherson wrote:
> On Fri, Feb 07, 2025, Yan Zhao wrote:
> > Always free obsolete roots when pre-faulting SPTEs in case it's called
> > after a root is invalidated (e.g., by memslot removal) but before any
> > vcpu_enter_guest() processing of KVM_REQ_MMU_FREE_OBSOLETE_ROOTS.
> > 
> > Lack of kvm_mmu_free_obsolete_roots() in this scenario can lead to
> > kvm_mmu_reload() failing to load a new root if the current root hpa is an
> > obsolete root (which is not INVALID_PAGE). Consequently,
> > kvm_arch_vcpu_pre_fault_memory() will retry infinitely due to the checking
> > of is_page_fault_stale().
> > 
> > It's safe to call kvm_mmu_free_obsolete_roots() even if there are no
> > obsolete roots or if it's called a second time when vcpu_enter_guest()
> > later processes KVM_REQ_MMU_FREE_OBSOLETE_ROOTS. This is because
> > kvm_mmu_free_obsolete_roots() sets an obsolete root to INVALID_PAGE and
> > will do nothing to an INVALID_PAGE.
> 
> Why is userspace changing memslots while prefaulting?
It currently only exists in the kvm selftest (written by myself...)
Not sure if there's any real use case like this.

> 
> > 
> > Signed-off-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> > ---
> >  arch/x86/kvm/mmu/mmu.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 47fd3712afe6..72f68458049a 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -4740,7 +4740,12 @@ long kvm_arch_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu,
> >  	/*
> >  	 * reload is efficient when called repeatedly, so we can do it on
> >  	 * every iteration.
> > +	 * Before reload, free obsolete roots in case the prefault is called
> > +	 * after a root is invalidated (e.g., by memslot removal) but
> > +	 * before any vcpu_enter_guest() processing of
> > +	 * KVM_REQ_MMU_FREE_OBSOLETE_ROOTS.
> >  	 */
> > +	kvm_mmu_free_obsolete_roots(vcpu);
> >  	r = kvm_mmu_reload(vcpu);
> >  	if (r)
> >  		return r;
> 
> I would prefer to do check for obsolete roots in kvm_mmu_reload() itself, but
Yes, it's better!
I previously considered doing in this way, but I was afraid to introduce
overhead (the extra compare) to kvm_mmu_reload(), which is called quite
frequently.

But maybe we can remove the check in vcpu_enter_guest() to reduce the overhead?

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index b2d9a16fd4d3..6a1f2780a094 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10731,8 +10731,6 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
                                goto out;
                        }
                }
-               if (kvm_check_request(KVM_REQ_MMU_FREE_OBSOLETE_ROOTS, vcpu))
-                       kvm_mmu_free_obsolete_roots(vcpu);
                if (kvm_check_request(KVM_REQ_MIGRATE_TIMER, vcpu))
                        __kvm_migrate_timers(vcpu);
                if (kvm_check_request(KVM_REQ_MASTERCLOCK_UPDATE, vcpu))

> keep the main kvm_check_request() so that the common case handles the resulting
> TLB flush without having to loop back around in vcpu_enter_guest().
Hmm, I'm a little confused.
What's is the resulting TLB flush?



> diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
> index 050a0e229a4d..f2b36d32ef40 100644
> --- a/arch/x86/kvm/mmu.h
> +++ b/arch/x86/kvm/mmu.h
> @@ -104,6 +104,9 @@ void kvm_mmu_track_write(struct kvm_vcpu *vcpu, gpa_t gpa, const u8 *new,
>  
>  static inline int kvm_mmu_reload(struct kvm_vcpu *vcpu)
>  {
> +       if (kvm_check_request(KVM_REQ_MMU_FREE_OBSOLETE_ROOTS, vcpu))
> +               kvm_mmu_free_obsolete_roots(vcpu);
> +
>         /*
>          * Checking root.hpa is sufficient even when KVM has mirror root.
>          * We can have either:
>