Re: [PATCH v7 07/14] KVM: Use gfn instead of hva for mmu_notifier_retry

[Chao Peng <chao.p.peng@linux.intel.com>]

On Fri, Jul 15, 2022 at 01:36:15PM +0200, Gupta, Pankaj wrote:
> > Currently in mmu_notifier validate path, hva range is recorded and then
> > checked in the mmu_notifier_retry_hva() from page fault path. However
> > for the to be introduced private memory, a page fault may not have a hva
> 
> As this patch appeared in v7, just wondering did you see an actual bug
> because of it? And not having corresponding 'hva' occurs only with private
> memory because its not mapped to host userspace?

The addressed problem is not new in this version, previous versions I
also had code to handle it (just in different way). But the problem is:
mmu_notifier/memfile_notifier may be in the progress of invalidating a
pfn that obtained earlier in the page fault handler, when happens, we
should retry the fault. In v6 I used global mmu_notifier_retry() for
memfile_notifier but that can block unrelated mmu_notifer invalidation
which has hva range specified.

Sean gave a comment at https://lkml.org/lkml/2022/6/17/1001 to separate
memfile_notifier from mmu_notifier but during the implementation I
realized we actually can reuse the same code for shared and private
memory if both using gpa range and that can simplify the code handling
in kvm_zap_gfn_range and some other code (e.g. we don't need two
versions for memfile_notifier/mmu_notifier).

Adding gpa range for private memory invalidation also relieves the
above blocking issue between private memory page fault and mmu_notifier.

Chao
> 
> Thanks,
> Pankaj
> 
> > associated, checking gfn(gpa) makes more sense. For existing non private
> > memory case, gfn is expected to continue to work.
> > 
> > The patch also fixes a potential bug in kvm_zap_gfn_range() which has
> > already been using gfn when calling kvm_inc/dec_notifier_count() in
> > current code.
> > 
> > Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> > ---
> >   arch/x86/kvm/mmu/mmu.c   |  2 +-
> >   include/linux/kvm_host.h | 18 ++++++++----------
> >   virt/kvm/kvm_main.c      |  6 +++---
> >   3 files changed, 12 insertions(+), 14 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index f7fa4c31b7c5..0d882fad4bc1 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -4182,7 +4182,7 @@ static bool is_page_fault_stale(struct kvm_vcpu *vcpu,
> >   		return true;
> >   	return fault->slot &&
> > -	       mmu_notifier_retry_hva(vcpu->kvm, mmu_seq, fault->hva);
> > +	       mmu_notifier_retry_gfn(vcpu->kvm, mmu_seq, fault->gfn);
> >   }
> >   static int direct_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index 0bdb6044e316..e9153b54e2a4 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -767,8 +767,8 @@ struct kvm {
> >   	struct mmu_notifier mmu_notifier;
> >   	unsigned long mmu_notifier_seq;
> >   	long mmu_notifier_count;
> > -	unsigned long mmu_notifier_range_start;
> > -	unsigned long mmu_notifier_range_end;
> > +	gfn_t mmu_notifier_range_start;
> > +	gfn_t mmu_notifier_range_end;
> >   #endif
> >   	struct list_head devices;
> >   	u64 manual_dirty_log_protect;
> > @@ -1362,10 +1362,8 @@ void kvm_mmu_free_memory_cache(struct kvm_mmu_memory_cache *mc);
> >   void *kvm_mmu_memory_cache_alloc(struct kvm_mmu_memory_cache *mc);
> >   #endif
> > -void kvm_inc_notifier_count(struct kvm *kvm, unsigned long start,
> > -				   unsigned long end);
> > -void kvm_dec_notifier_count(struct kvm *kvm, unsigned long start,
> > -				   unsigned long end);
> > +void kvm_inc_notifier_count(struct kvm *kvm, gfn_t start, gfn_t end);
> > +void kvm_dec_notifier_count(struct kvm *kvm, gfn_t start, gfn_t end);
> >   long kvm_arch_dev_ioctl(struct file *filp,
> >   			unsigned int ioctl, unsigned long arg);
> > @@ -1923,9 +1921,9 @@ static inline int mmu_notifier_retry(struct kvm *kvm, unsigned long mmu_seq)
> >   	return 0;
> >   }
> > -static inline int mmu_notifier_retry_hva(struct kvm *kvm,
> > +static inline int mmu_notifier_retry_gfn(struct kvm *kvm,
> >   					 unsigned long mmu_seq,
> > -					 unsigned long hva)
> > +					 gfn_t gfn)
> >   {
> >   	lockdep_assert_held(&kvm->mmu_lock);
> >   	/*
> > @@ -1935,8 +1933,8 @@ static inline int mmu_notifier_retry_hva(struct kvm *kvm,
> >   	 * positives, due to shortcuts when handing concurrent invalidations.
> >   	 */
> >   	if (unlikely(kvm->mmu_notifier_count) &&
> > -	    hva >= kvm->mmu_notifier_range_start &&
> > -	    hva < kvm->mmu_notifier_range_end)
> > +	    gfn >= kvm->mmu_notifier_range_start &&
> > +	    gfn < kvm->mmu_notifier_range_end)
> >   		return 1;
> >   	if (kvm->mmu_notifier_seq != mmu_seq)
> >   		return 1;
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index da263c370d00..4d7f0e72366f 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -536,8 +536,7 @@ static void kvm_mmu_notifier_invalidate_range(struct mmu_notifier *mn,
> >   typedef bool (*hva_handler_t)(struct kvm *kvm, struct kvm_gfn_range *range);
> > -typedef void (*on_lock_fn_t)(struct kvm *kvm, unsigned long start,
> > -			     unsigned long end);
> > +typedef void (*on_lock_fn_t)(struct kvm *kvm, gfn_t start, gfn_t end);
> >   typedef void (*on_unlock_fn_t)(struct kvm *kvm);
> > @@ -624,7 +623,8 @@ static __always_inline int __kvm_handle_hva_range(struct kvm *kvm,
> >   				locked = true;
> >   				KVM_MMU_LOCK(kvm);
> >   				if (!IS_KVM_NULL_FN(range->on_lock))
> > -					range->on_lock(kvm, range->start, range->end);
> > +					range->on_lock(kvm, gfn_range.start,
> > +							    gfn_range.end);
> >   				if (IS_KVM_NULL_FN(range->handler))
> >   					break;
> >   			}