Re: [PATCH v10 5/9] KVM: Use gfn instead of hva for mmu_notifier_retry

Chao Peng
2022-12-06T12:04:50Z

On Mon, Dec 05, 2022 at 09:23:49AM +0000, Fuad Tabba wrote:
> Hi Chao,
>
> On Fri, Dec 2, 2022 at 6:19 AM Chao Peng <chao.p.peng@linux.intel.com> wrote:
> >
> > Currently in mmu_notifier invalidate path, hva range is recorded and
> > then checked against by mmu_notifier_retry_hva() in the page fault
> > handling path. However, for the to be introduced private memory, a page
> > fault may not have a hva associated, checking gfn(gpa) makes more sense.
> >
> > For existing hva based shared memory, gfn is expected to also work. The
> > only downside is when aliasing multiple gfns to a single hva, the
> > current algorithm of checking multiple ranges could result in a much
> > larger range being rejected. Such aliasing should be uncommon, so the
> > impact is expected small.
> >
> > Suggested-by: Sean Christopherson <seanjc@google.com>
> > Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> > ---
> > arch/x86/kvm/mmu/mmu.c | 8 +++++---
> > include/linux/kvm_host.h | 33 +++++++++++++++++++++------------
> > virt/kvm/kvm_main.c | 32 +++++++++++++++++++++++---------
> > 3 files changed, 49 insertions(+), 24 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 4736d7849c60..e2c70b5afa3e 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -4259,7 +4259,7 @@ static bool is_page_fault_stale(struct kvm_vcpu *vcpu,
> > return true;
> >
> > return fault->slot &&
> > - mmu_invalidate_retry_hva(vcpu->kvm, mmu_seq, fault->hva);
> > + mmu_invalidate_retry_gfn(vcpu->kvm, mmu_seq, fault->gfn);
> > }
> >
> > static int direct_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> > @@ -6098,7 +6098,9 @@ void kvm_zap_gfn_range(struct kvm *kvm, gfn_t gfn_start, gfn_t gfn_end)
> >
> > write_lock(&kvm->mmu_lock);
> >
> > - kvm_mmu_invalidate_begin(kvm, gfn_start, gfn_end);
> > + kvm_mmu_invalidate_begin(kvm);
> > +
> > + kvm_mmu_invalidate_range_add(kvm, gfn_start, gfn_end);
> >
> > flush = kvm_rmap_zap_gfn_range(kvm, gfn_start, gfn_end);
> >
> > @@ -6112,7 +6114,7 @@ void kvm_zap_gfn_range(struct kvm *kvm, gfn_t gfn_start, gfn_t gfn_end)
> > kvm_flush_remote_tlbs_with_address(kvm, gfn_start,
> > gfn_end - gfn_start);
> >
> > - kvm_mmu_invalidate_end(kvm, gfn_start, gfn_end);
> > + kvm_mmu_invalidate_end(kvm);
> >
> > write_unlock(&kvm->mmu_lock);
> > }
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index 02347e386ea2..3d69484d2704 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -787,8 +787,8 @@ struct kvm {
> > struct mmu_notifier mmu_notifier;
> > unsigned long mmu_invalidate_seq;
> > long mmu_invalidate_in_progress;
> > - unsigned long mmu_invalidate_range_start;
> > - unsigned long mmu_invalidate_range_end;
> > + gfn_t mmu_invalidate_range_start;
> > + gfn_t mmu_invalidate_range_end;
> > #endif
> > struct list_head devices;
> > u64 manual_dirty_log_protect;
> > @@ -1389,10 +1389,9 @@ 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_mmu_invalidate_begin(struct kvm *kvm, unsigned long start,
> > - unsigned long end);
> > -void kvm_mmu_invalidate_end(struct kvm *kvm, unsigned long start,
> > - unsigned long end);
> > +void kvm_mmu_invalidate_begin(struct kvm *kvm);
> > +void kvm_mmu_invalidate_range_add(struct kvm *kvm, gfn_t start, gfn_t end);
> > +void kvm_mmu_invalidate_end(struct kvm *kvm);
> >
> > long kvm_arch_dev_ioctl(struct file *filp,
> > unsigned int ioctl, unsigned long arg);
> > @@ -1963,9 +1962,9 @@ static inline int mmu_invalidate_retry(struct kvm *kvm, unsigned long mmu_seq)
> > return 0;
> > }
> >
> > -static inline int mmu_invalidate_retry_hva(struct kvm *kvm,
> > +static inline int mmu_invalidate_retry_gfn(struct kvm *kvm,
> > unsigned long mmu_seq,
> > - unsigned long hva)
> > + gfn_t gfn)
> > {
> > lockdep_assert_held(&kvm->mmu_lock);
> > /*
> > @@ -1974,10 +1973,20 @@ static inline int mmu_invalidate_retry_hva(struct kvm *kvm,
> > * that might be being invalidated. Note that it may include some false
>
> nit: "might be" (or) "is being"
>
> > * positives, due to shortcuts when handing concurrent invalidations.
>
> nit: handling

Both are existing code, but I can fix it either.

>
> > */
> > - if (unlikely(kvm->mmu_invalidate_in_progress) &&
> > - hva >= kvm->mmu_invalidate_range_start &&
> > - hva < kvm->mmu_invalidate_range_end)
> > - return 1;
> > + if (unlikely(kvm->mmu_invalidate_in_progress)) {
> > + /*
> > + * Dropping mmu_lock after bumping mmu_invalidate_in_progress
> > + * but before updating the range is a KVM bug.
> > + */
> > + if (WARN_ON_ONCE(kvm->mmu_invalidate_range_start == INVALID_GPA ||
> > + kvm->mmu_invalidate_range_end == INVALID_GPA))
>
> INVALID_GPA is an x86-specific define in
> arch/x86/include/asm/kvm_host.h, so this doesn't build on other
> architectures. The obvious fix is to move it to
> include/linux/kvm_host.h.

Hmm, INVALID_GPA is defined as ZERO for x86, not 100% confident this is
correct choice for other architectures, but after search it has not been
used for other architectures, so should be safe to make it common.

Thanks,
Chao
>
> Cheers,
> /fuad
>
> > + return 1;
> > +
> > + if (gfn >= kvm->mmu_invalidate_range_start &&
> > + gfn < kvm->mmu_invalidate_range_end)
> > + return 1;
> > + }
> > +
> > if (kvm->mmu_invalidate_seq != mmu_seq)
> > return 1;
> > return 0;
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index b882eb2c76a2..ad55dfbc75d7 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -540,9 +540,7 @@ 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);
> > typedef void (*on_unlock_fn_t)(struct kvm *kvm);
> >
> > struct kvm_hva_range {
> > @@ -628,7 +626,8 @@ 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);
> > +
> > if (IS_KVM_NULL_FN(range->handler))
> > break;
> > }
> > @@ -715,8 +714,7 @@ static void kvm_mmu_notifier_change_pte(struct mmu_notifier *mn,
> > kvm_handle_hva_range(mn, address, address + 1, pte, kvm_set_spte_gfn);
> > }
> >
> > -void kvm_mmu_invalidate_begin(struct kvm *kvm, unsigned long start,
> > - unsigned long end)
> > +void kvm_mmu_invalidate_begin(struct kvm *kvm)
> > {
> > /*
> > * The count increase must become visible at unlock time as no
> > @@ -724,6 +722,17 @@ void kvm_mmu_invalidate_begin(struct kvm *kvm, unsigned long start,
> > * count is also read inside the mmu_lock critical section.
> > */
> > kvm->mmu_invalidate_in_progress++;
> > +
> > + if (likely(kvm->mmu_invalidate_in_progress == 1)) {
> > + kvm->mmu_invalidate_range_start = INVALID_GPA;
> > + kvm->mmu_invalidate_range_end = INVALID_GPA;
> > + }
> > +}
> > +
> > +void kvm_mmu_invalidate_range_add(struct kvm *kvm, gfn_t start, gfn_t end)
> > +{
> > + WARN_ON_ONCE(!kvm->mmu_invalidate_in_progress);
> > +
> > if (likely(kvm->mmu_invalidate_in_progress == 1)) {
> > kvm->mmu_invalidate_range_start = start;
> > kvm->mmu_invalidate_range_end = end;
> > @@ -744,6 +753,12 @@ void kvm_mmu_invalidate_begin(struct kvm *kvm, unsigned long start,
> > }
> > }
> >
> > +static bool kvm_mmu_unmap_gfn_range(struct kvm *kvm, struct kvm_gfn_range *range)
> > +{
> > + kvm_mmu_invalidate_range_add(kvm, range->start, range->end);
> > + return kvm_unmap_gfn_range(kvm, range);
> > +}
> > +
> > static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn,
> > const struct mmu_notifier_range *range)
> > {
> > @@ -752,7 +767,7 @@ static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn,
> > .start = range->start,
> > .end = range->end,
> > .pte = __pte(0),
> > - .handler = kvm_unmap_gfn_range,
> > + .handler = kvm_mmu_unmap_gfn_range,
> > .on_lock = kvm_mmu_invalidate_begin,
> > .on_unlock = kvm_arch_guest_memory_reclaimed,
> > .flush_on_ret = true,
> > @@ -791,8 +806,7 @@ static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn,
> > return 0;
> > }
> >
> > -void kvm_mmu_invalidate_end(struct kvm *kvm, unsigned long start,
> > - unsigned long end)
> > +void kvm_mmu_invalidate_end(struct kvm *kvm)
> > {
> > /*
> > * This sequence increase will notify the kvm page fault that
> > --
> > 2.25.1
> >