+Cc the other architectures, I'm guessing this would be a helpful optimization
for all archs.
Quite a few comments, but they're all little more than nits. Nice!
On Mon, Jan 25, 2021, David Stevens wrote:
> From: David Stevens <steve...@chromium.org>
>
> Use the range passed to mmu_notifer's invalidate_range_start to prevent
s/mmu_notifer/mmu_notifier.
And maybe avoid calling out invalidate_range_start() by name? It took me a few
reads to understand it's referring to the function, i.e. the start of the
invalidation, not the start of the range.
> spurious page fault retries due to changes in unrelated host virtual
> addresses.
This needs to elaborate on the exact scenario this is handling, as is it sounds
like KVM is tracking the history of invalidations or something. Understanding
this patch requires a priori knowledge of mmu_notifier_count. Something like:
Track the range being invalidated by mmu_notifier and skip page fault
retries if the fault address is not affected by the in-progress
invalidation. Disable the optimization if multiple invalidations are
in-progress to keep things simple, as tracking multiple ranges has
diminishing returns.
> This has the secondary effect of greatly reducing the likelihood of extreme
Out of curiosity, is this really the _secondary_ effect? I would expect this
change to primarily benefit scenarios where the invalidation has gotten
waylaid for whatever reason.
> latency when handing a page fault due to another thread having been preempted
> while modifying host virtual addresses.
>
> Signed-off-by: David Stevens <steve...@chromium.org>
> ---
...
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 6d16481aa29d..79166288ed03 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3658,8 +3658,8 @@ static bool kvm_arch_setup_async_pf(struct kvm_vcpu
> *vcpu, gpa_t cr2_or_gpa,
> }
>
> static bool try_async_pf(struct kvm_vcpu *vcpu, bool prefault, gfn_t gfn,
> - gpa_t cr2_or_gpa, kvm_pfn_t *pfn, bool write,
> - bool *writable)
> + gpa_t cr2_or_gpa, kvm_pfn_t *pfn, hva_t *hva,
> + bool write, bool *writable)
Side topic, I'm all for creating a 'struct kvm_page_fault' or whatever to hold
all these variables. The helper functions stacks are getting unwieldy.
Definitely doesn't need to be addressed here, this just reminded of how ugly
these stacks are.
> {
> struct kvm_memory_slot *slot = kvm_vcpu_gfn_to_memslot(vcpu, gfn);
> bool async;
> @@ -3672,7 +3672,8 @@ static bool try_async_pf(struct kvm_vcpu *vcpu, bool
> prefault, gfn_t gfn,
> }
>
> async = false;
> - *pfn = __gfn_to_pfn_memslot(slot, gfn, false, &async, write, writable);
> + *pfn = __gfn_to_pfn_memslot(slot, gfn, false, &async,
> + write, writable, hva);
> if (!async)
> return false; /* *pfn has correct page already */
>
> @@ -3686,7 +3687,8 @@ static bool try_async_pf(struct kvm_vcpu *vcpu, bool
> prefault, gfn_t gfn,
> return true;
> }
>
> - *pfn = __gfn_to_pfn_memslot(slot, gfn, false, NULL, write, writable);
> + *pfn = __gfn_to_pfn_memslot(slot, gfn, false, NULL,
> + write, writable, hva);
> return false;
> }
>
> @@ -3699,6 +3701,7 @@ static int direct_page_fault(struct kvm_vcpu *vcpu,
> gpa_t gpa, u32 error_code,
> gfn_t gfn = gpa >> PAGE_SHIFT;
> unsigned long mmu_seq;
> kvm_pfn_t pfn;
> + hva_t hva;
> int r;
>
> if (page_fault_handle_page_track(vcpu, error_code, gfn))
> @@ -3717,7 +3720,8 @@ static int direct_page_fault(struct kvm_vcpu *vcpu,
> gpa_t gpa, u32 error_code,
> mmu_seq = vcpu->kvm->mmu_notifier_seq;
> smp_rmb();
>
> - if (try_async_pf(vcpu, prefault, gfn, gpa, &pfn, write, &map_writable))
> + if (try_async_pf(vcpu, prefault, gfn, gpa, &pfn, &hva,
> + write, &map_writable))
> return RET_PF_RETRY;
>
> if (handle_abnormal_pfn(vcpu, is_tdp ? 0 : gpa, gfn, pfn, ACC_ALL, &r))
> @@ -3725,7 +3729,7 @@ static int direct_page_fault(struct kvm_vcpu *vcpu,
> gpa_t gpa, u32 error_code,
>
> r = RET_PF_RETRY;
> spin_lock(&vcpu->kvm->mmu_lock);
> - if (mmu_notifier_retry(vcpu->kvm, mmu_seq))
> + if (mmu_notifier_retry_hva(vcpu->kvm, mmu_seq, hva))
> goto out_unlock;
> r = make_mmu_pages_available(vcpu);
> if (r)
> diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
> index 50e268eb8e1a..3171784139a4 100644
> --- a/arch/x86/kvm/mmu/paging_tmpl.h
> +++ b/arch/x86/kvm/mmu/paging_tmpl.h
> @@ -790,6 +790,7 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gpa_t
> addr, u32 error_code,
> struct guest_walker walker;
> int r;
> kvm_pfn_t pfn;
> + hva_t hva;
> unsigned long mmu_seq;
> bool map_writable, is_self_change_mapping;
> int max_level;
> @@ -840,8 +841,8 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gpa_t
> addr, u32 error_code,
> mmu_seq = vcpu->kvm->mmu_notifier_seq;
> smp_rmb();
>
> - if (try_async_pf(vcpu, prefault, walker.gfn, addr, &pfn, write_fault,
> - &map_writable))
> + if (try_async_pf(vcpu, prefault, walker.gfn, addr, &pfn, &hva,
> + write_fault, &map_writable))
> return RET_PF_RETRY;
>
> if (handle_abnormal_pfn(vcpu, addr, walker.gfn, pfn, walker.pte_access,
> &r))
> @@ -869,7 +870,7 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gpa_t
> addr, u32 error_code,
>
> r = RET_PF_RETRY;
> spin_lock(&vcpu->kvm->mmu_lock);
> - if (mmu_notifier_retry(vcpu->kvm, mmu_seq))
> + if (mmu_notifier_retry_hva(vcpu->kvm, mmu_seq, hva))
> goto out_unlock;
>
> kvm_mmu_audit(vcpu, AUDIT_PRE_PAGE_FAULT);
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index f3b1013fb22c..b70097685249 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -502,6 +502,8 @@ 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;
> #endif
> long tlbs_dirty;
> struct list_head devices;
> @@ -729,7 +731,7 @@ kvm_pfn_t gfn_to_pfn_memslot(struct kvm_memory_slot
> *slot, gfn_t gfn);
> kvm_pfn_t gfn_to_pfn_memslot_atomic(struct kvm_memory_slot *slot, gfn_t gfn);
> kvm_pfn_t __gfn_to_pfn_memslot(struct kvm_memory_slot *slot, gfn_t gfn,
> bool atomic, bool *async, bool write_fault,
> - bool *writable);
> + bool *writable, hva_t *hva);
>
> void kvm_release_pfn_clean(kvm_pfn_t pfn);
> void kvm_release_pfn_dirty(kvm_pfn_t pfn);
> @@ -1203,6 +1205,24 @@ static inline int mmu_notifier_retry(struct kvm *kvm,
> unsigned long mmu_seq)
> return 1;
> return 0;
> }
> +
> +static inline int mmu_notifier_retry_hva(struct kvm *kvm,
> + unsigned long mmu_seq,
> + unsigned long hva)
> +{
> + /*
> + * Unlike mmu_notifier_retry, this function relies on
> + * kvm->mmu_lock for consistency.
mmu_notifier_retry is the outlier due to PPC behavior. Maybe just add a lockdep
annonation and call it good?
> + */
This needs a comment to explicitly state that 'count > 1' cannot be done at
this time. My initial thought is that it would be more intuitive to check for
'count > 1' here, but that would potentially check the wrong wrange when count
goes from 2->1. The comment about persistence in invalidate_range_start() is a
good hint, but I think it's worth being explicit to avoid bad "cleanup" in the
future.
> + if (unlikely(kvm->mmu_notifier_count)) {
> + if (kvm->mmu_notifier_range_start <= hva &&
> + hva < kvm->mmu_notifier_range_end)
Combine these into a single statement? I think the result is easier to read?
if (unlikely(kvm->mmu_notifier_count) &&
kvm->mmu_notifier_range_start <= hva &&
hva < kvm->mmu_notifier_range_end)
> + return 1;
> + }
> + if (kvm->mmu_notifier_seq != mmu_seq)
> + return 1;
> + return 0;
> +}
> #endif
>
> #ifdef CONFIG_HAVE_KVM_IRQ_ROUTING
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index fa9e3614d30e..d6e1ef5cb184 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -483,6 +483,18 @@ static int
> kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn,
> * count is also read inside the mmu_lock critical section.
> */
> kvm->mmu_notifier_count++;
> + if (likely(kvm->mmu_notifier_count == 1)) {
> + kvm->mmu_notifier_range_start = range->start;
> + kvm->mmu_notifier_range_end = range->end;
> + } else {
> + /**
> + * Tracking multiple concurrent ranges has diminishing returns,
> + * so just use the maximum range. This persists until after all
> + * outstanding invalidation operations complete.
> + */
> + kvm->mmu_notifier_range_start = 0;
> + kvm->mmu_notifier_range_end = ULONG_MAX;
Hrm, I don't think there's a corner case in practice, but ULONG_MAX is a legal
virtual address and range_end is exclusive. E.g. passing hva=-1ul would get a
false negative in mmu_notifier_retry_hva(). It's not an issue as written
because hva is generated from the gfn, and hva can't be a kernel address. I'm
guessing mmu_notifier also doesn't fire on kernel addresses. I assume that all
holds true for other architectures, and adding checks in
mmu_notifier_retry_hva()
feels like a waste of cycles, but it still bugs me. :-)
Maybe zero out range_end and explicitly check for that, just to be paranoid?
if (unlikely(kvm->mmu_notifier_count) &&
(!kvm->mmu_notifier_range_end ||
(kvm->mmu_notifier_range_start <= hva &&
hva < kvm->mmu_notifier_range_end))
> + }
> need_tlb_flush = kvm_unmap_hva_range(kvm, range->start, range->end,
> range->flags);
> /* we've to flush the tlb before the pages can be freed */
> @@ -2010,9 +2022,11 @@ static kvm_pfn_t hva_to_pfn(unsigned long addr, bool
> atomic, bool *async,
>
> kvm_pfn_t __gfn_to_pfn_memslot(struct kvm_memory_slot *slot, gfn_t gfn,
> bool atomic, bool *async, bool write_fault,
> - bool *writable)
> + bool *writable, hva_t *hva)
Hrm, it feels like we should really split gfn->hva and hva->pfn into separate
operations, but pretty much every arch needs the hva error handling. Splitting
it would probably do more harm than good, at least not without a lot of
additional refactoring. Bummer.
> {
> unsigned long addr = __gfn_to_hva_many(slot, gfn, NULL, write_fault);
Newline here.
> + if (hva)
> + *hva = addr;
>
> if (addr == KVM_HVA_ERR_RO_BAD) {
> if (writable)
> @@ -2041,19 +2055,19 @@ kvm_pfn_t gfn_to_pfn_prot(struct kvm *kvm, gfn_t gfn,
> bool write_fault,
> bool *writable)
> {
> return __gfn_to_pfn_memslot(gfn_to_memslot(kvm, gfn), gfn, false, NULL,
> - write_fault, writable);
> + write_fault, writable, NULL);
> }
> EXPORT_SYMBOL_GPL(gfn_to_pfn_prot);
>
> kvm_pfn_t gfn_to_pfn_memslot(struct kvm_memory_slot *slot, gfn_t gfn)
> {
> - return __gfn_to_pfn_memslot(slot, gfn, false, NULL, true, NULL);
> + return __gfn_to_pfn_memslot(slot, gfn, false, NULL, true, NULL, NULL);
> }
> EXPORT_SYMBOL_GPL(gfn_to_pfn_memslot);
>
> kvm_pfn_t gfn_to_pfn_memslot_atomic(struct kvm_memory_slot *slot, gfn_t gfn)
> {
> - return __gfn_to_pfn_memslot(slot, gfn, true, NULL, true, NULL);
> + return __gfn_to_pfn_memslot(slot, gfn, true, NULL, true, NULL, NULL);
> }
> EXPORT_SYMBOL_GPL(gfn_to_pfn_memslot_atomic);
>
> --
> 2.30.0.280.ga3ce27912f-goog
>
