On Mon, May 9, 2022 at 9:48 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Fri, Apr 22, 2022, David Matlack wrote:
> > +static bool need_topup_split_caches_or_resched(struct kvm *kvm)
> > +{
> > + if (need_resched() || rwlock_needbreak(&kvm->mmu_lock))
> > + return true;
> > +
> > + /*
> > + * In the worst case, SPLIT_DESC_CACHE_CAPACITY descriptors are needed
> > + * to split a single huge page. Calculating how many are actually needed
> > + * is possible but not worth the complexity.
> > + */
> > + return need_topup(&kvm->arch.split_desc_cache, SPLIT_DESC_CACHE_CAPACITY) ||
> > + need_topup(&kvm->arch.split_page_header_cache, 1) ||
> > + need_topup(&kvm->arch.split_shadow_page_cache, 1);
>
> Uber nit that Paolo will make fun of me for... please align indentiation
>
> return need_topup(&kvm->arch.split_desc_cache, SPLIT_DESC_CACHE_CAPACITY) ||
> need_topup(&kvm->arch.split_page_header_cache, 1) ||
> need_topup(&kvm->arch.split_shadow_page_cache, 1);
Will do.
>
> > +static void nested_mmu_split_huge_page(struct kvm *kvm,
> > + const struct kvm_memory_slot *slot,
> > + u64 *huge_sptep)
> > +
> > +{
> > + struct kvm_mmu_memory_cache *cache = &kvm->arch.split_desc_cache;
> > + u64 huge_spte = READ_ONCE(*huge_sptep);
> > + struct kvm_mmu_page *sp;
> > + bool flush = false;
> > + u64 *sptep, spte;
> > + gfn_t gfn;
> > + int index;
> > +
> > + sp = nested_mmu_get_sp_for_split(kvm, huge_sptep);
> > +
> > + for (index = 0; index < PT64_ENT_PER_PAGE; index++) {
> > + sptep = &sp->spt[index];
> > + gfn = kvm_mmu_page_get_gfn(sp, index);
> > +
> > + /*
> > + * The SP may already have populated SPTEs, e.g. if this huge
> > + * page is aliased by multiple sptes with the same access
> > + * permissions. These entries are guaranteed to map the same
> > + * gfn-to-pfn translation since the SP is direct, so no need to
> > + * modify them.
> > + *
> > + * However, if a given SPTE points to a lower level page table,
> > + * that lower level page table may only be partially populated.
> > + * Installing such SPTEs would effectively unmap a potion of the
> > + * huge page, which requires a TLB flush.
>
> Maybe explain why a TLB flush is required? E.g. "which requires a TLB flush as
> a subsequent mmu_notifier event on the unmapped region would fail to detect the
> need to flush".
Will do.
>
> > +static bool nested_mmu_skip_split_huge_page(u64 *huge_sptep)
>
> "skip" is kinda odd terminology. It reads like a command, but it's actually
> querying state _and_ it's returning a boolean, which I've learned to hate :-)
>
> I don't see any reason for a helper, there's one caller and it can just do
> "continue" directly.
Will do.
>
> > +static void kvm_nested_mmu_try_split_huge_pages(struct kvm *kvm,
> > + const struct kvm_memory_slot *slot,
> > + gfn_t start, gfn_t end,
> > + int target_level)
> > +{
> > + int level;
> > +
> > + /*
> > + * Split huge pages starting with KVM_MAX_HUGEPAGE_LEVEL and working
> > + * down to the target level. This ensures pages are recursively split
> > + * all the way to the target level. There's no need to split pages
> > + * already at the target level.
> > + */
> > + for (level = KVM_MAX_HUGEPAGE_LEVEL; level > target_level; level--) {
>
> Unnecessary braces.
The brace is unnecessary, but when the inner statement is split across
multiple lines I tend to prefer using braces. (That's why I did the
same in the other patch and you had the same feedback.) I couldn't
find any guidance about this in CodingStyle so I'm fine with getting
rid of the braces if that's what you prefer.
> > + slot_handle_level_range(kvm, slot,
> > + nested_mmu_try_split_huge_pages,
> > + level, level, start, end - 1,
> > + true, false);
>
> IMO it's worth running over by 4 chars to drop 2 lines:
Will do.
>
> for (level = KVM_MAX_HUGEPAGE_LEVEL; level > target_level; level--)
> slot_handle_level_range(kvm, slot, nested_mmu_try_split_huge_pages,
> level, level, start, end - 1, true, false);
> > + }
> > +}
> > +
> > /* Must be called with the mmu_lock held in write-mode. */
>
> Add a lockdep assertion, not a comment.
Agreed but this is an existing comment, so better left to a separate patch.
>
> > void kvm_mmu_try_split_huge_pages(struct kvm *kvm,
> > const struct kvm_memory_slot *memslot,
> > u64 start, u64 end,
> > int target_level)
> > {
> > - if (is_tdp_mmu_enabled(kvm))
> > - kvm_tdp_mmu_try_split_huge_pages(kvm, memslot, start, end,
> > - target_level, false);
> > + if (!is_tdp_mmu_enabled(kvm))
> > + return;
> > +
> > + kvm_tdp_mmu_try_split_huge_pages(kvm, memslot, start, end, target_level,
> > + false);
> > +
> > + if (kvm_memslots_have_rmaps(kvm))
> > + kvm_nested_mmu_try_split_huge_pages(kvm, memslot, start, end,
> > + target_level);
> >
> > /*
> > * A TLB flush is unnecessary at this point for the same resons as in
> > @@ -6051,10 +6304,19 @@ void kvm_mmu_slot_try_split_huge_pages(struct kvm *kvm,
> > u64 start = memslot->base_gfn;
> > u64 end = start + memslot->npages;
> >
> > - if (is_tdp_mmu_enabled(kvm)) {
> > - read_lock(&kvm->mmu_lock);
> > - kvm_tdp_mmu_try_split_huge_pages(kvm, memslot, start, end, target_level, true);
> > - read_unlock(&kvm->mmu_lock);
> > + if (!is_tdp_mmu_enabled(kvm))
> > + return;
> > +
> > + read_lock(&kvm->mmu_lock);
> > + kvm_tdp_mmu_try_split_huge_pages(kvm, memslot, start, end, target_level,
> > + true);
>
> Eh, let this poke out.
Will do :)
>
> > + read_unlock(&kvm->mmu_lock);
> > +
> > + if (kvm_memslots_have_rmaps(kvm)) {
> > + write_lock(&kvm->mmu_lock);
> > + kvm_nested_mmu_try_split_huge_pages(kvm, memslot, start, end,
> > + target_level);
> > + write_unlock(&kvm->mmu_lock);
>
> Super duper nit: all other flows do rmaps first, than TDP MMU. Might as well keep
> that ordering here, otherwise it suggests there's a reason to be different.
Will do.
>
> > }
> >
> > /*
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index ab336f7c82e4..e123e24a130f 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -12161,6 +12161,12 @@ static void kvm_mmu_slot_apply_flags(struct kvm *kvm,
> > * page faults will create the large-page sptes.
> > */
> > kvm_mmu_zap_collapsible_sptes(kvm, new);
> > +
> > + /*
> > + * Free any memory left behind by eager page splitting. Ignore
> > + * the module parameter since userspace might have changed it.
> > + */
> > + free_split_caches(kvm);
> > } else {
> > /*
> > * Initially-all-set does not require write protecting any page,
> > --
> > 2.36.0.rc2.479.g8af0fa9b8e-goog
> >
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