From: Ricardo Koller <ricarkol@google.com> To: Oliver Upton <oliver.upton@linux.dev> Cc: ricarkol@gmail.com, kvm@vger.kernel.org, catalin.marinas@arm.com, kvmarm@lists.linux.dev, andrew.jones@linux.dev, bgardon@google.com, maz@kernel.org, dmatlack@google.com, pbonzini@redhat.com, kvmarm@lists.cs.columbia.edu Subject: Re: [RFC PATCH 04/12] KVM: arm64: Add kvm_pgtable_stage2_split() Date: Tue, 15 Nov 2022 15:03:42 -0800 [thread overview] Message-ID: <Y3QazjAUVE+T6rHh@google.com> (raw) In-Reply-To: <Y3KrHG4WMXMUquUy@google.com> On Mon, Nov 14, 2022 at 08:54:52PM +0000, Oliver Upton wrote: > Hi Ricardo, > > On Sat, Nov 12, 2022 at 08:17:06AM +0000, Ricardo Koller wrote: > > [...] > > > +/** > > + * kvm_pgtable_stage2_split() - Split a range of huge pages into leaf PTEs pointing > > + * to PAGE_SIZE guest pages. > > + * @pgt: Page-table structure initialised by kvm_pgtable_stage2_init*(). > > + * @addr: Intermediate physical address from which to split. > > + * @size: Size of the range. > > + * @mc: Cache of pre-allocated and zeroed memory from which to allocate > > + * page-table pages. > > + * > > + * @addr and the end (@addr + @size) are effectively aligned down and up to > > + * the top level huge-page block size. This is an exampe using 1GB > > + * huge-pages and 4KB granules. > > + * > > + * [---input range---] > > + * : : > > + * [--1G block pte--][--1G block pte--][--1G block pte--][--1G block pte--] > > + * : : > > + * [--2MB--][--2MB--][--2MB--][--2MB--] > > + * : : > > + * [ ][ ][:][ ][ ][ ][ ][ ][:][ ][ ][ ] > > + * : : > > + * > > + * Return: 0 on success, negative error code on failure. Note that > > + * kvm_pgtable_stage2_split() is best effort: it tries to break as many > > + * blocks in the input range as allowed by the size of the memcache. It > > + * will fail it wasn't able to break any block. > > + */ > > +int kvm_pgtable_stage2_split(struct kvm_pgtable *pgt, u64 addr, u64 size, void *mc); > > + > > /** > > * kvm_pgtable_walk() - Walk a page-table. > > * @pgt: Page-table structure initialised by kvm_pgtable_*_init(). > > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c > > index d1f309128118..9c42eff6d42e 100644 > > --- a/arch/arm64/kvm/hyp/pgtable.c > > +++ b/arch/arm64/kvm/hyp/pgtable.c > > @@ -1267,6 +1267,80 @@ static int stage2_create_removed(kvm_pte_t *ptep, u64 phys, u32 level, > > return __kvm_pgtable_visit(&data, mm_ops, ptep, level); > > } > > > > +struct stage2_split_data { > > + struct kvm_s2_mmu *mmu; > > + void *memcache; > > + struct kvm_pgtable_mm_ops *mm_ops; > > You can also get at mm_ops through kvm_pgtable_visit_ctx > > > +}; > > + > > +static int stage2_split_walker(const struct kvm_pgtable_visit_ctx *ctx, > > + enum kvm_pgtable_walk_flags visit) > > +{ > > + struct stage2_split_data *data = ctx->arg; > > + struct kvm_pgtable_mm_ops *mm_ops = data->mm_ops; > > + kvm_pte_t pte = ctx->old, attr, new; > > + enum kvm_pgtable_prot prot; > > + void *mc = data->memcache; > > + u32 level = ctx->level; > > + u64 phys; > > + > > + if (WARN_ON_ONCE(kvm_pgtable_walk_shared(ctx))) > > + return -EINVAL; > > + > > + /* Nothing to split at the last level */ > > + if (level == KVM_PGTABLE_MAX_LEVELS - 1) > > + return 0; > > + > > + /* We only split valid block mappings */ > > + if (!kvm_pte_valid(pte) || kvm_pte_table(pte, ctx->level)) > > + return 0; > > + > > + phys = kvm_pte_to_phys(pte); > > + prot = kvm_pgtable_stage2_pte_prot(pte); > > + stage2_set_prot_attr(data->mmu->pgt, prot, &attr); > > + > > + /* > > + * Eager page splitting is best-effort, so we can ignore the error. > > + * The returned PTE (new) will be valid even if this call returns > > + * error: new will be a single (big) block PTE. The only issue is > > + * that it will affect dirty logging performance, as the huge-pages > > + * will have to be split on fault, and so we WARN. > > + */ > > + WARN_ON(stage2_create_removed(&new, phys, level, attr, mc, mm_ops)); > > I don't believe we should warn in this case, at least not > unconditionally. ENOMEM is an expected outcome, for example. Given that "eager page splitting" is best-effort, the error must be ignored somewhere: either here or by the caller (in mmu.c). It seems that ignoring the error here is not a very good idea. > > Additionally, I believe you'll want to bail out at this point to avoid > installing a potentially garbage PTE as well. It should be fine as stage2_create_removed() is also best-effort. The returned PTE is valid even when it fails; it just returns a big block PTE. > > > + stage2_put_pte(ctx, data->mmu, mm_ops); > > Ah, I see why you've relaxed the WARN in patch 1 now. > > I would recommend you follow the break-before-make pattern and use the > helpers here as well. stage2_try_break_pte() will demote the store to > WRITE_ONCE() if called from a non-shared context. > ACK, I can do that. The only reason why I didnt' is because I would have to handle the potential error from stage2_try_break_pte(). It would feel wrong not to, even if it's !shared. On the other hand, I would like to easily experiment with both the !shared and the shared approaches easily. > Then the WARN will behave as expected in stage2_make_pte(). > > > + /* > > + * Note, the contents of the page table are guaranteed to be made > > + * visible before the new PTE is assigned because > > + * stage2_make__pte() writes the PTE using smp_store_release(). > > typo: stage2_make_pte() > > -- > Thanks, > Oliver _______________________________________________ kvmarm mailing list kvmarm@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
WARNING: multiple messages have this Message-ID (diff)
From: Ricardo Koller <ricarkol@google.com> To: Oliver Upton <oliver.upton@linux.dev> Cc: pbonzini@redhat.com, maz@kernel.org, dmatlack@google.com, qperret@google.com, catalin.marinas@arm.com, andrew.jones@linux.dev, seanjc@google.com, alexandru.elisei@arm.com, suzuki.poulose@arm.com, eric.auger@redhat.com, gshan@redhat.com, reijiw@google.com, rananta@google.com, bgardon@google.com, kvm@vger.kernel.org, kvmarm@lists.linux.dev, kvmarm@lists.cs.columbia.edu, ricarkol@gmail.com Subject: Re: [RFC PATCH 04/12] KVM: arm64: Add kvm_pgtable_stage2_split() Date: Tue, 15 Nov 2022 15:03:42 -0800 [thread overview] Message-ID: <Y3QazjAUVE+T6rHh@google.com> (raw) Message-ID: <20221115230342.GUfI6XYZZMUh-ypv47DayaiBP0BLZfYbyFSE8xbfDfg@z> (raw) In-Reply-To: <Y3KrHG4WMXMUquUy@google.com> On Mon, Nov 14, 2022 at 08:54:52PM +0000, Oliver Upton wrote: > Hi Ricardo, > > On Sat, Nov 12, 2022 at 08:17:06AM +0000, Ricardo Koller wrote: > > [...] > > > +/** > > + * kvm_pgtable_stage2_split() - Split a range of huge pages into leaf PTEs pointing > > + * to PAGE_SIZE guest pages. > > + * @pgt: Page-table structure initialised by kvm_pgtable_stage2_init*(). > > + * @addr: Intermediate physical address from which to split. > > + * @size: Size of the range. > > + * @mc: Cache of pre-allocated and zeroed memory from which to allocate > > + * page-table pages. > > + * > > + * @addr and the end (@addr + @size) are effectively aligned down and up to > > + * the top level huge-page block size. This is an exampe using 1GB > > + * huge-pages and 4KB granules. > > + * > > + * [---input range---] > > + * : : > > + * [--1G block pte--][--1G block pte--][--1G block pte--][--1G block pte--] > > + * : : > > + * [--2MB--][--2MB--][--2MB--][--2MB--] > > + * : : > > + * [ ][ ][:][ ][ ][ ][ ][ ][:][ ][ ][ ] > > + * : : > > + * > > + * Return: 0 on success, negative error code on failure. Note that > > + * kvm_pgtable_stage2_split() is best effort: it tries to break as many > > + * blocks in the input range as allowed by the size of the memcache. It > > + * will fail it wasn't able to break any block. > > + */ > > +int kvm_pgtable_stage2_split(struct kvm_pgtable *pgt, u64 addr, u64 size, void *mc); > > + > > /** > > * kvm_pgtable_walk() - Walk a page-table. > > * @pgt: Page-table structure initialised by kvm_pgtable_*_init(). > > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c > > index d1f309128118..9c42eff6d42e 100644 > > --- a/arch/arm64/kvm/hyp/pgtable.c > > +++ b/arch/arm64/kvm/hyp/pgtable.c > > @@ -1267,6 +1267,80 @@ static int stage2_create_removed(kvm_pte_t *ptep, u64 phys, u32 level, > > return __kvm_pgtable_visit(&data, mm_ops, ptep, level); > > } > > > > +struct stage2_split_data { > > + struct kvm_s2_mmu *mmu; > > + void *memcache; > > + struct kvm_pgtable_mm_ops *mm_ops; > > You can also get at mm_ops through kvm_pgtable_visit_ctx > > > +}; > > + > > +static int stage2_split_walker(const struct kvm_pgtable_visit_ctx *ctx, > > + enum kvm_pgtable_walk_flags visit) > > +{ > > + struct stage2_split_data *data = ctx->arg; > > + struct kvm_pgtable_mm_ops *mm_ops = data->mm_ops; > > + kvm_pte_t pte = ctx->old, attr, new; > > + enum kvm_pgtable_prot prot; > > + void *mc = data->memcache; > > + u32 level = ctx->level; > > + u64 phys; > > + > > + if (WARN_ON_ONCE(kvm_pgtable_walk_shared(ctx))) > > + return -EINVAL; > > + > > + /* Nothing to split at the last level */ > > + if (level == KVM_PGTABLE_MAX_LEVELS - 1) > > + return 0; > > + > > + /* We only split valid block mappings */ > > + if (!kvm_pte_valid(pte) || kvm_pte_table(pte, ctx->level)) > > + return 0; > > + > > + phys = kvm_pte_to_phys(pte); > > + prot = kvm_pgtable_stage2_pte_prot(pte); > > + stage2_set_prot_attr(data->mmu->pgt, prot, &attr); > > + > > + /* > > + * Eager page splitting is best-effort, so we can ignore the error. > > + * The returned PTE (new) will be valid even if this call returns > > + * error: new will be a single (big) block PTE. The only issue is > > + * that it will affect dirty logging performance, as the huge-pages > > + * will have to be split on fault, and so we WARN. > > + */ > > + WARN_ON(stage2_create_removed(&new, phys, level, attr, mc, mm_ops)); > > I don't believe we should warn in this case, at least not > unconditionally. ENOMEM is an expected outcome, for example. Given that "eager page splitting" is best-effort, the error must be ignored somewhere: either here or by the caller (in mmu.c). It seems that ignoring the error here is not a very good idea. > > Additionally, I believe you'll want to bail out at this point to avoid > installing a potentially garbage PTE as well. It should be fine as stage2_create_removed() is also best-effort. The returned PTE is valid even when it fails; it just returns a big block PTE. > > > + stage2_put_pte(ctx, data->mmu, mm_ops); > > Ah, I see why you've relaxed the WARN in patch 1 now. > > I would recommend you follow the break-before-make pattern and use the > helpers here as well. stage2_try_break_pte() will demote the store to > WRITE_ONCE() if called from a non-shared context. > ACK, I can do that. The only reason why I didnt' is because I would have to handle the potential error from stage2_try_break_pte(). It would feel wrong not to, even if it's !shared. On the other hand, I would like to easily experiment with both the !shared and the shared approaches easily. > Then the WARN will behave as expected in stage2_make_pte(). > > > + /* > > + * Note, the contents of the page table are guaranteed to be made > > + * visible before the new PTE is assigned because > > + * stage2_make__pte() writes the PTE using smp_store_release(). > > typo: stage2_make_pte() > > -- > Thanks, > Oliver
next prev parent reply other threads:[~2022-11-15 23:03 UTC|newest] Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top 2022-11-12 8:17 [RFC PATCH 00/12] KVM: arm64: Eager huge-page splitting for dirty-logging Ricardo Koller 2022-11-12 8:17 ` Ricardo Koller 2022-11-12 8:17 ` [RFC PATCH 01/12] KVM: arm64: Relax WARN check in stage2_make_pte() Ricardo Koller 2022-11-12 8:17 ` Ricardo Koller 2022-11-14 20:59 ` Oliver Upton 2022-11-14 20:59 ` Oliver Upton 2022-11-12 8:17 ` [RFC PATCH 02/12] KVM: arm64: Allow visiting block PTEs in post-order Ricardo Koller 2022-11-12 8:17 ` Ricardo Koller 2022-11-14 18:48 ` Oliver Upton 2022-11-14 18:48 ` Oliver Upton 2023-01-13 3:44 ` Ricardo Koller 2022-11-12 8:17 ` [RFC PATCH 03/12] KVM: arm64: Add stage2_create_removed() Ricardo Koller 2022-11-12 8:17 ` Ricardo Koller 2022-11-12 8:17 ` [RFC PATCH 04/12] KVM: arm64: Add kvm_pgtable_stage2_split() Ricardo Koller 2022-11-12 8:17 ` Ricardo Koller 2022-11-14 20:54 ` Oliver Upton 2022-11-14 20:54 ` Oliver Upton 2022-11-15 23:03 ` Ricardo Koller [this message] 2022-11-15 23:03 ` Ricardo Koller 2022-11-15 23:27 ` Ricardo Koller 2022-11-15 23:27 ` Ricardo Koller 2022-11-15 23:54 ` Oliver Upton 2022-11-15 23:54 ` Oliver Upton 2022-11-17 21:50 ` Ricardo Koller 2022-11-17 21:50 ` Ricardo Koller 2022-11-12 8:17 ` [RFC PATCH 05/12] arm64: Add a capability for FEAT_BBM level 2 Ricardo Koller 2022-11-12 8:17 ` Ricardo Koller 2022-11-12 8:17 ` [RFC PATCH 06/12] KVM: arm64: Split block PTEs without using break-before-make Ricardo Koller 2022-11-12 8:17 ` Ricardo Koller 2022-11-14 18:56 ` Oliver Upton 2022-11-14 18:56 ` Oliver Upton 2022-11-12 8:17 ` [RFC PATCH 07/12] KVM: arm64: Refactor kvm_arch_commit_memory_region() Ricardo Koller 2022-11-12 8:17 ` Ricardo Koller 2022-11-12 8:17 ` [RFC PATCH 08/12] KVM: arm64: Add kvm_uninit_stage2_mmu() Ricardo Koller 2022-11-12 8:17 ` Ricardo Koller 2022-11-12 8:17 ` [RFC PATCH 09/12] KVM: arm64: Split huge pages when dirty logging is enabled Ricardo Koller 2022-11-12 8:17 ` Ricardo Koller 2022-11-12 8:17 ` [RFC PATCH 10/12] KVM: arm64: Open-code kvm_mmu_write_protect_pt_masked() Ricardo Koller 2022-11-12 8:17 ` Ricardo Koller 2022-11-12 8:17 ` [RFC PATCH 11/12] KVM: arm64: Split huge pages during KVM_CLEAR_DIRTY_LOG Ricardo Koller 2022-11-12 8:17 ` Ricardo Koller 2022-11-12 8:17 ` [RFC PATCH 12/12] KVM: arm64: Use local TLBI on permission relaxation Ricardo Koller 2022-11-12 8:17 ` Ricardo Koller 2022-11-14 18:42 ` [RFC PATCH 00/12] KVM: arm64: Eager huge-page splitting for dirty-logging Oliver Upton 2022-11-14 18:42 ` Oliver Upton 2023-01-13 3:42 ` Ricardo Koller
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