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[34.82.181.220]) by smtp.gmail.com with ESMTPSA id k13-20020a170902ce0d00b0017f5ad327casm10453887plg.103.2022.11.15.15.27.22 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 15 Nov 2022 15:27:22 -0800 (PST) Date: Tue, 15 Nov 2022 15:27:18 -0800 From: Ricardo Koller To: Oliver Upton 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() Message-ID: References: <20221112081714.2169495-1-ricarkol@google.com> <20221112081714.2169495-5-ricarkol@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Tue, Nov 15, 2022 at 03:03:42PM -0800, Ricardo Koller wrote: > 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. Actually, ignoring the error here simplifies the error handling. stage2_create_removed() is best-effort; here's an example. If stage2_create_removed() was called to split a 1G block PTE, and it wasn't able to split all 2MB blocks, it would return ENOMEM and a valid PTE pointing to a tree like this: [---------1GB-------------] : : [--2MB--][--2MB--][--2MB--] : : [ ][ ][ ] If we returned ENOMEM instead of ignoring the error, we would have to clean all the intermediate state. But stage2_create_removed() is designed to always return a valid PTE, even if the tree is not fully split (as above). So, there's no really need to clean it: it's a valid tree. Moreover, this valid tree would result in better dirty logging performance as it already has some 2M blocks split into 4K pages. > > > > > 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