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<20201130133421.GB24837@willie-the-truck> <67e9e393-1836-eca7-4235-6f4a19fed652@huawei.com> <20201130160119.GA25051@willie-the-truck> <868a4403-10d3-80f3-4ae1-a490813c55e2@huawei.com> <20201201134606.GB26973@willie-the-truck> User-Agent: Roundcube Webmail/1.4.9 Message-ID: <03ab1bdd8459831ad05993807e39e5bd@kernel.org> X-Sender: maz@kernel.org X-SA-Exim-Connect-IP: 51.254.78.96 X-SA-Exim-Rcpt-To: will@kernel.org, wangyanan55@huawei.com, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, catalin.marinas@arm.com, james.morse@arm.com, julien.thierry.kdev@gmail.com, suzuki.poulose@arm.com, gshan@redhat.com, qperret@google.com, wanghaibin.wang@huawei.com, yezengruan@huawei.com, zhukeqian1@huawei.com, yuzenghui@huawei.com, jiangkunkun@huawei.com, wangjingyi11@huawei.com, lushenming@huawei.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) 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Dec 2020 14:05:03 +0000 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Date: Tue, 01 Dec 2020 14:05:03 +0000 From: Marc Zyngier To: Will Deacon Cc: "wangyanan (Y)" , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Catalin Marinas , James Morse , Julien Thierry , Suzuki K Poulose , Gavin Shan , Quentin Perret , wanghaibin.wang@huawei.com, yezengruan@huawei.com, zhukeqian1@huawei.com, yuzenghui@huawei.com, jiangkunkun@huawei.com, wangjingyi11@huawei.com, lushenming@huawei.com Subject: Re: [RFC PATCH 2/3] KVM: arm64: Fix handling of merging tables into a block entry In-Reply-To: <20201201134606.GB26973@willie-the-truck> References: <20201130121847.91808-1-wangyanan55@huawei.com> <20201130121847.91808-3-wangyanan55@huawei.com> <20201130133421.GB24837@willie-the-truck> <67e9e393-1836-eca7-4235-6f4a19fed652@huawei.com> <20201130160119.GA25051@willie-the-truck> <868a4403-10d3-80f3-4ae1-a490813c55e2@huawei.com> <20201201134606.GB26973@willie-the-truck> User-Agent: Roundcube Webmail/1.4.9 Message-ID: <03ab1bdd8459831ad05993807e39e5bd@kernel.org> X-Sender: maz@kernel.org X-SA-Exim-Connect-IP: 51.254.78.96 X-SA-Exim-Rcpt-To: will@kernel.org, wangyanan55@huawei.com, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, catalin.marinas@arm.com, james.morse@arm.com, julien.thierry.kdev@gmail.com, suzuki.poulose@arm.com, gshan@redhat.com, qperret@google.com, wanghaibin.wang@huawei.com, yezengruan@huawei.com, zhukeqian1@huawei.com, yuzenghui@huawei.com, jiangkunkun@huawei.com, wangjingyi11@huawei.com, lushenming@huawei.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2020-12-01 13:46, Will Deacon wrote: > On Tue, Dec 01, 2020 at 10:30:41AM +0800, wangyanan (Y) wrote: >> On 2020/12/1 0:01, Will Deacon wrote: >> > On Mon, Nov 30, 2020 at 11:24:19PM +0800, wangyanan (Y) wrote: >> > > On 2020/11/30 21:34, Will Deacon wrote: >> > > > On Mon, Nov 30, 2020 at 08:18:46PM +0800, Yanan Wang wrote: >> > > > > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c >> > > > > index 696b6aa83faf..fec8dc9f2baa 100644 >> > > > > --- a/arch/arm64/kvm/hyp/pgtable.c >> > > > > +++ b/arch/arm64/kvm/hyp/pgtable.c >> > > > > @@ -500,6 +500,9 @@ static int stage2_map_walk_table_pre(u64 addr, u64 end, u32 level, >> > > > > return 0; >> > > > > } >> > > > > +static void stage2_flush_dcache(void *addr, u64 size); >> > > > > +static bool stage2_pte_cacheable(kvm_pte_t pte); >> > > > > + >> > > > > static int stage2_map_walk_leaf(u64 addr, u64 end, u32 level, kvm_pte_t *ptep, >> > > > > struct stage2_map_data *data) >> > > > > { >> > > > > @@ -507,9 +510,17 @@ static int stage2_map_walk_leaf(u64 addr, u64 end, u32 level, kvm_pte_t *ptep, >> > > > > struct page *page = virt_to_page(ptep); >> > > > > if (data->anchor) { >> > > > > - if (kvm_pte_valid(pte)) >> > > > > + if (kvm_pte_valid(pte)) { >> > > > > + kvm_set_invalid_pte(ptep); >> > > > > + kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, data->mmu, >> > > > > + addr, level); >> > > > > put_page(page); >> > > > This doesn't make sense to me: the page-table pages we're walking when the >> > > > anchor is set are not accessible to the hardware walker because we unhooked >> > > > the entire sub-table in stage2_map_walk_table_pre(), which has the necessary >> > > > TLB invalidation. >> > > > >> > > > Are you seeing a problem in practice here? >> > > Yes, I indeed find a problem in practice. >> > > >> > > When the migration was cancelled, a TLB conflic abort  was found in guest. >> > > >> > > This problem is fixed before rework of the page table code, you can have a >> > > look in the following two links: >> > > >> > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=3c3736cd32bf5197aed1410ae826d2d254a5b277 >> > > >> > > https://lists.cs.columbia.edu/pipermail/kvmarm/2019-March/035031.html >> > Ok, let's go through this, because I still don't see the bug. Please correct >> > me if you spot any mistakes: >> > >> > 1. We have a block mapping for X => Y >> > 2. Dirty logging is enabled, so the block mapping is write-protected and >> > ends up being split into page mappings >> > 3. Dirty logging is disabled due to a failed migration. >> > >> > --- At this point, I think we agree that the state of the MMU is alright --- >> > >> > 4. We take a stage-2 fault and want to reinstall the block mapping: >> > >> > a. kvm_pgtable_stage2_map() is invoked to install the block mapping >> > b. stage2_map_walk_table_pre() finds a table where we would like to >> > install the block: >> > >> > i. The anchor is set to point at this entry >> > ii. The entry is made invalid >> > iii. We invalidate the TLB for the input address. This is >> > TLBI IPAS2SE1IS without level hint and then TLBI VMALLE1IS. >> > >> > *** At this point, the page-table pointed to by the old table entry >> > is not reachable to the hardware walker *** >> > >> > c. stage2_map_walk_leaf() is called for each leaf entry in the >> > now-unreachable subtree, dropping page-references for each valid >> > entry it finds. >> > d. stage2_map_walk_table_post() is eventually called for the entry >> > which we cleared back in b.ii, so we install the new block mapping. >> > >> > You are proposing to add additional TLB invalidation to (c), but I don't >> > think that is necessary, thanks to the invalidation already performed in >> > b.iii. What am I missing here? >> >> The point is at b.iii where the TLBI is not enough. There are many >> page >> mappings that we need to merge into a block mapping. >> >> We invalidate the TLB for the input address without level hint at >> b.iii, but >> this operation just flush TLB for one page mapping, there >> >> are still some TLB entries for the other page mappings in the cache, >> the MMU >> hardware walker can still hit these entries next time. > > Ah, yes, I see. Thanks. I hadn't considered the case where there are > table > entries beneath the anchor. So how about the diff below? > > Will > > --->8 > > diff --git a/arch/arm64/kvm/hyp/pgtable.c > b/arch/arm64/kvm/hyp/pgtable.c > index 0271b4a3b9fe..12526d8c7ae4 100644 > --- a/arch/arm64/kvm/hyp/pgtable.c > +++ b/arch/arm64/kvm/hyp/pgtable.c > @@ -493,7 +493,7 @@ static int stage2_map_walk_table_pre(u64 addr, u64 > end, u32 level, > return 0; > > kvm_set_invalid_pte(ptep); > - kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, data->mmu, addr, 0); > + /* TLB invalidation is deferred until the _post handler */ > data->anchor = ptep; > return 0; > } > @@ -547,11 +547,21 @@ static int stage2_map_walk_table_post(u64 addr, > u64 end, u32 level, > struct stage2_map_data *data) > { > int ret = 0; > + kvm_pte_t pte = *ptep; > > if (!data->anchor) > return 0; > > - free_page((unsigned long)kvm_pte_follow(*ptep)); > + kvm_set_invalid_pte(ptep); > + > + /* > + * Invalidate the whole stage-2, as we may have numerous leaf > + * entries below us which would otherwise need invalidating > + * individually. > + */ > + kvm_call_hyp(__kvm_tlb_flush_vmid, data->mmu); That's a big hammer, and we so far have been pretty careful not to over-invalidate. Is the block-replacing-table *without* an unmap in between the only case where this triggers? Thanks, M. -- Jazz is not dead. It just smells funny...