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[2003:cb:c734:9600:af27:4326:a216:2bfb]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43aba532ba6sm186494095e9.12.2025.03.03.03.42.16 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 03 Mar 2025 03:42:17 -0800 (PST) Message-ID: <0ff27dbb-401b-4793-be70-1ff9c8010e4a@redhat.com> Date: Mon, 3 Mar 2025 12:42:11 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 3/4] arm64/mm: Elide tlbi in contpte_convert() under BBML2 To: =?UTF-8?Q?Miko=C5=82aj_Lenczewski?= Cc: ryan.roberts@arm.com, suzuki.poulose@arm.com, yang@os.amperecomputing.com, catalin.marinas@arm.com, will@kernel.org, joro@8bytes.org, jean-philippe@linaro.org, mark.rutland@arm.com, joey.gouly@arm.com, oliver.upton@linux.dev, james.morse@arm.com, broonie@kernel.org, maz@kernel.org, akpm@linux-foundation.org, jgg@ziepe.ca, nicolinc@nvidia.com, mshavit@google.com, jsnitsel@redhat.com, smostafa@google.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, iommu@lists.linux.dev References: <20250228182403.6269-2-miko.lenczewski@arm.com> <20250228182403.6269-5-miko.lenczewski@arm.com> <20250303094947.GB13345@e133081.arm.com> <7e987f17-ffcb-45e0-8588-2d569d90f776@redhat.com> <20250303105539.GA74129@e133081.arm.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 03.03.25 11:55, Mikołaj Lenczewski wrote: > On Mon, Mar 03, 2025 at 10:57:21AM +0100, David Hildenbrand wrote: >> On 03.03.25 10:49, Mikołaj Lenczewski wrote: >>> Hi David, >>> >>> Thanks for taking the time to review. >>> >>> On Mon, Mar 03, 2025 at 10:17:12AM +0100, David Hildenbrand wrote: >>>> On 28.02.25 19:24, Mikołaj Lenczewski wrote: >>>>> If we support bbml2 without conflict aborts, we can avoid the final >>>>> flush and have hardware manage the tlb entries for us. Avoiding flushes >>>>> is a win. >>>>> >>>>> Signed-off-by: Mikołaj Lenczewski >>>>> Reviewed-by: Ryan Roberts >>>>> --- >>>>> arch/arm64/mm/contpte.c | 3 --- >>>>> 1 file changed, 3 deletions(-) >>>>> >>>>> diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c >>>>> index 145530f706a9..77ed03b30b72 100644 >>>>> --- a/arch/arm64/mm/contpte.c >>>>> +++ b/arch/arm64/mm/contpte.c >>>>> @@ -72,9 +72,6 @@ static void contpte_convert(struct mm_struct *mm, unsigned long addr, >>>>> __flush_tlb_range(&vma, start_addr, addr, PAGE_SIZE, true, 3); >>>>> __set_ptes(mm, start_addr, start_ptep, pte, CONT_PTES); >>>>> - >>>>> - if (system_supports_bbml2_noabort()) >>>>> - __flush_tlb_range(&vma, start_addr, addr, PAGE_SIZE, true, 3); >>>>> } >>>>> void __contpte_try_fold(struct mm_struct *mm, unsigned long addr, >>>> >>>> What's the point of not squashing this into #2? :) >>>> >>>> If this split was requested during earlier review, at least seeing patch #2 >>>> on its own confused me. >>> >>> This split is a holdover from an earlier patchset, where it was still >>> unknown whether the removal of the second flush was permitted with >>> BBML2. Partly this was due to us being worried about conflict aborts >>> after the removal, and partly this was because the "delay" is a separate >>> optimisation that we could apply even if it turned out the final patch >>> was not architecturally sound. >>> >>> Now that we do not handle conflict aborts (preferring only systems that >>> handle BBML2 without ever raising aborts), the first issue is not a >>> problem. The reasoning behind the second patch is also a little bit >>> outdated, but I can see the logical split between a tlbi reorder, and >>> the removal of the tlbi. If this is truly redundant though, I would be >>> happy to squash the two into a single patch. >> >> Thanks for the information. >> >> Does patch #2 (reordering the tlbi) have any benefit on its own? I read >> "other threads will not see an invalid pagetable entry", but I am not sure >> that is correct. A concurrent HW page table walker would still find the >> invalid PTE? It's just a matter of TLB state. > > I think I understand what you mean. I agree that it is possible for a > concurrent walk to see an invalid TLBI state, if it is on the same TLB > that the repaint is happening on. For other TLBs, the flush has not yet > propagated our invalidated PTEs (from `__ptep_get_and_clear()`) though? What I am saying is: if there is no TLB entry yet, HW will walk the page table to find no present PTE and trigger a fault. > That invalidation will only be seen by other TLBs after the > `__flush_tlb_range()`, so we should save a few faults because only > "local" threads will ever see the invalid entry, as opposed to all > threads that try to read our modified range? So what you say is, that deferring the flush means that if there is already a TLB entry, flushing deferred reduces the likelihood that a page table walk is triggered that could find no present PTE: consequently, reducing the likelihood that a page fault is triggered. (I use the word likelihood, because I assume other action could result in a TLB entry getting flushed in the meantime, such as TLB entry reuse) Correct? Or it is the case that I > have misunderstood something basic here, or that I have misinterpreted > what you have written? No, that makes it clearer, thanks. -- Cheers, David / dhildenb