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Fri, 5 Dec 2025 17:23:30 +0000 (UTC) X-Farcaster-Flow-ID: ba644cd0-1d42-4328-b08c-52f3a96e0bd0 Received: from EX19D005EUB003.ant.amazon.com (10.252.51.31) by EX19MTAEUC002.ant.amazon.com (10.252.51.181) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA) id 15.2.2562.29; Fri, 5 Dec 2025 17:23:26 +0000 Received: from [192.168.14.68] (10.106.83.5) by EX19D005EUB003.ant.amazon.com (10.252.51.31) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA) id 15.2.2562.29; Fri, 5 Dec 2025 17:23:23 +0000 Message-ID: <356ece73-83ff-4c0f-a3fb-4b24df7953b3@amazon.com> Date: Fri, 5 Dec 2025 17:23:22 +0000 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Reply-To: Subject: Re: [PATCH v7 05/12] KVM: guest_memfd: Add flag to remove from direct map To: Aneesh Kumar K.V CC: "pbonzini@redhat.com" , "corbet@lwn.net" , "maz@kernel.org" , "oliver.upton@linux.dev" , "joey.gouly@arm.com" , "suzuki.poulose@arm.com" , "yuzenghui@huawei.com" , "catalin.marinas@arm.com" , "will@kernel.org" , "tglx@linutronix.de" , "mingo@redhat.com" , "bp@alien8.de" , "dave.hansen@linux.intel.com" , "x86@kernel.org" , "hpa@zytor.com" , "luto@kernel.org" , "peterz@infradead.org" , "willy@infradead.org" , "akpm@linux-foundation.org" , "lorenzo.stoakes@oracle.com" , "Liam.Howlett@oracle.com" , "vbabka@suse.cz" , "rppt@kernel.org" , "surenb@google.com" , "mhocko@suse.com" , "song@kernel.org" , "jolsa@kernel.org" , "ast@kernel.org" , "daniel@iogearbox.net" , "andrii@kernel.org" , "martin.lau@linux.dev" , "eddyz87@gmail.com" , "yonghong.song@linux.dev" , "john.fastabend@gmail.com" , "kpsingh@kernel.org" , "sdf@fomichev.me" , "haoluo@google.com" , "jgg@ziepe.ca" , "jhubbard@nvidia.com" , "peterx@redhat.com" , "jannh@google.com" , "pfalcato@suse.de" , "shuah@kernel.org" , "seanjc@google.com" , "kvm@vger.kernel.org" , "linux-doc@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" , "kvmarm@lists.linux.dev" , "linux-fsdevel@vger.kernel.org" , "linux-mm@kvack.org" , "bpf@vger.kernel.org" , "linux-kselftest@vger.kernel.org" , "Cali, Marco" , "Kalyazin, Nikita" , "Thomson, Jack" , "derekmn@amazon.co.uk" , "tabba@google.com" , "ackerleytng@google.com" , "david@kernel.org" , "patrick.roy@linux.dev" References: <20250924151101.2225820-4-patrick.roy@campus.lmu.de> <20250924152214.7292-1-roypat@amazon.co.uk> <20250924152214.7292-2-roypat@amazon.co.uk> Content-Language: en-US From: Nikita Kalyazin Autocrypt: addr=kalyazin@amazon.com; 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>> + >> + int r = set_direct_map_valid_noflush(folio_page(folio, 0), folio_nr_pages(folio), >> + false); >> + >> + if (!r) { >> + unsigned long addr = (unsigned long) folio_address(folio); >> + folio->private = (void *) ((u64) folio->private & KVM_GMEM_FOLIO_NO_DIRECT_MAP); >> + flush_tlb_kernel_range(addr, addr + folio_size(folio)); >> + } >> + >> + return r; >> +} > > These 'noflush' functions are actually doing flush_tlb_kernel > > [-] ∘ flush_tlb_kernel_range > |-[-] ← __change_memory_common > | `-[-] ← set_memory_valid > | `- ← set_direct_map_valid_noflush Hi Aneesh, Thanks for pointing at that. I ran internal tests and it appears that the second flush_tlb_kernel_range() call does add a latency similar to the one coming from the first call, even though it intuitively should be a no-op. I have to admit that I am not aware of a safe way to avoid the second flushing on ARM while keeping the guest_memfd code arch-agnostic. Perhaps I should seek Will's counsel for it. Nevertheless, I don't think there is a concern from the functional point of view. Nikita > > -aneesh