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Wed, 08 Jan 2025 18:47:32 -0800 (PST) Received: from [192.168.86.29] ([83.105.36.37]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38a8e4b81ccsm450052f8f.65.2025.01.08.18.47.30 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 08 Jan 2025 18:47:31 -0800 (PST) Message-ID: Date: Thu, 9 Jan 2025 02:47:30 +0000 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird To: yosryahmed@google.com Cc: akpm@linux-foundation.org, bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, jackmanb@google.com, kernel-team@meta.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, luto@kernel.org, mingo@redhat.com, nadav.amit@gmail.com, peterz@infradead.org, reijiw@google.com, riel@surriel.com, tglx@linutronix.de, x86@kernel.org, zhengqi.arch@bytedance.com References: Subject: Re: [PATCH v3 00/12] AMD broadcast TLB invalidation Content-Language: en-GB From: Andrew Cooper Autocrypt: addr=andrew.cooper3@citrix.com; keydata= xsFNBFLhNn8BEADVhE+Hb8i0GV6mihnnr/uiQQdPF8kUoFzCOPXkf7jQ5sLYeJa0cQi6Penp VtiFYznTairnVsN5J+ujSTIb+OlMSJUWV4opS7WVNnxHbFTPYZVQ3erv7NKc2iVizCRZ2Kxn srM1oPXWRic8BIAdYOKOloF2300SL/bIpeD+x7h3w9B/qez7nOin5NzkxgFoaUeIal12pXSR Q354FKFoy6Vh96gc4VRqte3jw8mPuJQpfws+Pb+swvSf/i1q1+1I4jsRQQh2m6OTADHIqg2E ofTYAEh7R5HfPx0EXoEDMdRjOeKn8+vvkAwhviWXTHlG3R1QkbE5M/oywnZ83udJmi+lxjJ5 YhQ5IzomvJ16H0Bq+TLyVLO/VRksp1VR9HxCzItLNCS8PdpYYz5TC204ViycobYU65WMpzWe LFAGn8jSS25XIpqv0Y9k87dLbctKKA14Ifw2kq5OIVu2FuX+3i446JOa2vpCI9GcjCzi3oHV e00bzYiHMIl0FICrNJU0Kjho8pdo0m2uxkn6SYEpogAy9pnatUlO+erL4LqFUO7GXSdBRbw5 gNt25XTLdSFuZtMxkY3tq8MFss5QnjhehCVPEpE6y9ZjI4XB8ad1G4oBHVGK5LMsvg22PfMJ ISWFSHoF/B5+lHkCKWkFxZ0gZn33ju5n6/FOdEx4B8cMJt+cWwARAQABzSlBbmRyZXcgQ29v cGVyIDxhbmRyZXcuY29vcGVyM0BjaXRyaXguY29tPsLBegQTAQgAJAIbAwULCQgHAwUVCgkI CwUWAgMBAAIeAQIXgAUCWKD95wIZAQAKCRBlw/kGpdefoHbdD/9AIoR3k6fKl+RFiFpyAhvO 59ttDFI7nIAnlYngev2XUR3acFElJATHSDO0ju+hqWqAb8kVijXLops0gOfqt3VPZq9cuHlh IMDquatGLzAadfFx2eQYIYT+FYuMoPZy/aTUazmJIDVxP7L383grjIkn+7tAv+qeDfE+txL4 SAm1UHNvmdfgL2/lcmL3xRh7sub3nJilM93RWX1Pe5LBSDXO45uzCGEdst6uSlzYR/MEr+5Z JQQ32JV64zwvf/aKaagSQSQMYNX9JFgfZ3TKWC1KJQbX5ssoX/5hNLqxMcZV3TN7kU8I3kjK mPec9+1nECOjjJSO/h4P0sBZyIUGfguwzhEeGf4sMCuSEM4xjCnwiBwftR17sr0spYcOpqET ZGcAmyYcNjy6CYadNCnfR40vhhWuCfNCBzWnUW0lFoo12wb0YnzoOLjvfD6OL3JjIUJNOmJy RCsJ5IA/Iz33RhSVRmROu+TztwuThClw63g7+hoyewv7BemKyuU6FTVhjjW+XUWmS/FzknSi dAG+insr0746cTPpSkGl3KAXeWDGJzve7/SBBfyznWCMGaf8E2P1oOdIZRxHgWj0zNr1+ooF /PzgLPiCI4OMUttTlEKChgbUTQ+5o0P080JojqfXwbPAyumbaYcQNiH1/xYbJdOFSiBv9rpt TQTBLzDKXok86M7BTQRS4TZ/ARAAkgqudHsp+hd82UVkvgnlqZjzz2vyrYfz7bkPtXaGb9H4 Rfo7mQsEQavEBdWWjbga6eMnDqtu+FC+qeTGYebToxEyp2lKDSoAsvt8w82tIlP/EbmRbDVn 7bhjBlfRcFjVYw8uVDPptT0TV47vpoCVkTwcyb6OltJrvg/QzV9f07DJswuda1JH3/qvYu0p vjPnYvCq4NsqY2XSdAJ02HrdYPFtNyPEntu1n1KK+gJrstjtw7KsZ4ygXYrsm/oCBiVW/OgU g/XIlGErkrxe4vQvJyVwg6YH653YTX5hLLUEL1NS4TCo47RP+wi6y+TnuAL36UtK/uFyEuPy wwrDVcC4cIFhYSfsO0BumEI65yu7a8aHbGfq2lW251UcoU48Z27ZUUZd2Dr6O/n8poQHbaTd 6bJJSjzGGHZVbRP9UQ3lkmkmc0+XCHmj5WhwNNYjgbbmML7y0fsJT5RgvefAIFfHBg7fTY/i kBEimoUsTEQz+N4hbKwo1hULfVxDJStE4sbPhjbsPCrlXf6W9CxSyQ0qmZ2bXsLQYRj2xqd1 bpA+1o1j2N4/au1R/uSiUFjewJdT/LX1EklKDcQwpk06Af/N7VZtSfEJeRV04unbsKVXWZAk uAJyDDKN99ziC0Wz5kcPyVD1HNf8bgaqGDzrv3TfYjwqayRFcMf7xJaL9xXedMcAEQEAAcLB XwQYAQgACQUCUuE2fwIbDAAKCRBlw/kGpdefoG4XEACD1Qf/er8EA7g23HMxYWd3FXHThrVQ HgiGdk5Yh632vjOm9L4sd/GCEACVQKjsu98e8o3ysitFlznEns5EAAXEbITrgKWXDDUWGYxd pnjj2u+GkVdsOAGk0kxczX6s+VRBhpbBI2PWnOsRJgU2n10PZ3mZD4Xu9kU2IXYmuW+e5KCA vTArRUdCrAtIa1k01sPipPPw6dfxx2e5asy21YOytzxuWFfJTGnVxZZSCyLUO83sh6OZhJkk b9rxL9wPmpN/t2IPaEKoAc0FTQZS36wAMOXkBh24PQ9gaLJvfPKpNzGD8XWR5HHF0NLIJhgg 4ZlEXQ2fVp3XrtocHqhu4UZR4koCijgB8sB7Tb0GCpwK+C4UePdFLfhKyRdSXuvY3AHJd4CP 4JzW0Bzq/WXY3XMOzUTYApGQpnUpdOmuQSfpV9MQO+/jo7r6yPbxT7CwRS5dcQPzUiuHLK9i nvjREdh84qycnx0/6dDroYhp0DFv4udxuAvt1h4wGwTPRQZerSm4xaYegEFusyhbZrI0U9tJ B8WrhBLXDiYlyJT6zOV2yZFuW47VrLsjYnHwn27hmxTC/7tvG3euCklmkn9Sl9IAKFu29RSo d5bD8kMSCYsTqtTfT6W4A3qHGvIDta3ptLYpIAOD2sY3GYq2nf3Bbzx81wZK14JdDDHUX2Rs 6+ahAA== In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam05 X-Stat-Signature: 77pn9gqpjo9ae6fagphnfc3qx1mrx9fr X-Rspamd-Queue-Id: AA0DD100002 X-Rspam-User: X-HE-Tag: 1736390853-209113 X-HE-Meta: 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 O7Ws5Kh1 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: >> I suspect AMD wouldn't tell us exactly ;) > > Well, ideally they would just tell us the conditions under which CPUs > respond to the broadcast TLB flush or the expectations around latency. [Resend, complete this time] Disclaimer.  I'm not at AMD; I don't know how they implement it; I'm just a random person on the internet.  But, here are a few things that might be relevant to know. AMD's SEV-SNP whitepaper [1] states that RMP permissions "are cached in the CPU TLB and related structures" and also "When required, hardware automatically performs TLB invalidations to ensure that all processors in the system see the updated RMP entry information." That sentence doesn't use "broadcast" or "remote", but "all processors" is a pretty clear clue.  Broadcast TLB invalidations are a building block of all the RMP-manipulation instructions. Furthermore, to be useful in this context, they need to be ordered with memory.  Specifically, a new pagewalk mustn't start after an invalidation, yet observe the stale RMP entry. x86 CPUs do have reasonable forward-progress guarantees, but in order to achieve forward progress, they need to e.g. guarantee that one memory access doesn't displace the TLB entry backing a different memory access from the same instruction, or you could livelock while trying to complete a single instruction. A consequence is that you can't safely invalidate a TLB entry of an in-progress instruction (although this means only the oldest instruction in the pipeline, because everything else is speculative and potentially transient). INVLPGB invalidations are interrupt-like from the point of view of the remote core, but are microarchitectural and can be taken irrespective of the architectural Interrupt and Global Interrupt Flags.  As a consequence, they'll need wait until an instruction boundary to be processed.  While not AMD, the Intel RAR whitepaper [2] discusses the handling of RARs on the remote processor, and they share a number of constraints in common with INVLPGB. Overall, I'd expect the INVLPGB instructions to be pretty quick in and of themselves; interestingly, they're not identified as architecturally serialising.  The broadcast is probably posted, and will be dealt with by remote processors on the subsequent instruction boundary.  TLBSYNC is the barrier to wait until the invalidations have been processed, and this will block for an unspecified length of time, probably bounded by the "longest" instruction in progress on a remote CPU.  e.g. I expect it probably will suck if you have to wait for a WBINVD instruction to complete on a remote CPU. That said, architectural IPIs have the same conditions too, except on top of that you've got to run a whole interrupt handler.  So, with reasonable confidence, however slow TLBSYNC might be in the worst case, it's got absolutely nothing on the overhead of doing invalidations the old fashioned way. ~Andrew [1] https://www.amd.com/content/dam/amd/en/documents/epyc-business-docs/white-papers/SEV-SNP-strengthening-vm-isolation-with-integrity-protection-and-more.pdf [2] https://www.intel.com/content/dam/develop/external/us/en/documents/341431-remote-action-request-white-paper.pdf