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Mon, 13 Jul 2026 14:03:19 +0000 Message-ID: <8c0c099d-47aa-4bf0-ab7a-c682b0dccdf5@arm.com> Date: Mon, 13 Jul 2026 16:03:18 +0200 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 1/3] KVM: guest_memfd: Use memslot id to keep track of associated memslots To: Alexandru Elisei , Sean Christopherson Cc: pbonzini@redhat.com, kvm@vger.kernel.org, maz@kernel.org, oupton@kernel.org, joey.gouly@arm.com, seiden@linux.ibm.com, suzuki.poulose@arm.com, yuzenghui@huawei.com, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, fuad.tabba@linux.dev, mark.rutland@arm.com References: <20260702142912.6395-1-alexandru.elisei@arm.com> <20260702142912.6395-2-alexandru.elisei@arm.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david.hildenbrand@arm.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS1EYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWQuaGlsZGVuYnJhbmRAYXJtLmNvbT7CwY0EEwEIADcWIQQb2cqtc1xM OkYN/MpN3hD3AP+DWgUCaYJpgwIbAwUJJlgIpAILCQQVCgkIAhYCAh4FAheAAAoJEE3eEPcA /4NaGMsP/iLFeIScMoXBTDNlEyrfQQ83cTPahTGF4H+RXK/EdTOezXgCUzADIAyQYuOVZu9A bRBGw4F/mLeCfbFUhdMQnZ7XtHNmKHT3eOfylFiznkzUHAvFs4CaRptXVL2eEn1l6DmV7Sd0 N4yKy4BJoJ01arJOAGKLMTSXPt94SIo1r70bDuiiDHMAcOQ/4pka9D1X6w9aptTYsOWGu0AK 2enfm/KYpBEJlepvKkg4UsdeRlDwRUXxFma8Gd4PhoTD8wcXkeaMk3Qupkg7ge8oaU0gvaOk 8WxY+sIDGXf2xvVqZk8klzX8uGydW0GOxw3h0TIz12GCgi3uXaN4Y3C2CQJlM+ThqBrhJGNE KPsme2RzknpcuKsJAj5DByZyT0FZdrLF3NsrgwEM72A/gIKSDkzhW9OOfg7mnIhF53LfrKVd /RYUYVq4p2mGkcSGo/aolitmLxflbXcPNypT+S66i4+sgyExUsv+QRNacXtuacdPyQNk3pnM n37ZANdADP9KLVtfIBvI6iYyTH+B2C8U9JcrMGgaqkBONOEz03mfbBnBCLA8oBZqudBLOF3s amcIE5dVBj3OP8o2vuU8FKgiP/6cWSNUY/+Mg95XLGUAl8xTN+n4sohkPrGzR+1t8oVLsab0 Y+absub5T4UIl09wk29ClBMYz82uyDzwDAqxf4XuzkSUzsFNBFXLn5EBEADn1959INH2cwYJ v0tsxf5MUCghCj/CA/lc/LMthqQ773gauB9mN+F1rE9cyyXb6jyOGn+GUjMbnq1o121Vm0+n eKHUCBtHyseBfDXHA6m4B3mUTWo13nid0e4AM71r0DS8+KYh6zvweLX/LL5kQS9GQeT+QNro XcC1NzWbitts6TZ+IrPOwT1hfB4WNC+X2n4AzDqp3+ILiVST2DT4VBc11Gz6jijpC/KI5Al8 ZDhRwG47LUiuQmt3yqrmN63V9wzaPhC+xbwIsNZlLUvuRnmBPkTJwwrFRZvwu5GPHNndBjVp AfaSTOfppyKBTccu2AXJXWAE1Xjh6GOC8mlFjZwLxWFqdPHR1n2aPVgoiTLk34LR/bXO+e0G pzFXT7enwyvFFFyAS0Nk1q/7EChPcbRbhJqEBpRNZemxmg55zC3GLvgLKd5A09MOM2BrMea+ l0FUR+PuTenh2YmnmLRTro6eZ/qYwWkCu8FFIw4pT0OUDMyLgi+GI1aMpVogTZJ70FgV0pUA lpmrzk/bLbRkF3TwgucpyPtcpmQtTkWSgDS50QG9DR/1As3LLLcNkwJBZzBG6PWbvcOyrwMQ UF1nl4SSPV0LLH63+BrrHasfJzxKXzqgrW28CTAE2x8qi7e/6M/+XXhrsMYG+uaViM7n2je3 qKe7ofum3s4vq7oFCPsOgwARAQABwsF8BBgBCAAmAhsMFiEEG9nKrXNcTDpGDfzKTd4Q9wD/ g1oFAmic2qsFCSZYCKEACgkQTd4Q9wD/g1oq0xAAsAnw/OmsERdtdwRfAMpC74/++2wh9RvV Q0x8xXvoGJwZrk0Jmck1ABIM//5sWDo7eDHk1uEcc95pbP9XGU6ZgeiQeh06+0vRYILwDk8Q /y06TrTb1n4n7FRwyskKU1UWnNW86lvWUJuGPABXjrkfL41RJttSJHF3M1C0u2BnM5VnDuPF QKzhRRktBMK4GkWBvXlsHFhn8Ev0xvPE/G99RAg9ufNAxyq2lSzbUIwrY918KHlziBKwNyLo Pn9kgHD3hRBaYakz87WKUZd17ZnPMZiXriCWZxwPx7zs6cSAqcfcVucmdPiIlyG1K/HIk2LX 63T6oO2Libzz7/0i4+oIpvpK2X6zZ2cu0k2uNcEYm2xAb+xGmqwnPnHX/ac8lJEyzH3lh+pt 2slI4VcPNnz+vzYeBAS1S+VJc1pcJr3l7PRSQ4bv5sObZvezRdqEFB4tUIfSbDdEBCCvvEMB goisDB8ceYxOcFAM8nBWrEmNU2vvIGJzjJ/NVYYIY0TgOc5bS9wh6jKHL2+chrfDW5neLJjY 2x3snF8q7U9GEIbBfNHDlOV8SyhEjtX0DyKxQKioTYPOHcW9gdV5fhSz5tEv+ipqt4kIgWqB gzK8ePtDTqRMqZq457g1/SXSoSQi4jN+gsneqvlTJdzaEu1bJP0iv6ViVf15+qHuY5iojCz8 fa0= In-Reply-To: Content-Type: text/plain; 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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: KhR9zSTncAG3WhrKD43LWTcmH7RXGK2rjLeTWVqjClfznBOkFKPbmFbXDYzsgOYpYHdgFYIAAt6nbRVr4jzBJ499Hi93XgQ8GjNCatvwNhRLxD7qJhLPDl63nbT40OJIG3oYShx7me24DjU0qm4pxDRYqQO1xs2Dre4eoyfZa7wsWdnz0hlcCgnuFWjGeXGZjN52XYgf68lmcq603FGAz9/8UQLXAqX0JrTjxVeN6kRHFo+x0Zr3/vMk++OcHMzLXXmWgmi8G7kgGG1VCVekmfpvykJMmTsrSYqAEKz3Wy9ljwDGZRtZ0/td79XnAutQJCzfy4My9xCj0/+iplDuUMGWdI+IzKfozVDg2nyIG7QTdjNbSddnsfRdG8JebY6WbTyaFPhyfbMLVKiCQvq2ERctnUpGsYf8BVWKUFAJwz9P0aLne3cTnmFiPlisTfMg X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 13 Jul 2026 14:04:21.8625 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 1d42fa0e-b981-4fb8-7047-08dee0e7a393 X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[4.158.2.129];Helo=[outbound-uk1.az.dlp.m.darktrace.com] X-MS-Exchange-CrossTenant-AuthSource: AM2PEPF0001C713.eurprd05.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: PR3PR08MB5593 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260713_150428_173206_8F9710B3 X-CRM114-Status: GOOD ( 10.70 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 7/7/26 19:05, Alexandru Elisei wrote: > Hi Sean, > > On Mon, Jul 06, 2026 at 02:43:23PM -0700, Sean Christopherson wrote: >> On Thu, Jul 02, 2026, Alexandru Elisei wrote: >>> To enable memslot operations, KVM maintains two arrays of memslots, and an >>> RCU pointer to the active (in use) array. Changes are made first to the >>> inactive array, and the RCU pointer is updated to point to the inactive >>> array, which becomes active. >>> >>> The guest_memfd file maintains an xarray of pointers to memslots that use >>> it as the memory provider. After the RCU pointer to the active memslots is >>> updated and until SRCU is synchronized, readers can observe the old or the >>> new value for the active array, and therefore the old or the new pointer >>> for a given memslot. For memslot creation or deletion that is not an issue >>> for guest_memfd, as readers will either read the same memslot pointer saved >>> by the guest_memfd file, or a non-existing memslot. >>> >>> But when changing the flags for a memslot, readers can read two different >>> and non-NULL memslot pointers. >> >> And? Why does that matter? KVM memslot updates aren't atomic. Practically >> speaking, they _can't_ be made atomic. Userspace is required to quiesce all >> activity that must not observe inconsistent state, i.e. userspace must pause >> (stop running) vCPUs when performing a memslot update. > > Is that true when KVM_MEM_LOG_DIRTY_PAGES is toggled for a memslot? Good point. > > As far as I can tell, KVM today tolerates VCPUs running while the > KVM_MEM_LOG_DIRTY_PAGES flags is being changed for a memslot. And by > tolarate I mean that VCPUs that are running when the flag is changed don't > return an error from KVM_RUN. If changing a memslot while VCPUs are running > were fatal, I would think that KVM would want to take vcpu->mutex for all > VCPUs to keep them from running. Or is it a case of KVM allowing userspace > to shoot themselves in the foot if they really want it? > > When the KVM_MEM_LOG_DIRTY_PAGES flags is being changed, VCPUs handling a > guest fault can observe either the old memslot, with the old flags, or the > new memslot, with the flag changed, but they still continue running without > returning an error. Staring at QEMU, kvm_log_start()+kvm_log_stop() do not call accel_ioctl_inhibit_begin() etc. So at least QEMU does not force VCPUs out of KVM when only updating flags. One option would be to require user space to do that also when starting+stopping dirty page logging. (I'd assume that should work, but it might be tricky depending on in which context it is called from QEMU migration code -- whether we hold the BQL) -- Cheers, David