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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: 56ZAIDhcqIdY8DMUNXvqUK0JVkr0laVbu++1ChEmljseixC7vz3U3rhXS/gD4CKE/xHGHCRwnsE8VXFwH6RzLxSwtjHp6g6rfuaVpgy8e5YbdK0agTNZRxfi9QlkKt0zlpWEKMEcJQEXnVvLu296UAyjFRzEJtk/RrYZfb/yNyry52KySyi5FtZHdDt5bezBAnbXgyN8Vhzk9JcAqDaZJqoLuFjr3FAlJrJWh+OqW/WFWPfPLp/E2qGxKLTTSJ5/d5gUu3+s7quuWC0cmfJw8ejJBtX8n5JeybtT5YIbOqbao/vIGlgVksjydd0t3+AuyqaTsCOingDDWKSKUmUvlY1akmIrsfG0sqfZAnU+yN0kBzVDFhY7A/f5AvfedOagE9bmCS/bstefJWtIm4WPyMKPEVIaYzSa17XZhHN17XaCG90luqAqg7NP0M9QBaWM X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 13 Jul 2026 13:43:44.7847 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: ee846e4c-b533-4f91-0bf4-08dee0e4c238 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: AMS1EPF0000004D.eurprd04.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: DB5PR08MB10114 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260713_064357_646791_68120BC7 X-CRM114-Status: GOOD ( 11.86 ) 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 7/6/26 23:43, 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. I agree with that. It took us quite an effort to sort that out in QEMU. I was especially when nasty when, some guest activity resulted in memslot splits or merges, while some VCPUs were still active. -- Cheers, David