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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 16 Sep 2025 14:16:12.7806 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: e31d5fa6-b0ab-4eb8-4d3e-08ddf52b9765 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: DB5PEPF00014B98.eurprd02.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: PAXPR08MB7365 On Tue, Sep 16, 2025 at 03:07:09PM +0100, Mark Rutland wrote: > On Tue, Sep 16, 2025 at 02:58:16PM +0100, Yeoreum Yun wrote: > > Hi Mark, > > > > [...] > > > > I think it's enough to use usafe_get_user() instead of get_user() in here > > > > since when FEAT_LSUI enabled, it doeesn't need to call > > > > uaccess_ttbr0_enable()/disable(). > > > > > > Regardless of uaccess_ttbr0_enable() and uaccess_ttbr0_disable() > > > specifically, API-wise unsafe_get_user() is only supposed to be called > > > between user_access_begin() and user_access_end(), and there's some > > > stuff we probably want to add there (e.g. might_fault(), which > > > unsafe_get_user() lacks today). > > > > > > Do we call those? > > > > Yes when you're available. > > As you mention, the difference seems might_fault(), > > But I'm not sure whether that would be a reason to validate to use > > get_user() instead of unsafe_get_user() taking a increase of instruction > > of "nop" -- uaccess_ttbr0_enable()/disable() in LSUI > > except the reason for DEUBG purpose. > > I think the practical impact of those NOPs is going to be neglible, and > not worth optimizing for unless/until we have data demonstrating > otherwise. > > If we want to strictly avoid those NOPs, I think that we should do a > more general cleanup, and e.g. have variants of user_access_begin() and > user_access_end() that do not mess with TTBR0. I don't think we need to > do that for this series. > > For now, I think that you should either: > > * Use get_user(). > > * Use user_access_begin() .. user_access_end() wrapping both > unsafe_get_user() and the user cmpxchg. Understood. I'll use get_user() in this series after getting some more comments from other about C version which I sent before your suggestion (Sorry to miss your email before I sent). Thanks :D -- Sincerely, Yeoreum Yun