On Wed, Jan 12, 2022 at 03:15:27PM +0000, David Laight wrote: > From: Mathieu Desnoyers > > Sent: 12 January 2022 15:06 > > > > ----- On Jan 12, 2022, at 9:58 AM, David Laight David.Laight@xxxxxxxxxx wrote: > > > > >> * [*] The openrisc, powerpc64 and x86-64 architectures define a "redzone" as a > > >> * stack area beyond the stack pointer which can be used by the compiler > > >> * to store local variables in leaf functions. > > > > > > I wonder if that is really worth the trouble it causes! > > > By the time a function is spilling values to stack the cost > > > of a %sp update is almost certainly noise. > > > > > > Someone clearly thought it was a 'good idea (tm)'. > > > > I must admit that I've been surprised to learn about these redzones. Thanks for > > pointing them out to me, it was clearly a blind spot. I suspect it would be useful > > to introduce per-architecture KERNEL_REDZONE, USER_REDZONE and COMPAT_USER_REDZONE > > with a asm-generic version defining them to 0, with proper documentation. It would > > make it clearer to kernel developers working on stuff similar to signal handler > > delivery that they need to consider these carefully. > > They can never be used in kernel - any ISR would overwrite them. That depends on how the architecture does exceptions; also consider: https://www.intel.com/content/www/us/en/develop/download/flexible-return-and-event-delivery-specification.html
