On Wed, May 23, 2018 at 02:31:59PM +0100, Dave P Martin wrote: > On Wed, May 23, 2018 at 01:48:12PM +0200, Christoffer Dall wrote: > > On Tue, May 22, 2018 at 05:05:08PM +0100, Dave Martin wrote: > > > This is true by construction however: TIF_FOREIGN_FPSTATE is never > > > cleared except when returning to userspace or returning from a > > > signal: thus, for a true kernel thread no FPSIMD context is ever > > > loaded, TIF_FOREIGN_FPSTATE will remain set and no context will > > > ever be saved. > > > > I don't understand this construction proof; from looking at the patch > > below it is not obvious to me why fpsimd_thread_switch() can never have > > !wrong_task && !wrong_cpu and therefore clear TIF_FOREIGN_FPSTATE for a > > kernel thread? > > Looking at this again, I think it is poorly worded. This patch aims to > make it true by construction, but it isn't prior to the patch. > > I'm tempted to delete the paragraph: the assertion of both untrue and > not the best way to justify that this patch works. > > > How about: > > -8<- > > The context switch logic already isolates user threads from each other. > This, it is sufficient for isolating user threads from the kernel, > since the goal either way is to ensure that code executing in userspace > cannot see any FPSIMD state except its own. Thus, there is no special > property of kernel threads that we care about except that it is > pointless to save or load FPSIMD register state for them. > > At worst, the removal of all the kernel thread special cases by this > patch would thus spuriously load and save state for kernel threads when > unnecessary. > > But the context switch logic is already deliberately optimised to defer > reloads of the regs until ret_to_user (or sigreturn as a special case), > which kernel threads by definition never reach. > > ->8- The "at worst" paragraph makes it look like it could happen (at least until you reach the last paragraph). Maybe you can just say that wrong_task and wrong_cpu (with the fpsimd_cpu = NR_CPUS addition) are always true for kernel threads. You should probably mention this in a comment in the code as well. -- Catalin _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
