On Fri, May 1, 2015 at 12:11 PM, Rik van Riel <[email protected]> wrote: > On 05/01/2015 02:40 PM, Ingo Molnar wrote: > >> Or we could do that in the syscall path with a single store of a >> constant flag to a location in the task struct. We have a number of >> natural flags that get written on syscall entry, such as: >> >> pushq_cfi $__USER_DS /* pt_regs->ss */ >> >> That goes to a constant location on the kernel stack. On return from >> system calls we could write 0 to that location.
Huh? IRET with zero there will fault, and we can't guarantee that all syscalls return using sysret. Also, we'd have to audit all the entries, and system_call_after_swapgs currently enables interrupts early enough that an interrupt before all the pushes will do unpredictable things to pt_regs. We could further abuse orig_ax, but that would require a lot of auditing. Honestly, though, I think keeping a flag in an otherwise-hot cache line is a better bet. Also, making it per-cpu instead of per-task will probably be easier on the RCU code, since otherwise the RCU code will have to do some kind of synchronization (even if it's a wait-free probe of the rq lock or similar) to figure out which pt_regs to look at. If we went that route, I'd advocate sticking the flag in tss->sp1. That cacheline is unconditionally read on kernel entry already, and sp1 is available in tip:x86/asm (and maybe even in Linus' tree -- I lost track). [1] Alternatively, I'd suggest that we actually add a whole new word to pt_regs. [1] It's not unconditionally accessed yet, but it wil be once Denys' latest patches are in. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
