On Tue, May 6, 2014 at 2:25 PM, Linus Torvalds <[email protected]> wrote: > On Tue, May 6, 2014 at 2:00 PM, Linus Torvalds > <[email protected]> wrote: >> >> I'll do profiles and test the kernel compile too, but the raw timings >> are certainly promising. The "sysret" hack is pretty disgusting, and >> it's broken too. sysret doesn't do some things iret does (like TF flag >> etc), so it's not complete, but it's clearly good enough to run tests >> on. It will definitely break ptrace() and friends. > > It clearly breaks other things too, and there seems to be bugs in > there. I got this, for example: > > WARNING: CPU: 2 PID: 1174 at kernel/smp.c:230 > smp_call_function_single+0x81/0xa0() > CPU: 2 PID: 1174 Comm: gdbus Tainted: G W > 3.15.0-rc4-00260-g38583f095c5a-dirty #2 > Call Trace: > dump_stack+0x45/0x56 > warn_slowpath_common+0x73/0x90 > warn_slowpath_null+0x15/0x20 > smp_call_function_single+0x81/0xa0 > smp_call_function_many+0x21c/0x260 > flush_tlb_page+0x6d/0xb0 > ptep_clear_flush+0x2c/0x40 > do_wp_page+0x208/0x6e0 > handle_mm_fault+0x79c/0x1060 > __do_page_fault+0x15e/0x510 > do_page_fault+0xc/0x10 > page_fault+0x22/0x30 > retint_swapgs+0x6/0x10 > > which is because interrupts are disabled in that > install_sysret_trampoline() path that touches user space. It's > possibly sufficient to just move the "cli" to below the call to it. I > eventually ended up with a hung machine, possibly related to this, > possibly something else.
I don't think that's enough to fix this -- interrupts may not have been on in the first place, I think. I wonder if __put_user_inatomic would work here. Also, sysexit might be better than sysret. And I categorically refuse to add any new feature that requires vsyscall=native, so this would have to use the vdso instead. Plus it's awful. > > Whatever. I got enough profile data to say that it seems to have cut > 'iret' overhead by at least two thirds. So it may not *work*, but from > a "hey look, some random numbers" standpoint it is worth playing with. > :) Is there actual interest in turning something like this into a real patch? It would almost certainly have to default off and no one sane would ever use it except for special-purpose machines. --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/
