Re: [ABOMINATION] x86: Fast interrupt return to userspace

[Andy Lutomirski]

On Tue, May 6, 2014 at 2:25 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, May 6, 2014 at 2:00 PM, Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> 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
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