Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable & enable from context tracking on syscall entry

[Andy Lutomirski]

On Fri, May 1, 2015 at 12:11 PM, Rik van Riel <riel@xxxxxxxxxx> 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
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