Mark Rutland <mark.rutl...@arm.com> writes:
> On Fri, May 23, 2014 at 03:51:07PM +0100, Catalin Marinas wrote:
>> On Fri, May 23, 2014 at 01:11:38AM +0100, Kevin Hilman wrote:
>> > Christopher Covington <c...@codeaurora.org> writes:
>> > > On 05/22/2014 03:27 PM, Larry Bassel wrote:
>> > >> Make calls to ct_user_enter when the kernel is exited
>> > >> and ct_user_exit when the kernel is entered (in el0_da,
>> > >> el0_ia, el0_svc, el0_irq and all of the "error" paths).
>> > >>
>> > >> These macros expand to function calls which will only work
>> > >> properly if el0_sync and related code has been rearranged
>> > >> (in a previous patch of this series).
>> > >>
>> > >> The calls to ct_user_exit are made after hw debugging has been
>> > >> enabled (enable_dbg_and_irq).
>> > >>
>> > >> The call to ct_user_enter is made at the beginning of the
>> > >> kernel_exit macro.
>> > >>
>> > >> This patch is based on earlier work by Kevin Hilman.
>> > >> Save/restore optimizations were also done by Kevin.
>> > >
>> > >> --- a/arch/arm64/kernel/entry.S
>> > >> +++ b/arch/arm64/kernel/entry.S
>> > >> @@ -30,6 +30,44 @@
>> > >> #include <asm/unistd32.h>
>> > >>
>> > >> /*
>> > >> + * Context tracking subsystem. Used to instrument transitions
>> > >> + * between user and kernel mode.
>> > >> + */
>> > >> + .macro ct_user_exit, restore = 0
>> > >> +#ifdef CONFIG_CONTEXT_TRACKING
>> > >> + bl context_tracking_user_exit
>> > >> + .if \restore == 1
>> > >> + /*
>> > >> + * Save/restore needed during syscalls. Restore syscall
>> > >> arguments from
>> > >> + * the values already saved on stack during kernel_entry.
>> > >> + */
>> > >> + ldp x0, x1, [sp]
>> > >> + ldp x2, x3, [sp, #S_X2]
>> > >> + ldp x4, x5, [sp, #S_X4]
>> > >> + ldp x6, x7, [sp, #S_X6]
>> > >> + .endif
>> > >> +#endif
>> > >> + .endm
>> > >> +
>> > >> + .macro ct_user_enter, save = 0
>> > >> +#ifdef CONFIG_CONTEXT_TRACKING
>> > >> + .if \save == 1
>> > >> + /*
>> > >> + * Save/restore only needed on syscall fastpath, which uses
>> > >> + * x0-x2.
>> > >> + */
>> > >> + push x2, x3
>> > >
>> > > Why is x3 saved?
>> >
>> > I'll respond here since I worked with Larry on the context save/restore
>> > part.
>> >
>> > [insert rather embarassing disclamer of ignorance of arm64 assembly]
>> >
>> > Based on my reading of the code, I figured only x0-x2 needed to be
>> > saved. However, based on some experiments with intentionally clobbering
>> > the registers[1] (as suggested by Mark Rutland) in order to make sure
>> > we're saving/restoring the right things, I discovered x3 was needed too
>> > (I missed updating the comment to mention x0-x3.)
>> >
>> > Maybe Will/Catalin/Mark R. can shed some light here?
>>
>> I haven't checked all the code paths but at least for pushing onto the
>> stack we must keep it 16-bytes aligned (architecture requirement).
>
> Sure -- if modifying the stack we need to push/pop pairs of registers to
> keep it aligned. It might be better to use xzr as the dummy value in
> that case to make it clear that the value doesn't really matter.
>
> That said, ct_user_enter is only called in kernel_exit before we restore
> the values off the stack, and the only register I can spot that we need
> to preserve is x0 for the syscall return value. I can't see x1 or x2
> being used any more specially than the rest of the remaining registers.
> Am I missing something,
I don't think you're missing something. I had thought my experiment in
clobbering registers uncovered that x1-x3 were also in use somewhere,
but in trying to reproduce that now, it's clear only x0 is important.
> or would it be sufficient to do the following?
> push x0, xzr
> bl context_tacking_user_enter
> pop x0, xzr
Yes, this seems to work.
Following Will's suggestion of using a callee-saved register to save x0,
the updated version now looks like this:
.macro ct_user_enter, save = 0
#ifdef CONFIG_CONTEXT_TRACKING
.if \save == 1
/*
* We only have to save/restore x0 on the fast syscall path where
* x0 contains the syscall return.
*/
mov x19, x0
.endif
bl context_tracking_user_enter
.if \save == 1
mov x0, x19
.endif
#endif
.endm
We'll update this as well as address the comments on PATCH 1/2 and send
a v5.
Thanks guys for the review and guidance as I'm wandering a bit in the
dark here in arm64 assembler land.
Cheers,
Kevin
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