Vince Weaver wrote:
> On Sat, 24 Oct 2009, nathan binkert wrote:
>
>> I don't know the code super well, but I'd think it possible to process
>> arguments in order without too much trouble. Given the C calling
>> convention, it would be extraordinarily odd to have a later argument
>> define the semantics of an earlier one.
>>
>
> The problem is like this:
>
> int syscall_blah(int a, long long b);
>
> On x86 arg0=%ebx arg1-low=%ecx arg1-high=%edx
> So on x86 regs are consecutive like you'd expect
>
> On arm, arg0=%r0 arg1-low=%r2 arg1-high=%r3
> *note r1 was skipped because 64-bit value has to be passed in
> even/odd register pair!*
>
> But, if a function is as so:
>
> int syscall_blah2(int a, int b, long long c)
>
> Then there is no gap, because argument c naturally fits on
> an even/odd pair.
>
> So it depends on whether you are a 32-bit architecture, whether that
> architecure places restrictions on how a 64-bit value is passed in
> registers, and then if there are an odd or even number of 32-bit
> values passed before it.
>
> It can get more complicated if there are multiple 64-bit values (the ARM
> kernel maintainers try to stop things like this from happening but
> somtimes syscalls slip through w/o them noticing).
>
> It might be possible to come up with some elaborate way of handling this
> automatically, but I don't think it's worth the trouble if a few simple
> ifdefs will handle it.
>
> >From a look at qemu, syscalls that have this problem, at least for ARM
> are only:
> truncate64
> ftruncate64
> pread
> pwrite
> readahead
>
> I think m5 only supports the first two of those, and that's only within
> the past few days, which is why this problem hasn't shown up before. And
> it's probably a small enough problem that a few ifdefs (while ugly) can
> deal with the problem.
>
> Vince
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>
Accessing the arguments like they're on a stack solves exactly that
problem. #ifdefs should be avoided unless there's simply no alternative,
and even then revisited later to see if they can be removed.
x86:
getFirstSyscallArg returns %ebx and sets the "stack pointer" to 1.
getNextSyscallArg(64) returns %ecx combined with %edx and sets the
"stack pointer" to 3.
arm:
getFirstSyscallArg returns %r0 and sets the "stack pointer" to 1.
getNextSyscallArg(64) aligns the "stack pointer" to 2, combines %r2 and
%r3, and sets it to 4.
Gabe
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