Re: [PATCH -tip -v10 5/7] x86: add pt_regs register and stack access APIs

[Masami Hiramatsu <mhiramat@redhat.com>]

Andi Kleen wrote:
> Masami Hiramatsu <mhiramat@redhat.com> writes:
> 
>> Add following APIs for accessing registers and stack entries from pt_regs.
> 
> You forgot to state who calls these functions/why are they added?
> Who only has strings for registers?

Oh, yes. This patch is needed for kprobes based event tracer on ftrace.
Some other debugging tools might be able to use it.

> I can see the point of having a function for nth argument though,
> that's useful.
> 
>> +static inline unsigned long regs_get_argument_nth(struct pt_regs *regs,
>> +						  unsigned n)
>> +{
>> +	if (n < NR_REGPARMS) {
>> +		switch (n) {
>> +		case 0:
>> +			return regs->ax;
>> +		case 1:
>> +			return regs->dx;
>> +		case 2:
>> +			return regs->cx;
> 
> 
> [....]
> 
> That could be done shorter with a offsetof table.
> 
>> +	if (n < NR_REGPARMS) {
>> +		switch (n) {
>> +		case 0:
>> +			return regs->di;
>> +		case 1:
>> +			return regs->si;
>> +		case 2:
>> +			return regs->dx;
>> +		case 3:
>> +			return regs->cx;
>> +		case 4:
>> +			return regs->r8;
>> +		case 5:
>> +			return regs->r9;
> 
> and that too.

I'm not so sure about your idea.
Would you mean below code?

int offs_table[NR_REGPARMS] = {
	[0] = offsetof(struct pt_regs, di),
	...
};
if (n < NR_REGPARMS)
	return *((unsigned long *)regs + offs_table[n]);


Thank you,

-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America), Inc.
Software Solutions Division

e-mail: mhiramat@redhat.com