[DTrace-devel] [PATCH 2/2] Implement the pcap action

[Eugene Loh]

On 10/10/23 00:35, Kris Van Hees wrote:
> On Mon, Oct 09, 2023 at 11:54:32PM -0400, Eugene Loh via DTrace-devel wrote:
>> I am not familiar with pcap...

In legacy DTrace, in dtrace/dtrace_probe.c, dtrace_probe_pcap() deals 
with some complications around fragments.  Why are there no such 
complications in this patch?

And, another comment on the patch.  In dt_cg_act_pcap(), pcapsz is 
defined as dtp->dt_options[DTRACEOPT_PCAPSIZE].  Later, data_off is 
defined in terms of dtp->dt_options[DTRACEOPT_PCAPSIZE].  This is an 
opportunity to use pcapsz.

>> That's "a lot" of instructions simply to read a word from an unverified
>> address.  Not too many, but such a read is relatively common.  So, we should
>> have a simple way of generating these instructions.  How about
>> dt_cg_load_scalar()?  Bottom line is that we should look at the various
>> sites that read words in this manner and put the associated code generation
>> under a function that can be called from code like this.  Code reuse.
> Yes, we do need to look into better (more) utility functions that help with
> common code like this.  I am not sure whether we ought to do that right now,
> but I'll look at it because it is becoming more common (here and also in
> some provider trampolines).

Yeah.  One issue with trampolines is that the context for reg management 
is different.  Anyhow, yes, and my patch for the io provider, fwiw, has 
a utility function for this stuff.  There, it's especially useful, 
because there are a lot of such reads, including pointer chasing and 
stuff.  So there is a big incentive to simplify the cg.