Date: Tue, 18 Dec 2001 14:54:13 -0500 From: Jake Burkholder <jake@locore.ca> To: Andrew Gallatin <gallatin@cs.duke.edu> Cc: John Baldwin <jhb@FreeBSD.ORG>, alpha@FreeBSD.ORG Subject: Re: mdproc flags in KSE Message-ID: <20011218145413.L39205@locore.ca> In-Reply-To: <15391.37056.22312.225550@grasshopper.cs.duke.edu>; from gallatin@cs.duke.edu on Tue, Dec 18, 2001 at 01:53:52PM -0500 References: <XFMail.011218100048.jhb@FreeBSD.org> <15391.37056.22312.225550@grasshopper.cs.duke.edu>
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Apparently, On Tue, Dec 18, 2001 at 01:53:52PM -0500,
Andrew Gallatin said words to the effect of;
>
> John Baldwin writes:
> > The alpha had several fields in its mdproc struct prior to KSE, and KSE stuck
> > them all in mdthread, which isn't quite correct. Specifically, we have the
> > following for mdthread now:
> >
> > struct mdthread {
> > u_long md_flags;
> > struct pcb *md_pcbpaddr; /* phys addr of the pcb */
> > struct mdbpt md_sstep[2]; /* two single step breakpoints */
> > u_int64_t md_hae; /* user HAE register value */
> > void *osf_sigtramp; /* user-level signal trampoline */
> > u_int md_kernnest; /* nesting level in the kernel */
> > };
> >
> > #define MDP_FPUSED 0x0001 /* Process used the FPU */
> > #define MDP_STEP1 0x0002 /* Single step normal instruction */
> > #define MDP_STEP2 0x0004 /* Single step branch instruction */
> > #define MDP_HAEUSED 0x0008 /* Process used the HAE */
> > #define MDP_UAC_NOPRINT 0x0010 /* Don't print unaligned traps */
> > #define MDP_UAC_NOFIX 0x0020 /* Don't fixup unaligned traps */
> > #define MDP_UAC_SIGBUS 0x0040 /* Deliver SIGBUS upon
> > unaligned access */
> > #define MDP_UAC_MASK (MDP_UAC_NOPRINT | MDP_UAC_NOFIX | MDP_UAC_SIGBUS)
> >
> > Out of that structure, it seems obvious that md_pcbpaddr and md_kernnest need
> > to be per-thread. It also seems that the UAC flags need to be
> > per-process and need to move back into a md_flags in mdproc. However, I'm not
> > sure about the other ones. What is the HAE and in what contexts is it used?
>
> Its used by programs, like the X server, which grovel around in PCI
> memory space... Hmm.. In a quick browse through the code, I don't see
> where its actually used anymore..
>
> > Also, what are the single stepping fields used for exactly (details, not just
> > "to implement single stepping :)"). Is the osf_sigtramp just setup once for
>
> Single-stepping -- no clue.
My take on this is that alpha doesn't have a hardware single stepping
flag like i386 (correct me if I'm wrong). So the only way to implement
single stepping is to replace the next instruction to be executed with
a breakpoint each time. You need 2 because of branches, both paths
need to have a breakpoint poked in. This is inherently evil.
Sparc64 needs the same kind of support.
Where this goes depends a lot on how debugging works in multithreaded
processes. ie, do we allow single stepping 2 threads from the same
process simultaneously.
Jake
>
> > the entire process, or does it have to be regenerated on the fly all the time?
>
> Its used only by OSFF/ECOFF binaries. It is setup each time osf1_sigaction()
> is called & used at sigreturn time to point to that process's
> sigtramp code. Tru64 does'nt just keep it in a standard place like we
> do.
>
> Drew
>
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