svn commit: r337127 - head/sys/riscv/riscv

[John Baldwin]

On 8/2/18 8:50 AM, Konstantin Belousov wrote:
> On Thu, Aug 02, 2018 at 08:16:59AM -0700, John Baldwin wrote:
>> On 8/2/18 5:13 AM, Ruslan Bukin wrote:
>>> Author: br
>>> Date: Thu Aug  2 12:13:52 2018
>>> New Revision: 337127
>>> URL: https://svnweb.freebsd.org/changeset/base/337127
>>>
>>> Log:
>>>   Don't overwrite tp in set_mcontext().
>>>   
>>>   This makes libthr/swapcontext_test:swapcontext1 happy.
>>>   
>>>   Sponsored by:	DARPA, AFRL
>>
>> This seems a bit odd.  Is get_mcontext() not setting gp_tp correctly, or is
>> makecontext() not preserving it perhaps?
>>
>> In general I feel like we should be saving/restoring 'tp' in the trapframe
>> on kernel entry/exit like other GPRs instead of in the pcb and then
>> including it in 'struct reg'.  This makes it possible for debuggers to
>> handle TLS variables more sanely (and in general we need to fix the various
>> struct reg in FreeBSD architectures to include TLS registers when possible,
>> such as adding fs_base and gs_base on x86).  It would be nice if RISC-V
>> started off on the correct foot here.
> 
> I wanted to do this on x86, but I gave up due to the ABI issues.
> When FSGSBASE instructions are supported on amd64, we save base
> on context switch and restore on return to usermode.

For struct reg Linux has done the equivalent of adding fs_base/gs_base if
that is the ABI issues.  Cores are pretty easy as debuggers can use the
note size to determine if fs_base/gs_base are present and old debuggers
will just ignore the larger notes.  However, PT_GETREGS and PT_SETREGS is
more of a pain.  Probably we should define a "register set" API for
ptrace() which I've been kicking around in my head that includes a way
to communicate the size of the register block.

I think Linux uses 'data' for it's register set ptrace ops to specify
the register size.  I think I'd like to be able to use 'data' for the
userland size and instead borrow N upper bits in the ptrace op as the
register set identifier.  Either that or the 'addr' would need to start
pointing to an indirect structure when working with a register set.

-- 
John Baldwin