Re: [Qemu-devel] [PATCH v1 2/2] gdbstub: don't fail on vCont; C04:0; c packets

[Claudio Imbrenda <imbrenda@linux.vnet.ibm.com>]

On Wed, 31 May 2017 19:06:29 +0200
Greg Kurz <groug@kaod.org> wrote:

> On Wed, 31 May 2017 18:51:06 +0200
> Claudio Imbrenda <imbrenda@linux.vnet.ibm.com> wrote:
> [...]
> > > > 
> > > > This is strange. cpu_index() is defined as:
> > > > 
> > > > static inline int cpu_index(CPUState *cpu)
> > > > {
> > > > #if defined(CONFIG_USER_ONLY)
> > > >     return cpu->host_tid;
> > > > #else
> > > >     return cpu->cpu_index + 1;
> > > > #endif
> > > > }
> > > > 
> > > > therefore it shouldn't return 0 under any circumstance,
> > > > and      
> > > 
> > > I think it is 0 for first_cpu in user mode.    
> > 
> > in linux-user/syscall.c:
> > 
> > info->tid = gettid();
> > cpu->host_tid = info->tid;
> > 
> > kernel thread-ids are system-wide unique and can't be 0
> >    
> 
> This is correct but these lines are in clone_func(). This gets called
> for all threads but the "main" thread which I believe to be
> associated to first_cpu.

then IMHO that is a bug and it needs to be corrected. the host_tid
should be, well, the host tid, and not 0, which is never a valid
tid for Linux.

the current behaviour is simply the easiest for the "any CPU" case.
Picking the last CPU or a random one would still be correct, and in
that case there would be no way to explicitly address the first CPU.