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

[Greg Kurz <groug@kaod.org>]

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On Wed, 31 May 2017 19:40:46 +0200
Claudio Imbrenda <imbrenda@linux.vnet.ibm.com> wrote:

> 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.
> 

I tend to agree indeed. It isn't a problem for user mode though since it
doesn't use @host_tid. Only gdbstub does.

$ git grep host_tid
include/exec/gdbstub.h:    return cpu->host_tid;
include/qom/cpu.h: * @host_tid: Host thread ID.
include/qom/cpu.h:    uint32_t host_tid;
linux-user/syscall.c:    cpu->host_tid = info->tid;

> 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.
> 

I'm not familiar enough with gdbstub to know if this is a real problem.
But I guess it is possible to add a "first_cpu->host_tid = gettid();" line
somewhere in linux-user/main.c.

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