Re: [Qemu-devel] [PATCH RFC 1/4] spapr-hcall: take iothread lock during handler call

[Greg Kurz]

On Fri, 02 Sep 2016 14:58:12 +0530
Nikunj A Dadhania <address@hidden> wrote:

> Greg Kurz <address@hidden> writes:
> 
> > On Fri,  2 Sep 2016 12:02:53 +0530
> > Nikunj A Dadhania <address@hidden> wrote:
> >  
> >> Signed-off-by: Nikunj A Dadhania <address@hidden>
> >> ---
> >>  hw/ppc/spapr_hcall.c | 11 +++++++++--
> >>  1 file changed, 9 insertions(+), 2 deletions(-)
> >> 
> >> diff --git a/hw/ppc/spapr_hcall.c b/hw/ppc/spapr_hcall.c
> >> index e5eca67..daea7a0 100644
> >> --- a/hw/ppc/spapr_hcall.c
> >> +++ b/hw/ppc/spapr_hcall.c
> >> @@ -1075,20 +1075,27 @@ target_ulong spapr_hypercall(PowerPCCPU *cpu, 
> >> target_ulong opcode,
> >>                               target_ulong *args)
> >>  {
> >>      sPAPRMachineState *spapr = SPAPR_MACHINE(qdev_get_machine());
> >> +    target_ulong ret;
> >>  
> >>      if ((opcode <= MAX_HCALL_OPCODE)
> >>          && ((opcode & 0x3) == 0)) {
> >>          spapr_hcall_fn fn = papr_hypercall_table[opcode / 4];
> >>  
> >>          if (fn) {
> >> -            return fn(cpu, spapr, opcode, args);
> >> +            qemu_mutex_lock_iothread();
> >> +            ret = fn(cpu, spapr, opcode, args);
> >> +            qemu_mutex_unlock_iothread();
> >> +            return ret;
> >>          }
> >>      } else if ((opcode >= KVMPPC_HCALL_BASE) &&
> >>                 (opcode <= KVMPPC_HCALL_MAX)) {
> >>          spapr_hcall_fn fn = kvmppc_hypercall_table[opcode - 
> >> KVMPPC_HCALL_BASE];
> >>  
> >>          if (fn) {
> >> -            return fn(cpu, spapr, opcode, args);
> >> +            qemu_mutex_lock_iothread();
> >> +            ret = fn(cpu, spapr, opcode, args);
> >> +            qemu_mutex_unlock_iothread();
> >> +            return ret;
> >>          }
> >>      }
> >>    
> >
> > This will serialize all hypercalls, even when it is not needed... Isn't that
> > too much coarse grain locking ?  
> 
> You are right, I was thinking to do this only for emulation case, as
> this is not needed for hardware acceleration.
> 

Yes, at the very least. And even in the MTTCG case, shouldn't we serialize only
when we know I/O will actually happen ?

> Regards
> Nikunj
> 
> 

Cheers.

--
Greg