Re: [Qemu-devel] [PATCH] kvm: First step to push iothread lock out of inner run loop

[Marcelo Tosatti <mtosatti@redhat.com>]

On Fri, Jun 22, 2012 at 09:22:59PM -0300, Marcelo Tosatti wrote:
> On Sat, Jun 23, 2012 at 12:55:49AM +0200, Jan Kiszka wrote:
> > Should have declared this [RFC] in the subject and CC'ed kvm...
> > 
> > On 2012-06-23 00:45, Jan Kiszka wrote:
> > > This sketches a possible path to get rid of the iothread lock on vmexits
> > > in KVM mode. On x86, the the in-kernel irqchips has to be used because
> > > we otherwise need to synchronize APIC and other per-cpu state accesses
> > > that could be changed concurrently. Not yet fully analyzed is the NMI
> > > injection path in the absence of an APIC.
> > > 
> > > s390x should be fine without specific locking as their pre/post-run
> > > callbacks are empty. Power requires locking for the pre-run callback.
> > > 
> > > This patch is untested, but a similar version was successfully used in
> > > a x86 setup with a network I/O path that needed no central iothread
> > > locking anymore (required special MMIO exit handling).
> > > ---
> > >  kvm-all.c         |   18 ++++++++++++++++--
> > >  target-i386/kvm.c |    7 +++++++
> > >  target-ppc/kvm.c  |    4 ++++
> > >  3 files changed, 27 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/kvm-all.c b/kvm-all.c
> > > index f8e4328..9c3e26f 100644
> > > --- a/kvm-all.c
> > > +++ b/kvm-all.c
> > > @@ -1460,6 +1460,8 @@ int kvm_cpu_exec(CPUArchState *env)
> > >          return EXCP_HLT;
> > >      }
> > >  
> > > +    qemu_mutex_unlock_iothread();
> > > +
> > >      do {
> > >          if (env->kvm_vcpu_dirty) {
> > >              kvm_arch_put_registers(env, KVM_PUT_RUNTIME_STATE);
> > > @@ -1476,14 +1478,16 @@ int kvm_cpu_exec(CPUArchState *env)
> > >               */
> > >              qemu_cpu_kick_self();
> > >          }
> > > -        qemu_mutex_unlock_iothread();
> > >  
> > >          run_ret = kvm_vcpu_ioctl(env, KVM_RUN, 0);
> > >  
> > > -        qemu_mutex_lock_iothread();
> > >          kvm_arch_post_run(env, run);
> 
> target-i386/kvm.c
> 
> void kvm_arch_post_run(CPUX86State *env, struct kvm_run *run)
> {       
>     if (run->if_flag) {
>         env->eflags |= IF_MASK;
>     } else {
>         env->eflags &= ~IF_MASK;
>     }
>     cpu_set_apic_tpr(env->apic_state, run->cr8);
>     cpu_set_apic_base(env->apic_state, run->apic_base);
> }
> 
> Clearly there is no structure to any of the writes around the writes
> in x86's kvm_arch_post_run, so it is unsafe.

No access protocol to the CPUState and apic devices (who can write when,
who can read when).