[Qemu-devel] [PATCH] target-ppc: Correct KVM synchronization for ppc_hash64_set_external_hpt()

[Qemu-devel] [PATCH] target-ppc: Correct KVM synchronization for ppc_hash64_set_external_hpt()

[David Gibson]

ppc_hash64_set_external_hpt() was added in e5c0d3c "target-ppc: Add helpers
for updating a CPU's SDR1 and external HPT".  This helper contains a
cpu_synchronize_state() since it may need to push state back to KVM
afterwards.

This turns out to break things when it is used in the reset path, which is
the only current user.  It appears that kvm_vcpu_dirty is not being set
early in the reset path, so the cpu_synchronize_state() is clobbering state
set up by the early part of the cpu reset path with stale state from KVM.

To fix this, remove the cpu_synchronize_state() from
ppc_hash64_set_external_hpt().  Any future non-reset-path users will need
to manually invoke cpu_synchronize_state().

Reported-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 target-ppc/mmu-hash64.c | 2 --
 1 file changed, 2 deletions(-)

Paolo, Peter,

This seems like the right minimal fix in the qemu-2.6 timeframe to fix
the actual bug.  However, longer term it seems like the correct thing
to do might be to set kvm_vcpu_dirty early in the reset path.  Thoughts?

diff --git a/target-ppc/mmu-hash64.c b/target-ppc/mmu-hash64.c
index 72c4ab5..caf41ce 100644
--- a/target-ppc/mmu-hash64.c
+++ b/target-ppc/mmu-hash64.c
@@ -283,8 +283,6 @@ void ppc_hash64_set_external_hpt(PowerPCCPU *cpu, void *hpt, int shift,
     CPUPPCState *env = &cpu->env;
     Error *local_err = NULL;
 
-    cpu_synchronize_state(CPU(cpu));
-
     if (hpt) {
         env->external_htab = hpt;
     } else {
-- 
2.5.5

Re: [Qemu-devel] [PATCH] target-ppc: Correct KVM synchronization for ppc_hash64_set_external_hpt()

[Paolo Bonzini]

On 01/04/2016 05:52, David Gibson wrote:
> This seems like the right minimal fix in the qemu-2.6 timeframe to fix
> the actual bug.  However, longer term it seems like the correct thing
> to do might be to set kvm_vcpu_dirty early in the reset path.  Thoughts?

Isn't it done already? vl.c does:

        pause_all_vcpus();
        cpu_synchronize_all_states();
        qemu_system_reset(VMRESET_REPORT);
        resume_all_vcpus();

Thanks,

Paolo

> diff --git a/target-ppc/mmu-hash64.c b/target-ppc/mmu-hash64.c
> index 72c4ab5..caf41ce 100644
> --- a/target-ppc/mmu-hash64.c
> +++ b/target-ppc/mmu-hash64.c
> @@ -283,8 +283,6 @@ void ppc_hash64_set_external_hpt(PowerPCCPU *cpu, void *hpt, int shift,
>      CPUPPCState *env = &cpu->env;
>      Error *local_err = NULL;
>  
> -    cpu_synchronize_state(CPU(cpu));
> -
>      if (hpt) {
>          env->external_htab = hpt;
>      } else {

Re: [Qemu-devel] [PATCH] target-ppc: Correct KVM synchronization for ppc_hash64_set_external_hpt()

[David Gibson]

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On Fri, Apr 01, 2016 at 12:28:31PM +0200, Paolo Bonzini wrote:
> 
> 
> On 01/04/2016 05:52, David Gibson wrote:
> > This seems like the right minimal fix in the qemu-2.6 timeframe to fix
> > the actual bug.  However, longer term it seems like the correct thing
> > to do might be to set kvm_vcpu_dirty early in the reset path.  Thoughts?
> 
> Isn't it done already? vl.c does:
> 
>         pause_all_vcpus();
>         cpu_synchronize_all_states();
>         qemu_system_reset(VMRESET_REPORT);
>         resume_all_vcpus();


Huh.. now I'm really confused.  Given this I would indeed have
expected kvm_vpcu_dirty to be set in the reset path, but that would
make the cpu_synchronize_state() causing the problem to act as a
no-op, so it shouldn't be causing the problem.

Investigating...

> 
> Thanks,
> 
> Paolo
> 
> > diff --git a/target-ppc/mmu-hash64.c b/target-ppc/mmu-hash64.c
> > index 72c4ab5..caf41ce 100644
> > --- a/target-ppc/mmu-hash64.c
> > +++ b/target-ppc/mmu-hash64.c
> > @@ -283,8 +283,6 @@ void ppc_hash64_set_external_hpt(PowerPCCPU *cpu, void *hpt, int shift,
> >      CPUPPCState *env = &cpu->env;
> >      Error *local_err = NULL;
> >  
> > -    cpu_synchronize_state(CPU(cpu));
> > -
> >      if (hpt) {
> >          env->external_htab = hpt;
> >      } else {
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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Re: [Qemu-devel] [PATCH] target-ppc: Correct KVM synchronization for ppc_hash64_set_external_hpt()

[David Gibson]

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On Mon, Apr 04, 2016 at 11:10:56AM +1000, David Gibson wrote:
> On Fri, Apr 01, 2016 at 12:28:31PM +0200, Paolo Bonzini wrote:
> > 
> > 
> > On 01/04/2016 05:52, David Gibson wrote:
> > > This seems like the right minimal fix in the qemu-2.6 timeframe to fix
> > > the actual bug.  However, longer term it seems like the correct thing
> > > to do might be to set kvm_vcpu_dirty early in the reset path.  Thoughts?
> > 
> > Isn't it done already? vl.c does:
> > 
> >         pause_all_vcpus();
> >         cpu_synchronize_all_states();
> >         qemu_system_reset(VMRESET_REPORT);
> >         resume_all_vcpus();
> 
> 
> Huh.. now I'm really confused.  Given this I would indeed have
> expected kvm_vpcu_dirty to be set in the reset path, but that would
> make the cpu_synchronize_state() causing the problem to act as a
> no-op, so it shouldn't be causing the problem.
> 
> Investigating...

Ok, I've found it.

So, if you reset the system after it's up and running, the fragment
above is indeed what will happen and I think everything will work
correct from there.

The problem occurs only on the very first reset in main() (vl.c:4624
as of de1d099): kvm_vcpu_dirty is false during this call.
Specifically, although kvm_vcpu_dirty was initialized to true in
kvm_vcpu_init(), it gets set to false in cpu_synchronize_post_init()
and never set true again before the qemu_system_reset().

Seems to me we either need a cpu_synchronize_all_states() before that
qemu_system_reset(), or we need to fold the
cpu_synchronize_all_states() right into qemu_system_reset() itself.

Opinion on which option is preferred?

> 
> > 
> > Thanks,
> > 
> > Paolo
> > 
> > > diff --git a/target-ppc/mmu-hash64.c b/target-ppc/mmu-hash64.c
> > > index 72c4ab5..caf41ce 100644
> > > --- a/target-ppc/mmu-hash64.c
> > > +++ b/target-ppc/mmu-hash64.c
> > > @@ -283,8 +283,6 @@ void ppc_hash64_set_external_hpt(PowerPCCPU *cpu, void *hpt, int shift,
> > >      CPUPPCState *env = &cpu->env;
> > >      Error *local_err = NULL;
> > >  
> > > -    cpu_synchronize_state(CPU(cpu));
> > > -
> > >      if (hpt) {
> > >          env->external_htab = hpt;
> > >      } else {
> > 
> 



-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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