* David Gibson (da...@gibson.dropbear.id.au) wrote:
> On Mon, May 29, 2017 at 10:46:05PM +0200, Greg Kurz wrote:
> > On Fri, 26 May 2017 15:23:16 +1000
> > David Gibson <da...@gibson.dropbear.id.au> wrote:
> >
> > > As a rule, CPU internal state should never be updated when
> > > !cpu->kvm_vcpu_dirty (or the HAX equivalent). If that is done, then
> > > subsequent calls to cpu_synchronize_state() - usually safe and idempotent
> > > -
> > > will clobber state.
> > >
> > > However, we routinely do this during a loadvm or incoming migration.
> > > Usually this is called shortly after a reset, which will clear all the cpu
> > > dirty flags with cpu_synchronize_all_post_reset(). Nothing is expected
> > > to set the dirty flags again before the cpu state is loaded from the
> > > incoming stream.
> > >
> > > This means that it isn't safe to call cpu_synchronize_state() from a
> > > post_load handler, which is non-obvious and potentially inconvenient.
> > >
> > > We could cpu_synchronize_all_state() before the loadvm, but that would be
> > > overkill since a) we expect the state to already be synchronized from the
> > > reset and b) we expect to completely rewrite the state with a call to
> > > cpu_synchronize_all_post_init() at the end of qemu_loadvm_state().
> > >
> > > To clear this up, this patch introduces cpu_synchronize_pre_loadvm() and
> > > associated helpers, which simply marks the cpu state as dirty without
> > > actually changing anything. i.e. it says we want to discard any existing
> > > KVM (or HAX) state and replace it with what we're going to load.
> > >
> >
> > This makes sense and looks nicer than adding a post-load specific path to
> > ppc_set_compat() indeed.
> >
> > Just one remark below.
> >
> > > Cc: Juan Quintela <quint...@redhat.com>
> > > Cc: Dave Gilbert <dgilb...@redhat.com>
> > > Signed-off-by: David Gibson <da...@gibson.dropbear.id.au>
> > > ---
> > > cpus.c | 9 +++++++++
> > > include/sysemu/cpus.h | 1 +
> > > include/sysemu/hax.h | 1 +
> > > include/sysemu/hw_accel.h | 10 ++++++++++
> > > include/sysemu/kvm.h | 1 +
> > > kvm-all.c | 10 ++++++++++
> > > migration/savevm.c | 2 ++
> > > target/i386/hax-all.c | 10 ++++++++++
> > > 8 files changed, 44 insertions(+)
> > >
> > > diff --git a/cpus.c b/cpus.c
> > > index 516e5cb..6398439 100644
> > > --- a/cpus.c
> > > +++ b/cpus.c
> > > @@ -921,6 +921,15 @@ void cpu_synchronize_all_post_init(void)
> > > }
> > > }
> > >
> > > +void cpu_synchronize_all_pre_loadvm(void)
> > > +{
> > > + CPUState *cpu;
> > > +
> > > + CPU_FOREACH(cpu) {
> > > + cpu_synchronize_pre_loadvm(cpu);
> > > + }
> > > +}
> > > +
> > > static int do_vm_stop(RunState state)
> > > {
> > > int ret = 0;
> > > diff --git a/include/sysemu/cpus.h b/include/sysemu/cpus.h
> > > index a8053f1..731756d 100644
> > > --- a/include/sysemu/cpus.h
> > > +++ b/include/sysemu/cpus.h
> > > @@ -27,6 +27,7 @@ void qemu_timer_notify_cb(void *opaque, QEMUClockType
> > > type);
> > > void cpu_synchronize_all_states(void);
> > > void cpu_synchronize_all_post_reset(void);
> > > void cpu_synchronize_all_post_init(void);
> > > +void cpu_synchronize_all_pre_loadvm(void);
> > >
> > > void qtest_clock_warp(int64_t dest);
> > >
> > > diff --git a/include/sysemu/hax.h b/include/sysemu/hax.h
> > > index d9f0239..232a68a 100644
> > > --- a/include/sysemu/hax.h
> > > +++ b/include/sysemu/hax.h
> > > @@ -33,6 +33,7 @@ int hax_populate_ram(uint64_t va, uint32_t size);
> > > void hax_cpu_synchronize_state(CPUState *cpu);
> > > void hax_cpu_synchronize_post_reset(CPUState *cpu);
> > > void hax_cpu_synchronize_post_init(CPUState *cpu);
> > > +void hax_cpu_synchronize_pre_loadvm(CPUState *cpu);
> > >
> > > #ifdef CONFIG_HAX
> > >
> > > diff --git a/include/sysemu/hw_accel.h b/include/sysemu/hw_accel.h
> > > index c9b3105..469ffda 100644
> > > --- a/include/sysemu/hw_accel.h
> > > +++ b/include/sysemu/hw_accel.h
> > > @@ -45,4 +45,14 @@ static inline void cpu_synchronize_post_init(CPUState
> > > *cpu)
> > > }
> > > }
> > >
> > > +static inline void cpu_synchronize_pre_loadvm(CPUState *cpu)
> > > +{
> > > + if (kvm_enabled()) {
> > > + kvm_cpu_synchronize_pre_loadvm(cpu);
> > > + }
> > > + if (hax_enabled()) {
> > > + hax_cpu_synchronize_pre_loadvm(cpu);
> > > + }
> > > +}
> > > +
> > > #endif /* QEMU_HW_ACCEL_H */
> > > diff --git a/include/sysemu/kvm.h b/include/sysemu/kvm.h
> > > index 5cc83f2..a45c145 100644
> > > --- a/include/sysemu/kvm.h
> > > +++ b/include/sysemu/kvm.h
> > > @@ -459,6 +459,7 @@ int kvm_physical_memory_addr_from_host(KVMState *s,
> > > void *ram_addr,
> > > void kvm_cpu_synchronize_state(CPUState *cpu);
> > > void kvm_cpu_synchronize_post_reset(CPUState *cpu);
> > > void kvm_cpu_synchronize_post_init(CPUState *cpu);
> > > +void kvm_cpu_synchronize_pre_loadvm(CPUState *cpu);
> > >
> > > void kvm_init_cpu_signals(CPUState *cpu);
> > >
> > > diff --git a/kvm-all.c b/kvm-all.c
> > > index 90b8573..a8485bd 100644
> > > --- a/kvm-all.c
> > > +++ b/kvm-all.c
> > > @@ -1896,6 +1896,16 @@ void kvm_cpu_synchronize_post_init(CPUState *cpu)
> > > run_on_cpu(cpu, do_kvm_cpu_synchronize_post_init, RUN_ON_CPU_NULL);
> > > }
> > >
> > > +static void do_kvm_cpu_synchronize_pre_loadvm(CPUState *cpu,
> > > run_on_cpu_data arg)
> > > +{
> > > + cpu->kvm_vcpu_dirty = true;
> > > +}
> > > +
> > > +void kvm_cpu_synchronize_pre_loadvm(CPUState *cpu)
> > > +{
> > > + run_on_cpu(cpu, do_kvm_cpu_synchronize_pre_loadvm, RUN_ON_CPU_NULL);
> >
> > Do we really need to run_on_cpu() since we only set the dirty flag ?
>
> Um.. yeah.. I'm not sure. I left it in out of paranoia, because I
> wasn't sure if there could be memory ordering issues between the qemu
> threads if we just set it from the migration thread.
>
> I'm hoping Dave or Juan will have a better idea.
I don't know the kvm cpu sync stuff well enough.
Dave
> --
> 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
--
Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
