[kvm-unit-tests PATCH v9 19/31] powerpc: Avoid using larx/stcx. in spinlocks when only one CPU is running

[Thomas Huth]

On 04/05/2024 14.28, Nicholas Piggin wrote:
> The test harness uses spinlocks if they are implemented with larx/stcx.
> it can prevent some test scenarios such as testing migration of a
> reservation.

I'm having a hard time to understand that patch description. Maybe you could 
rephrase it / elaborate what's the exact problem here?

  Thanks,
   Thomas


> Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
> ---
>   lib/powerpc/asm/smp.h    |  1 +
>   lib/powerpc/smp.c        |  5 +++++
>   lib/powerpc/spinlock.c   | 29 +++++++++++++++++++++++++++++
>   lib/ppc64/asm/spinlock.h |  7 ++++++-
>   powerpc/Makefile.common  |  1 +
>   5 files changed, 42 insertions(+), 1 deletion(-)
>   create mode 100644 lib/powerpc/spinlock.c
> 
> diff --git a/lib/powerpc/asm/smp.h b/lib/powerpc/asm/smp.h
> index c45988bfa..66188b9dd 100644
> --- a/lib/powerpc/asm/smp.h
> +++ b/lib/powerpc/asm/smp.h
> @@ -15,6 +15,7 @@ struct cpu {
>   
>   extern int nr_cpus_present;
>   extern int nr_cpus_online;
> +extern bool multithreaded;
>   extern struct cpu cpus[];
>   
>   register struct cpu *__current_cpu asm("r13");
> diff --git a/lib/powerpc/smp.c b/lib/powerpc/smp.c
> index 27b169841..73c0ef214 100644
> --- a/lib/powerpc/smp.c
> +++ b/lib/powerpc/smp.c
> @@ -276,6 +276,8 @@ static void start_each_secondary(int fdtnode, u64 regval __unused, void *info)
>   	start_core(fdtnode, datap->entry);
>   }
>   
> +bool multithreaded = false;
> +
>   /*
>    * Start all stopped cpus on the guest at entry with register 3 set to r3
>    * We expect that we come in with only one thread currently started
> @@ -290,6 +292,7 @@ bool start_all_cpus(secondary_entry_fn entry)
>   
>   	assert(nr_cpus_online == 1);
>   	assert(nr_started == 1);
> +	multithreaded = true;
>   	ret = dt_for_each_cpu_node(start_each_secondary, &data);
>   	assert(ret == 0);
>   	assert(nr_started == nr_cpus_present);
> @@ -361,10 +364,12 @@ static void wait_each_secondary(int fdtnode, u64 regval __unused, void *info)
>   
>   void stop_all_cpus(void)
>   {
> +	assert(multithreaded);
>   	while (nr_cpus_online > 1)
>   		cpu_relax();
>   
>   	dt_for_each_cpu_node(wait_each_secondary, NULL);
>   	mb();
>   	nr_started = 1;
> +	multithreaded = false;
>   }
> diff --git a/lib/powerpc/spinlock.c b/lib/powerpc/spinlock.c
> new file mode 100644
> index 000000000..623a1f2c1
> --- /dev/null
> +++ b/lib/powerpc/spinlock.c
> @@ -0,0 +1,29 @@
> +/* SPDX-License-Identifier: LGPL-2.0 */
> +#include <asm/spinlock.h>
> +#include <asm/smp.h>
> +
> +/*
> + * Skip the atomic when single-threaded, which helps avoid larx/stcx. in
> + * the harness when testing tricky larx/stcx. sequences (e.g., migration
> + * vs reservation).
> + */
> +void spin_lock(struct spinlock *lock)
> +{
> +	if (!multithreaded) {
> +		assert(lock->v == 0);
> +		lock->v = 1;
> +	} else {
> +		while (__sync_lock_test_and_set(&lock->v, 1))
> +			;
> +	}
> +}
> +
> +void spin_unlock(struct spinlock *lock)
> +{
> +	assert(lock->v == 1);
> +	if (!multithreaded) {
> +		lock->v = 0;
> +	} else {
> +		__sync_lock_release(&lock->v);
> +	}
> +}
> diff --git a/lib/ppc64/asm/spinlock.h b/lib/ppc64/asm/spinlock.h
> index f59eed191..b952386da 100644
> --- a/lib/ppc64/asm/spinlock.h
> +++ b/lib/ppc64/asm/spinlock.h
> @@ -1,6 +1,11 @@
>   #ifndef _ASMPPC64_SPINLOCK_H_
>   #define _ASMPPC64_SPINLOCK_H_
>   
> -#include <asm-generic/spinlock.h>
> +struct spinlock {
> +	unsigned int v;
> +};
> +
> +void spin_lock(struct spinlock *lock);
> +void spin_unlock(struct spinlock *lock);
>   
>   #endif /* _ASMPPC64_SPINLOCK_H_ */
> diff --git a/powerpc/Makefile.common b/powerpc/Makefile.common
> index b98f71c2f..1ee9c25d6 100644
> --- a/powerpc/Makefile.common
> +++ b/powerpc/Makefile.common
> @@ -49,6 +49,7 @@ cflatobjs += lib/powerpc/rtas.o
>   cflatobjs += lib/powerpc/processor.o
>   cflatobjs += lib/powerpc/handlers.o
>   cflatobjs += lib/powerpc/smp.o
> +cflatobjs += lib/powerpc/spinlock.o
>   
>   OBJDIRS += lib/powerpc
>