Re: [Qemu-devel] [PATCH 40/64] PPC: Fix sync instructions problem in SMP

[Elie Richa]

Hello,

Actually the test case that I suggested is a bit imprecise because
creating a reservation on a CPU does not cause loss of reservation
on other CPUs (I mean in the specification). It is writing to the
memory location that causes loss of reservation. Therefore the correct
test case would be to perform a write on the second step rather than just
a reservation.

But aside from that the problem remains the same and the patch does
solve it.

On Oct 6, 2011, at 10:05 AM, Alexander Graf wrote:

> From: Elie Richa <address@hidden>
> 
> In the current emulation of the load-and-reserve (lwarx) and
> store-conditional (stwcx.) instructions, the internal reservation
> mechanism is taken into account, however each CPU has its own
> reservation information and this information is not synchronized between
> CPUs to perform proper synchronization.
> The following test case with 2 CPUs shows that the semantics of the
> "lwarx" and "stwcx." instructions are not preserved by the emulation.
> The test case does the following :
>       - CPU0: reserve a memory location
>       - CPU1: reserve the same memory location
>       - CPU0: perform stwcx. on the location
> The last store-conditional operation succeeds while it is supposed to
> fail since the reservation was supposed to be lost at the second reserve
> operation.
> 
> This (one line) patch fixes this problem in a very simple manner by
> removing the reservation of a CPU every time it is scheduled (in
> cpu_exec()). While this is a harsh workaround, it does not affect the
> guest code much because reservations are usually held for a very short
> time, that is an lwarx is almost always followed by an stwcx. a few
> instructions below. Therefore, in most cases, the reservation will be
> taken and consumed before a CPU switch occurs. However in the rare case
> where a CPU switch does occur between the lwarx and its corresponding
> stwcx.  this patch solves a potential erroneous behavior of the
> synchronization instructions.
> 
> Signed-off-by: Elie Richa <address@hidden>
> Signed-off-by: Alexander Graf <address@hidden>
> ---
> cpu-exec.c |    1 +
> 1 files changed, 1 insertions(+), 0 deletions(-)
> 
> diff --git a/cpu-exec.c b/cpu-exec.c
> index aef66f2..a9fa608 100644
> --- a/cpu-exec.c
> +++ b/cpu-exec.c
> @@ -217,6 +217,7 @@ int cpu_exec(CPUState *env)
> #elif defined(TARGET_ARM)
> #elif defined(TARGET_UNICORE32)
> #elif defined(TARGET_PPC)
> +    env->reserve_addr = -1;
> #elif defined(TARGET_LM32)
> #elif defined(TARGET_MICROBLAZE)
> #elif defined(TARGET_MIPS)
> -- 
> 1.6.0.2
>