[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Xen-devel] [PATCH 3/7] xen/arm/psci: Implement CPU_OFF PSCI call (physical interface)

On 12/04/18 12:33, Mirela Simonovic wrote:
On Wed, Apr 11, 2018 at 4:46 PM, Julien Grall <julien.grall@xxxxxxx> wrote:
On 11/04/18 14:19, Mirela Simonovic wrote:

       cpu_is_dead = true;
       /* Make sure the write happens before we sleep forever */
+    /* PSCI cpu off call will return only in case of an error */
+    errno = call_psci_cpu_off();
+    printk(XENLOG_DEBUG "PSCI cpu off call failed for CPU#%d err=%d\n",
+           get_processor_id(), errno);
+    isb();

What are you trying to achieve with the isb() here?

I use to have a problem that the wfi below gets executed before the
call_psci_cpu_off(). Adding isb() fixed the issue. However, I tried
now to reproduce the problem and it doesn't show up. I still believe
isb() should be here, please let me know if you disagree (I obviously
can't prove the claim now).

The problem you describe can't be possible with the code you have because call_psci_cpu_off() is issuing a SMC. SMC will lead to change exception level and therefore have a context-synchronization barrier.

This is obviously based on the assumption you don't have an errata on your CPU exposing the behavior you describe. For that you would need to check errata notice for your CPU and/or try to reproduce.

However, what you would need is a dsb(sy); isb(); to drain the write buffer if you print a message.

Furthermore, now on platform without CPU off support (e.g non-PSCI platform and PSCI 0.1) you will log an error message that may worry people. In reality, PSCI cpu_off will unlikely fail, so you probably want to add a panic in call_psci_cpu_off instead.


Julien Grall

Xen-devel mailing list



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.