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Re: [Xen-devel] [PATCH v2 2/6] arm: move GIC SGI kicking into separate function



On 12/04/2013 01:28 PM, Ian Campbell wrote:
On Wed, 2013-12-04 at 13:15 +0100, Andre Przywara wrote:
On 12/02/2013 04:01 PM, Ian Campbell wrote:
On Mon, 2013-12-02 at 12:08 +0100, Andre Przywara wrote:
Currently we unconditionally send SGIs to all cores on SMP bringup.
PSCI will not need this, so we move this into a function and call it
explicitly from the platforms that need it. This gets us get rid of
the empty cpu_up() platform functions in ARM32 and the comment in
there.

I don't think this is quite true -- even on a PSCI system the kick is
required to get past the gate in head.S.

Right, but this is the responsibility of the PSCI handler in the
firmware, right?

Note that I am talking about a gate which is implemented in Xen's
head.S, not in the firmware. It is Xen's reponsibility to get the CPU
past that point, which is somewhat independent from the firmware wakeup,
except in reality it is intertwined because they use the same mechanism.

However, I think I was mistaken. In the case of PSCI we are able to wake
up specific targetted CPUs individually and we do so having already
opened the gate in out head.S, so the CPU must necessarily fall through
without waiting. I think this is worth mentioning in the commit log if
you don't mind.

  I was under the assumption that the semantics of cpu_on
is to start executing code at the given address, whatever this takes
internally.

Right, the issue here is a gate which we have subsequent to that
happening.

Calxeda firmware for instance does the SGI kick.


I wonder how this interacts with PSCI implementations which use an SGI
themselves internally...

@@ -376,11 +386,6 @@ int __cpu_up(unsigned int cpu)
           return rc;
       }

-    /* We don't know the GIC ID of the CPU until it has woken up, so just 
signal
-     * everyone and rely on our own smp_up_cpu gate to ensure only the one we
-     * want gets through. */
-    send_SGI_allbutself(GIC_SGI_EVENT_CHECK);
-

So, I was saying in the 00 mail I'm not sure we can get rid of this
altogether.

Please note that we do not get rid of this, but just move it. ARM64
calls it in arm64/smpboot.c,

How does this interact with the sev() In smp_spin_table_cpu_up I wonder.

Me, too ;-)
The original code does the sev() in arm/arm64/smpboot.c, then returns to the caller in arm/smpboot.c, which does the GIC kick. I was already wondering if that is correct, I guess you could answer this much better.

I think the GIC send should be only for the non-PSCI case here too.

It is. The arm64 code has the GIC kick moved into the spin_table function only. PSCI is a different beast, not using the GIC. Also arm32 does it only in platform specific functions, which don't get called when PSCI is available.

Regards,
Andre.


  ARM32 non-PSCI platforms call this now
explicitly by pointing to that function in their platforms/foo.c file.

OK.



But I suppose it is the intention that the platform code always has both
its own logic and this SGI kick (possibly coalesced) in such
circumstances? Which is probably ok?

That was my thinking, yes.

Regards,
Andre.


       while ( !cpu_online(cpu) )
       {
           cpu_relax();
diff --git a/xen/include/asm-arm/smp.h b/xen/include/asm-arm/smp.h
index 1485cc6..a1de03c 100644
--- a/xen/include/asm-arm/smp.h
+++ b/xen/include/asm-arm/smp.h
@@ -21,6 +21,8 @@ extern int arch_smp_init(void);
   extern int arch_cpu_init(int cpu, struct dt_device_node *dn);
   extern int arch_cpu_up(int cpu);

+int cpu_up_send_sgi(int cpu);
+
   /* Secondary CPU entry point */
   extern void init_secondary(void);








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