Re: [PATCH v2 1/8] powerpc/xive: Use cpu_to_node() instead of ibm,chip-id property

On 3/8/21 6:13 PM, Greg Kurz wrote:

On Wed, 3 Mar 2021 18:48:50 +0100
Cédric Le Goater [off-list ref] wrote:

quoted

The 'chip_id' field of the XIVE CPU structure is used to choose a
target for a source located on the same chip when possible. This field
is assigned on the PowerNV platform using the "ibm,chip-id" property
on pSeries under KVM when NUMA nodes are defined but it is undefined

This sentence seems to have a syntax problem... like it is missing an
'and' before 'on pSeries'.

ah yes, or simply a comma.

quoted

under PowerVM. The XIVE source structure has a similar field
'src_chip' which is only assigned on the PowerNV platform.

cpu_to_node() returns a compatible value on all platforms, 0 being the
default node. It will also give us the opportunity to set the affinity
of a source on pSeries when we can localize them.

IIUC this relies on the fact that the NUMA node id is == to chip id
on PowerNV, i.e. xc->chip_id which is passed to OPAL remain stable
with this change.

Linux sets the NUMA node in numa_setup_cpu(). On pseries, the hcall 
H_HOME_NODE_ASSOCIATIVITY returns the node id if I am correct (Daniel
in Cc:)

On PowerNV, Linux uses "ibm,associativity" property of the CPU to find
the node id. This value is built from the chip id in OPAL, so the 
value returned by cpu_to_node(cpu) and the value of the "ibm,chip-id"
property are unlikely to be different.

cpu_to_node(cpu) is used in many places to allocate the structures 
locally to the owning node. XIVE is not an exception (see below in the 
same patch), it is better to be consistent and get the same information 
(node id) using the same routine.


In Linux, "ibm,chip-id" is only used in low level PowerNV drivers : 
LPC, XSCOM, RNG, VAS, NX. XIVE should be in that list also but skiboot
unifies the controllers of the system to only expose one the OS. This
is problematic and should be changed but it's another topic.

On the other hand, you have the pSeries case under PowerVM that
doesn't xc->chip_id, which isn't passed to any hcall AFAICT.

yes "ibm,chip-id" is an OPAL concept unfortunately and it has no meaning 
under PAPR. xc->chip_id on pseries (PowerVM) will contains an invalid 
chip id. 

QEMU/KVM exposes "ibm,chip-id" but it's not used. (its value is not
always correct btw)

It looks like the chip id is only used for localization purpose in
this case, right ?

Yes and PAPR sources are not localized. So it's not used. MSI sources 
could be if we rewrote the MSI driver.

In this case, what about doing this change for pSeries only,
somewhere in spapr.c ?

The IPI code is common to all platforms and all have the same issue. 
I rather not.

Thanks,

C.
 

quoted

Signed-off-by: Cédric Le Goater <clg@kaod.org>
---
 arch/powerpc/sysdev/xive/common.c | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/arch/powerpc/sysdev/xive/common.c b/arch/powerpc/sysdev/xive/common.c
index 595310e056f4..b8e456da28aa 100644
--- a/arch/powerpc/sysdev/xive/common.c
+++ b/arch/powerpc/sysdev/xive/common.c

@@ -1335,16 +1335,11 @@ static int xive_prepare_cpu(unsigned int cpu)
 
 	xc = per_cpu(xive_cpu, cpu);
 	if (!xc) {
-		struct device_node *np;
-
 		xc = kzalloc_node(sizeof(struct xive_cpu),
 				  GFP_KERNEL, cpu_to_node(cpu));
 		if (!xc)
 			return -ENOMEM;
-		np = of_get_cpu_node(cpu, NULL);
-		if (np)
-			xc->chip_id = of_get_ibm_chip_id(np);
-		of_node_put(np);
+		xc->chip_id = cpu_to_node(cpu);
 		xc->hw_ipi = XIVE_BAD_IRQ;
 
 		per_cpu(xive_cpu, cpu) = xc;