Re: [PATCH v3 1/9] powerpc/xive: Use cpu_to_node() instead of "ibm,chip-id" property

[David Gibson <david@gibson.dropbear.id.au>]

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On Wed, Mar 31, 2021 at 04:45:06PM +0200, Cédric Le Goater wrote:
> 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. The XIVE
> driver queries the chip id value from the "ibm,chip-id" DT property
> but this property is not available on all platforms. It was first
> introduced on the PowerNV platform and later, under QEMU for pseries.
> However, the property does not exist under PowerVM since it is not
> specified in PAPR.
> 
> cpu_to_node() is a better alternative. On the PowerNV platform, the
> node id is computed from the "ibm,associativity" property of the CPU.
> Its value is built in the OPAL firmware from the physical chip id and
> is equivalent to "ibm,chip-id".

Hrm... I mean, for powernv this is certainly correct, but seems to be
relying on pretty specific specifics of the OPAL / chip behaviour,
namely that the NUMA id == chip ID.

> On pSeries, the hcall
> H_HOME_NODE_ASSOCIATIVITY returns the node id.

AFAICT, the chip_id field is never actually used in the PAPR version
of XIVE.  The only access to the field outside native.c is in
xive_pick_irq_target(), and it only looks at chip_id if src_chip is
valid.  But src_chip is initialized to XIVE_INVALID_CHIP_ID in papr.c

So it would make more sense to me to also initialize chip_id to
XIVE_INVALID_CHIP_ID for PAPR to make it clearer that it's not
relevant.

> Also to be noted that under QEMU/KVM "ibm,chip-id" is badly calculated
> with unusual SMT configuration. This leads to a bogus chip id value
> being returned by of_get_ibm_chip_id().

I *still* don't clearly understand what you think is bogus about the
chip id value that qemu generates.  It's clearly not a problem for
XIVE, since PAPR XIVE never uses it.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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