Hi Stefano,
> -----Original Message-----
> From: Xen-devel <xen-devel-boun...@lists.xenproject.org> On Behalf Of
> Stefano Stabellini
> Sent: 2021年9月1日 5:36
> To: Wei Chen <wei.c...@arm.com>
> Cc: Stefano Stabellini <sstabell...@kernel.org>; xen-
> de...@lists.xenproject.org; jul...@xen.org; Bertrand Marquis
> <bertrand.marq...@arm.com>
> Subject: RE: [XEN RFC PATCH 24/40] xen/arm: introduce a helper to parse
> device tree NUMA distance map
>
> On Tue, 31 Aug 2021, Wei Chen wrote:
> > Hi Stefano,
> >
> > > -----Original Message-----
> > > From: Stefano Stabellini <sstabell...@kernel.org>
> > > Sent: 2021年8月31日 8:48
> > > To: Wei Chen <wei.c...@arm.com>
> > > Cc: xen-devel@lists.xenproject.org; sstabell...@kernel.org;
> jul...@xen.org;
> > > jbeul...@suse.com; Bertrand Marquis <bertrand.marq...@arm.com>
> > > Subject: Re: [XEN RFC PATCH 24/40] xen/arm: introduce a helper to
> parse
> > > device tree NUMA distance map
> > >
> > > On Wed, 11 Aug 2021, Wei Chen wrote:
> > > > A NUMA aware device tree will provide a "distance-map" node to
> > > > describe distance between any two nodes. This patch introduce a
> > > > new helper to parse this distance map.
> > > >
> > > > Signed-off-by: Wei Chen <wei.c...@arm.com>
> > > > ---
> > > > xen/arch/arm/numa_device_tree.c | 67
> +++++++++++++++++++++++++++++++++
> > > > 1 file changed, 67 insertions(+)
> > > >
> > > > diff --git a/xen/arch/arm/numa_device_tree.c
> > > b/xen/arch/arm/numa_device_tree.c
> > > > index bbe081dcd1..6e0d1d3d9f 100644
> > > > --- a/xen/arch/arm/numa_device_tree.c
> > > > +++ b/xen/arch/arm/numa_device_tree.c
> > > > @@ -200,3 +200,70 @@ device_tree_parse_numa_memory_node(const void
> *fdt,
> > > int node,
> > > >
> > > > return 0;
> > > > }
> > > > +
> > > > +/* Parse NUMA distance map v1 */
> > > > +int __init
> > > > +device_tree_parse_numa_distance_map_v1(const void *fdt, int node)
> > > > +{
> > > > + const struct fdt_property *prop;
> > > > + const __be32 *matrix;
> > > > + int entry_count, len, i;
> > > > +
> > > > + printk(XENLOG_INFO "NUMA: parsing numa-distance-map\n");
> > > > +
> > > > + prop = fdt_get_property(fdt, node, "distance-matrix", &len);
> > > > + if ( !prop )
> > > > + {
> > > > + printk(XENLOG_WARNING
> > > > + "NUMA: No distance-matrix property in distance-
> map\n");
> > > > +
> > > > + return -EINVAL;
> > > > + }
> > > > +
> > > > + if ( len % sizeof(uint32_t) != 0 )
> > > > + {
> > > > + printk(XENLOG_WARNING
> > > > + "distance-matrix in node is not a multiple of
> u32\n");
> > > > + return -EINVAL;
> > > > + }
> > > > +
> > > > + entry_count = len / sizeof(uint32_t);
> > > > + if ( entry_count <= 0 )
> > > > + {
> > > > + printk(XENLOG_WARNING "NUMA: Invalid distance-matrix\n");
> > > > +
> > > > + return -EINVAL;
> > > > + }
> > > > +
> > > > + matrix = (const __be32 *)prop->data;
> > > > + for ( i = 0; i + 2 < entry_count; i += 3 )
> > > > + {
> > > > + uint32_t from, to, distance;
> > > > +
> > > > + from = dt_read_number(matrix, 1);
> > > > + matrix++;
> > > > + to = dt_read_number(matrix, 1);
> > > > + matrix++;
> > > > + distance = dt_read_number(matrix, 1);
> > > > + matrix++;
> > > > +
> > > > + if ( (from == to && distance != NUMA_LOCAL_DISTANCE) ||
> > > > + (from != to && distance <= NUMA_LOCAL_DISTANCE) )
> > > > + {
> > > > + printk(XENLOG_WARNING
> > > > + "Invalid nodes' distance from node#%d to node#%d
> > > = %d\n",
> > > > + from, to, distance);
> > > > + return -EINVAL;
> > > > + }
> > > > +
> > > > + printk(XENLOG_INFO "NUMA: distance from node#%d to node#%d
> > > = %d\n",
> > > > + from, to, distance);
> > > > + numa_set_distance(from, to, distance);
> > > > +
> > > > + /* Set default distance of node B->A same as A->B */
> > > > + if (to > from)
> > > > + numa_set_distance(to, from, distance);
> > >
> > > I am a bit unsure about this last 2 lines: why calling
> numa_set_distance
> > > in the opposite direction only when to > from? Wouldn't it be OK to
> > > always do both:
> > >
> > > numa_set_distance(from, to, distance);
> > > numa_set_distance(to, from, distance);
> > >
> > > ?
> > >
> > I borrowed this code from Linux, but here is my understanding:
> >
> > First, I read some notes in Documentation/devicetree/bindings/numa.txt
> > 1. Each entry represents distance from first node to second node.
> > The distances are equal in either direction.
> > 2. distance-matrix should have entries in lexicographical ascending
> > order of nodes.
> >
> > Here is an example of distance-map node in DTB:
> > Sample#1, full list:
> > distance-map {
> > compatible = "numa-distance-map-v1";
> > distance-matrix = <0 0 10>,
> > <0 1 20>,
> > <0 2 40>,
> > <0 3 20>,
> > <1 0 20>,
> > <1 1 10>,
> > <1 2 20>,
> > <1 3 40>,
> > <2 0 40>,
> > <2 1 20>,
> > <2 2 10>,
> > <2 3 20>,
> > <3 0 20>,
> > <3 1 40>,
> > <3 2 20>,
> > <3 3 10>;
> > };
> >
> > Call numa_set_distance when "to > from" will prevent Xen to call
> > numa_set_distance(0, 1, 20) again when it's setting distance for <1 0
> 20>.
> > But, numa_set_distance(1, 0, 20) will be call twice.
> >
> > Normally, distance-map node will be optimized in following sample#2,
> > all redundant entries are removed:
> > Sample#2, partial list:
> > distance-map {
> > compatible = "numa-distance-map-v1";
> > distance-matrix = <0 0 10>,
> > <0 1 20>,
> > <0 2 40>,
> > <0 3 20>,
> > <1 1 10>,
> > <1 2 20>,
> > <1 3 40>,
> > <2 2 10>,
> > <2 3 20>,
> > <3 3 10>;
> > };
> >
> > There is not any "from > to" entry in the map. But using this partial
> map
> > still can set all distances for all pairs. And numa_set_distance(1, 0,
> 20)
> > will be only once.
>
> I see. I can't find in Documentation/devicetree/bindings/numa.txt where
> it says that "from > to" nodes can be omitted. If it is not written
> down, then somebody could easily optimize it the opposite way:
>
> distance-matrix = <0 0 10>,
> <1 0 20>,
> <2 0 40>,
> <3 0 20>,
> <1 1 10>,
> <2 1 20>,
> <3 1 40>,
> <2 2 10>,
> <3 2 20>,
> <3 3 10>;
>
Yes, you're right. Spec doesn't say opposite way is unallowed.
> I think the code in Xen should be resilient and able to cope with a
> device tree like the one you wrote or the one I wrote. From a code
> perspective, it should be very easy to do. If nothing else it would make
> Xen more resilient against buggy firmware.
>
>
I don't disagree with that.
> > > But in any case, I have a different suggestion. The binding states
> that
> > > "distances are equal in either direction". Also it has an example
> where
> > > only one direction is expressed unfortunately (at the end of the
> > > document).
> > >
> >
> > Oh, I should see this comment first, then I will not post above
> > comment : )
> >
> > > So my suggestion is to parse it as follows:
> > >
> > > - call numa_set_distance just once from
> > > device_tree_parse_numa_distance_map_v1
> > >
> > > - in numa_set_distance:
> > > - set node_distance_map[from][to] = distance;
> > > - check node_distance_map[to][from]
> > > - if unset, node_distance_map[to][from] = distance;
> > > - if already set to the same value, return success;
> > > - if already set to a different value, return error;
> >
> > I don't really like this implementation. I want the behavior of
> > numa_set_distance just like the function name, do not include
> > implicit operations. Otherwise, except the user read this function
> > implementation before he use it, he probably doesn't know this
> > function has done so many things.
>
> You can leave numa_set_distance as-is without any implicit operations.
>
> In that case, just call numa_set_distance twice from numa_set_distance
> for both from/to and to/from. numa_set_distance could return error is
I am OK for the first sentence. But...
> the entry was already set to a different value or success otherwise
> (also in the case it was already set to the same value). This would
... I prefer not to check the previous value. Subsequent numa_set_distance
call will override previous calls. Keep numa_set_distance as simple as
it can. And when you pass new data to numa_set_distance, it doesn't
know whether the previous data was correct or the new data is correct.
Only caller may have known.
> allow Xen to cope with both scenarios above.
