[RFC] dom0less vcpu affinity bindings

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

Hi all,

We have received requests to introduce Dom0less vCPU affinity bindings
to allow configuring which pCPUs a given vCPU is allowed to run on.

After considering different approaches, I am thinking of using the
following binding format:

    vcpu0 {
           compatible = "xen,vcpu-affinity"; // compatible string
           id = <0>;  // vcpu id
           hard-affinity = "1,4-7"; // pcpu ranges
    };

Notably, the hard-affinity property is represented as a string.

We also considered using a bitmask, such as:

           hard-affinity = <0x0f>;

However, I decided against this approach because, on large server
systems, the number of physical CPUs can be very high, making the
bitmask size potentially very large. The string representation is more
practical for large systems and is also easier to understand and write.
It is also fully aligned with the way we have already implemented the
llc-colors option (see docs/misc/arm/device-tree/booting.txt and
docs/misc/cache-coloring.rst:).

What do you think?

diff --git a/xen/arch/arm/dom0less-build.c b/xen/arch/arm/dom0less-build.c
index 49d1f14d65..12379ecb20 100644
--- a/xen/arch/arm/dom0less-build.c
+++ b/xen/arch/arm/dom0less-build.c
@@ -818,6 +818,8 @@ void __init create_domUs(void)
     const struct dt_device_node *cpupool_node,
                                 *chosen = dt_find_node_by_path("/chosen");
     const char *llc_colors_str = NULL;
+    const char *hard_affinity_str = NULL;
+    struct dt_device_node *np;
 
     BUG_ON(chosen == NULL);
     dt_for_each_child_node(chosen, node)
@@ -992,6 +994,55 @@ void __init create_domUs(void)
         if ( rc )
             panic("Could not set up domain %s (rc = %d)\n",
                   dt_node_name(node), rc);
+
+        dt_for_each_child_node(node, np)
+        {
+            const char *s;
+            struct vcpu *v;
+            cpumask_t affinity;
+
+            if ( !dt_device_is_compatible(np, "xen,vcpu-affinity") )
+                continue;
+
+            if ( !dt_property_read_u32(np, "id", &val) )
+                continue;
+            if ( val >= d->max_vcpus )
+                panic("Invalid vcpu_id %u for domain %s\n", val, 
dt_node_name(node));
+
+            v = d->vcpu[val];
+            rc = dt_property_read_string(np, "hard-affinity", 
&hard_affinity_str);
+            if ( rc < 0 )
+                continue;
+            
+            s = hard_affinity_str;
+            cpumask_clear(&affinity);
+            while ( *s != '\0' )
+            {
+                unsigned int start, end;
+
+                start = simple_strtoul(s, &s, 0);
+
+                if ( *s == '-' )    /* Range */
+                {
+                    s++;
+                    end = simple_strtoul(s, &s, 0);
+                }
+                else                /* Single value */
+                    end = start;
+
+                for ( ; start <= end; start++ )
+                    cpumask_set_cpu(start, &affinity);
+
+                if ( *s == ',' )
+                    s++;
+                else if ( *s != '\0' )
+                    break;
+            }
+
+            rc = vcpu_set_hard_affinity(v, &affinity);
+            if ( rc )
+                panic("vcpu%d: failed to set hard affinity\n", v->vcpu_id);
+        }
     }
 }