[PATCH v3 1/1] xen/arm: populate a default P2M pool for new domains

[Julian Vetter <julian.vetter@xxxxxxxxxx>]

On x86, a domain can be populated directly after the
XEN_DOMCTL_createdomain call. Sizing the pool is thus an optional tuning
step for the toolstack (libxl: "the Xen minimum is for safety, not
performance").

On ARM however, the P2M pool of a non-hardware domain starts empty. So,
any toolstack must size the pool before populating the domain's physmap,
otherwise no P2M tables can be allocated. This is an ARM specific step,
which every toolstack has to know about.

Instead, populate a default pool of 256 pages (1MByte) in p2m_init(),
matching what x86 HAP does in hap_enable(). The size is intentionally
fixed, because p2m_init() isn't preemptible, so the allocation must stay
bounded. Then dom0less and libxl set the full size, as they do today.

Skip the hardware domain, which has no dedicated P2M pool. Also skip LLC
colored setups, as the domain's colors are only configured after domain
creation, so the pool pages would be allocated with the default colors.

Signed-off-by: Julian Vetter <julian.vetter@xxxxxxxxxx>
---
Changes in v3:
- Use a fixed default of 256 pages, as x86 HAP does
- Drop the domain_p2m_default_kb() helper and the dom0less change
- Adapt the commit message to explain why a fixed size is used

Changes in v2:
- Size the default pool based on the exisiting formula
- Rewrite commit message to explain why a default pool is desirable
---
 xen/arch/arm/mmu/p2m.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/xen/arch/arm/mmu/p2m.c b/xen/arch/arm/mmu/p2m.c
index 70d7c9fed9..9579e905d8 100644
--- a/xen/arch/arm/mmu/p2m.c
+++ b/xen/arch/arm/mmu/p2m.c
@@ -3,6 +3,7 @@
 #include <xen/domain_page.h>
 #include <xen/ioreq.h>
 #include <xen/lib.h>
+#include <xen/llc-coloring.h>
 #include <xen/sched.h>
 #include <xen/softirq.h>
 
@@ -1555,6 +1556,26 @@ int p2m_init(struct domain *d)
     if ( rc )
         return rc;
 
+    /*
+     * Populate a default of 256 pages (1MB) to the P2M pages pool here, as
+     * x86 HAP does. So, the domain is usable without the pool being sized
+     * first. The toolstack, or dom0less, still sets the full size later,
+     * taking the domain's RAM into account. A fixed size keeps this
+     * non-preemptible allocation bounded, independent of the number of vCPUs.
+     *
+     * Skip the hardware domain, which has no dedicated P2M pool, and LLC
+     * colored setups, as the domain's colors are only configured after domain
+     * creation.
+     */
+    if ( !is_hardware_domain(d) && !llc_coloring_enabled )
+    {
+        spin_lock(&d->arch.paging.lock);
+        rc = p2m_set_allocation(d, 256, /* can_preempt */ false);
+        spin_unlock(&d->arch.paging.lock);
+        if ( rc )
+            return rc;
+    }
+
     return 0;
 }
 
-- 
2.53.0



--
Julian Vetter | Vates Hypervisor & Kernel Developer

XCP-ng & Xen Orchestra - Vates solutions

web: https://vates.tech