[RFCv2 03/38] x86/hyperlaunch: convert max vcpu determination to domain builder

["Daniel P. Smith" <dpsmith@apertussolutions.com>]

The domain configuration may request more vcpus than are present in the system.
For dom0, the function dom0_max_vcpus() was used to clamp down to physically
available vcpus. Here we are introducing a generalized version,
dom_max_vcpus(), that takes a boot domain and sets the max vcpus based on the
lesser of the requested max and the available vcpus.

Signed-off-by: Daniel P. Smith <dpsmith@apertussolutions.com>
---
 xen/arch/x86/Makefile                |  1 +
 xen/arch/x86/domain-builder/Makefile |  1 +
 xen/arch/x86/domain-builder/domain.c | 40 ++++++++++++++++++++++++++++
 xen/arch/x86/setup.c                 |  4 +--
 xen/include/xen/domain-builder.h     |  3 +++
 5 files changed, 47 insertions(+), 2 deletions(-)
 create mode 100644 xen/arch/x86/domain-builder/Makefile
 create mode 100644 xen/arch/x86/domain-builder/domain.c

diff --git a/xen/arch/x86/Makefile b/xen/arch/x86/Makefile
index c2f1dcf301d6..80269c5e6b45 100644
--- a/xen/arch/x86/Makefile
+++ b/xen/arch/x86/Makefile
@@ -31,6 +31,7 @@ obj-y += desc.o
 obj-bin-y += dmi_scan.init.o
 obj-y += domain.o
 obj-bin-y += dom0_build.init.o
+obj-y += domain-builder/
 obj-y += domain_page.o
 obj-y += e820.o
 obj-y += emul-i8254.o
diff --git a/xen/arch/x86/domain-builder/Makefile b/xen/arch/x86/domain-builder/Makefile
new file mode 100644
index 000000000000..0c2e7085e21b
--- /dev/null
+++ b/xen/arch/x86/domain-builder/Makefile
@@ -0,0 +1 @@
+obj-y += domain.init.o
diff --git a/xen/arch/x86/domain-builder/domain.c b/xen/arch/x86/domain-builder/domain.c
new file mode 100644
index 000000000000..d8eba73dac28
--- /dev/null
+++ b/xen/arch/x86/domain-builder/domain.c
@@ -0,0 +1,40 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright (C) 2025, Apertus Solutions, LLC
+ */
+
+#include <xen/cpumask.h>
+#include <xen/domain.h>
+#include <xen/init.h>
+#include <xen/sched.h>
+
+#include <public/bootfdt.h>
+
+#include <asm/bootinfo.h>
+
+unsigned int __init dom_max_vcpus(struct boot_domain *bd)
+{
+    unsigned int limit = bd->mode & BUILD_MODE_PARAVIRT ?
+                                    MAX_VIRT_CPUS : HVM_MAX_VCPUS;
+
+    if ( bd->capabilities & DOMAIN_CAPS_CONTROL )
+        limit = dom0_max_vcpus();
+    else
+        limit = min(limit,
+                    (uint32_t)cpumask_weight(cpupool_valid_cpus(cpupool0)));
+
+    if ( bd->max_vcpus == 0 || bd->max_vcpus > limit )
+        bd->max_vcpus = limit;
+
+    return bd->max_vcpus;
+}
+
+/*
+ * Local variables:
+ * mode: C
+ * c-file-style: "BSD"
+ * c-basic-offset: 4
+ * tab-width: 4
+ * indent-tabs-mode: nil
+ * End:
+ */
diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index afc133b4d562..d3bde6c43e8d 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -1020,18 +1020,18 @@ static struct domain *__init create_dom0(struct boot_info *bi)
 {
     char *cmdline = NULL;
     size_t cmdline_size;
+    struct boot_domain *bd = &bi->domains[0];
     struct xen_domctl_createdomain dom0_cfg = {
         .flags = IS_ENABLED(CONFIG_TBOOT) ? XEN_DOMCTL_CDF_s3_integrity : 0,
         .max_evtchn_port = -1,
         .max_grant_frames = -1,
         .max_maptrack_frames = -1,
         .grant_opts = XEN_DOMCTL_GRANT_version(opt_gnttab_max_version),
-        .max_vcpus = dom0_max_vcpus(),
+        .max_vcpus = dom_max_vcpus(bd),
         .arch = {
             .misc_flags = opt_dom0_msr_relaxed ? XEN_X86_MSR_RELAXED : 0,
         },
     };
-    struct boot_domain *bd = &bi->domains[0];
     struct domain *d;
 
     if ( opt_dom0_pvh ||
diff --git a/xen/include/xen/domain-builder.h b/xen/include/xen/domain-builder.h
index 350e2eb2af8c..82c929cc48a1 100644
--- a/xen/include/xen/domain-builder.h
+++ b/xen/include/xen/domain-builder.h
@@ -5,6 +5,7 @@
 #include <xen/types.h>
 
 struct boot_info;
+struct boot_domain;
 
 /* Return status of builder_init(). Shows which boot mechanism was detected */
 enum fdt_kind
@@ -34,4 +35,6 @@ size_t builder_get_cmdline_size(const struct boot_info *bi, int offset);
 int builder_get_cmdline(const struct boot_info *bi, int offset,
                         char *cmdline, size_t size);
 
+unsigned int dom_max_vcpus(struct boot_domain *bd);
+
 #endif /* __XEN_DOMAIN_BUILDER_H__ */
-- 
2.30.2