[PATCH v5 for-4.19? 1/2] amend 'cmdline: document and enforce "extra_guest_irqs" upper bounds'

[Jan Beulich <jbeulich@suse.com>]

Address late review comments for what is now commit 17f6d398f765:
- bound max_irqs right away against nr_irqs
- introduce a #define for a constant used twice

Signed-off-by: Jan Beulich <jbeulich@suse.com>
---
v5: New.

--- a/xen/arch/x86/io_apic.c
+++ b/xen/arch/x86/io_apic.c
@@ -2663,15 +2663,16 @@ void __init ioapic_init(void)
 unsigned int __hwdom_init arch_hwdom_irqs(const struct domain *d)
 {
     unsigned int n = fls(num_present_cpus());
-    /* Bounded by the domain pirq EOI bitmap gfn. */
-    const unsigned int max_irqs = PAGE_SIZE * BITS_PER_BYTE;
+    /* Bounding by the domain pirq EOI bitmap capacity. */
+    const unsigned int max_irqs = min_t(unsigned int, nr_irqs,
+                                        PAGE_SIZE * BITS_PER_BYTE);
 
     if ( is_system_domain(d) )
         return max_irqs;
 
     if ( !d->domain_id )
         n = min(n, dom0_max_vcpus());
-    n = min(nr_irqs_gsi + n * NR_DYNAMIC_VECTORS, min(nr_irqs, max_irqs));
+    n = min(nr_irqs_gsi + n * NR_DYNAMIC_VECTORS, max_irqs);
 
     printk("%pd has maximum %u PIRQs\n", d, n);
 
--- a/xen/common/domain.c
+++ b/xen/common/domain.c
@@ -353,7 +353,8 @@ static int late_hwdom_init(struct domain
 #ifdef CONFIG_HAS_PIRQ
 
 static unsigned int __read_mostly extra_hwdom_irqs;
-static unsigned int __read_mostly extra_domU_irqs = 32;
+#define DEFAULT_EXTRA_DOMU_IRQS 32U
+static unsigned int __read_mostly extra_domU_irqs = DEFAULT_EXTRA_DOMU_IRQS;
 
 static int __init cf_check parse_extra_guest_irqs(const char *s)
 {
@@ -839,7 +840,8 @@ void __init setup_system_domains(void)
             extra_hwdom_irqs = n - nr_static_irqs;
             printk(XENLOG_WARNING "hwdom IRQs bounded to %u\n", n);
         }
-        if ( extra_domU_irqs > max(32U, n - nr_static_irqs) )
+        if ( extra_domU_irqs >
+             max(DEFAULT_EXTRA_DOMU_IRQS, n - nr_static_irqs) )
         {
             extra_domU_irqs = n - nr_static_irqs;
             printk(XENLOG_WARNING "domU IRQs bounded to %u\n", n);