xen: rework xen_pv_domain()

Rework xen_pv_domain() to no longer use the xen_domain_type variable,
but the artificial X86_FEATURE_XENPV cpu feature.

On non-x86 architectures xen_pv_domain() can be defined as "0".

This has the advantage that a kernel not built with CONFIG_XEN_PV
will be smaller due to dead code elimination.

Set the X86_FEATURE_XENPV feature very early, as xen_pv_domain() is
used rather early, too.

Reviewed-by: Jason Andryuk <jason.andryuk@amd.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Message-ID: <20250826145608.10352-2-jgross@suse.com>

Author: jgross1

arch/x86/xen/enlighten_pv.c

@@ -382,7 +382,6 @@ static bool __init xen_check_xsave(void)
 
 static void __init xen_init_capabilities(void)
 {
-	setup_force_cpu_cap(X86_FEATURE_XENPV);
 	setup_clear_cpu_cap(X86_FEATURE_DCA);
 	setup_clear_cpu_cap(X86_FEATURE_APERFMPERF);
 	setup_clear_cpu_cap(X86_FEATURE_MTRR);
@@ -1402,6 +1401,7 @@ asmlinkage __visible void __init xen_start_kernel(struct start_info *si)
 			JMP32_INSN_SIZE);
 
 	xen_domain_type = XEN_PV_DOMAIN;
+	setup_force_cpu_cap(X86_FEATURE_XENPV);
 	xen_start_flags = xen_start_info->flags;
 	/* Interrupts are guaranteed to be off initially. */
 	early_boot_irqs_disabled = true;

include/xen/xen.h

@@ -22,8 +22,15 @@ extern bool xen_pvh;
 #define xen_pvh			0
 #endif
 
+#ifdef CONFIG_X86
+#include <asm/cpufeature.h>
+
+#define xen_pv_domain()		(cpu_feature_enabled(X86_FEATURE_XENPV))
+#else
+#define xen_pv_domain()		0
+#endif
+
 #define xen_domain()		(xen_domain_type != XEN_NATIVE)
-#define xen_pv_domain()		(xen_domain_type == XEN_PV_DOMAIN)
 #define xen_hvm_domain()	(xen_domain_type == XEN_HVM_DOMAIN)
 #define xen_pvh_domain()	(xen_pvh)