x86/setup: Do not reserve a crash kernel region if booted on Xen PV

Xen PV domains cannot shut down and start a crash kernel. Instead,
the crashing kernel makes a SCHEDOP_shutdown hypercall with the
reason code SHUTDOWN_crash, cf. xen_crash_shutdown() machine op in
arch/x86/xen/enlighten_pv.c.

A crash kernel reservation is merely a waste of RAM in this case. It
may also confuse users of kexec_load(2) and/or kexec_file_load(2).
When flags include KEXEC_ON_CRASH or KEXEC_FILE_ON_CRASH,
respectively, these syscalls return success, which is technically
correct, but the crash kexec image will never be actually used.

Signed-off-by: Petr Tesarik <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Reviewed-by: Juergen Gross <[e-mail]>
Cc: Tom Lendacky <[e-mail]>
Cc: Dou Liyang <[e-mail]>
Cc: Mikulas Patocka <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: "H. Peter Anvin" <[e-mail]>
Cc: [e-mail]
Cc: Boris Ostrovsky <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Jean Delvare <[e-mail]>
Link: https://lkml.kernel.org/r/[e-mail]

arch/x86/kernel/setup.c

···
 #include <linux/init_ohci1394_dma.h>
 #include <linux/kvm_para.h>
 #include <linux/dma-contiguous.h>
+#include <xen/xen.h>
 
 #include <linux/errno.h>
 #include <linux/kernel.h>
···
 		if (ret != 0 || crash_size <= 0)
 			return;
 		high = true;
+	}
+
+	if (xen_pv_domain()) {
+		pr_info("Ignoring crashkernel for a Xen PV domain\n");
+		return;
 	}
 
 	/* 0 means: find the address automatically */