[PATCH v2 3/3] x86/xen: add a dummy trampoline for Xen PV guests

[Juergen Gross <jgross@xxxxxxxx>]

When running as a Xen PV guest there is no need for setting up the
realmode trampoline, as realmode isn't supported in this environment.

Trying to setup the trampoline has been proven to be problematic in
some cases, especially when trying to debug early boot problems with
Xen requiring to keep the EFI boot-services memory mapped (some
firmware variants seem to claim basically all memory below 1M for boot
services).

Setup a dummy trampoline in order to make init_real_mode() happy.
In order to avoid too tight coupling between the Xen PV specific code
and the trampoline handling, modify the trampoline handling to do
nothing if the detected trampoline size is 0.

Fixes: 084ee1c641a0 ("x86, realmode: Relocator for realmode code")
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
 arch/x86/realmode/init.c    |  6 ++++++
 arch/x86/xen/enlighten_pv.c | 17 +++++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/arch/x86/realmode/init.c b/arch/x86/realmode/init.c
index 5a670a9ed2f7..538328ce5ff5 100644
--- a/arch/x86/realmode/init.c
+++ b/arch/x86/realmode/init.c
@@ -110,6 +110,9 @@ static void __init setup_real_mode(void)
        int i;
 #endif
 
+       if (!size)
+               return;
+
        base = (unsigned char *)real_mode_header;
 
        /*
@@ -204,6 +207,9 @@ static void __init set_real_mode_permissions(void)
        unsigned long text_start =
                (unsigned long) __va(real_mode_header->text_start);
 
+       if (!size)
+               return;
+
        set_memory_nx((unsigned long) base, size >> PAGE_SHIFT);
        set_memory_ro((unsigned long) base, ro_size >> PAGE_SHIFT);
        set_memory_x((unsigned long) text_start, text_size >> PAGE_SHIFT);
diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
index 038da45f057a..a6194a6a6806 100644
--- a/arch/x86/xen/enlighten_pv.c
+++ b/arch/x86/xen/enlighten_pv.c
@@ -71,6 +71,7 @@
 #include <asm/mwait.h>
 #include <asm/pci_x86.h>
 #include <asm/cpu.h>
+#include <asm/realmode.h>
 #ifdef CONFIG_X86_IOPL_IOPERM
 #include <asm/io_bitmap.h>
 #endif
@@ -137,12 +138,28 @@ static void __init xen_pv_init_platform(void)
        xen_init_time_ops();
 }
 
+static struct real_mode_header xen_rm_header;
+
+static __initdata struct trampoline_ref xen_dummy_trampoline = {
+       .blob = (unsigned char *)&xen_rm_header,
+       .blob_end = (unsigned char *)&xen_rm_header,
+};
+
+/* Setup dummy trampoline of size 0 with no relocations. */
+static void __init xen_setup_trampoline(void)
+{
+       real_mode_header = &xen_rm_header;
+       real_mode_trampoline = &xen_dummy_trampoline;
+}
+
 static void __init xen_pv_guest_late_init(void)
 {
 #ifndef CONFIG_SMP
        /* Setup shared vcpu info for non-smp configurations */
        xen_setup_vcpu_info_placement();
 #endif
+
+       xen_setup_trampoline();
 }
 
 static __read_mostly unsigned int cpuid_leaf5_ecx_val;
-- 
2.35.3