linux-kernel - [PATCH] x86/boot: skip realmode init code when running as Xen PV guest

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Date:   Mon, 21 Nov 2022 17:24:33 +0100
From:   Juergen Gross <jgross@...e.com>
To:     linux-kernel@...r.kernel.org, x86@...nel.org
Cc:     Juergen Gross <jgross@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH] x86/boot: skip realmode init code when running as Xen PV guest

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).

Skip the trampoline setup code for Xen PV guests.

Fixes: 084ee1c641a0 ("x86, realmode: Relocator for realmode code")
Signed-off-by: Juergen Gross <jgross@...e.com>
---
 arch/x86/include/asm/realmode.h | 4 ++--
 arch/x86/realmode/init.c        | 3 +++
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/realmode.h b/arch/x86/include/asm/realmode.h
index fd6f6e5b755a..5bfce58f1bab 100644
--- a/arch/x86/include/asm/realmode.h
+++ b/arch/x86/include/asm/realmode.h
@@ -78,8 +78,8 @@ extern unsigned char secondary_startup_64_no_verify[];
 
 static inline size_t real_mode_size_needed(void)
 {
-	if (real_mode_header)
-		return 0;	/* already allocated. */
+	if (real_mode_header || cpu_feature_enabled(X86_FEATURE_XENPV))
+		return 0;	/* already allocated or not needed. */
 
 	return ALIGN(real_mode_blob_end - real_mode_blob, PAGE_SIZE);
 }
diff --git a/arch/x86/realmode/init.c b/arch/x86/realmode/init.c
index 41d7669a97ad..1826700b156e 100644
--- a/arch/x86/realmode/init.c
+++ b/arch/x86/realmode/init.c
@@ -202,6 +202,9 @@ static void __init set_real_mode_permissions(void)
 
 static int __init init_real_mode(void)
 {
+	if (cpu_feature_enabled(X86_FEATURE_XENPV))
+		return 0;
+
 	if (!real_mode_header)
 		panic("Real mode trampoline was not allocated");
 
-- 
2.35.3