linux-kernel - [PATCH] x86/xen: Fix __xen_hypercall

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Message-ID: <20250410193106.16353-1-jason.andryuk@amd.com>
Date: Thu, 10 Apr 2025 15:31:05 -0400
From: Jason Andryuk <jason.andryuk@....com>
To: Juergen Gross <jgross@...e.com>, Boris Ostrovsky
	<boris.ostrovsky@...cle.com>, Thomas Gleixner <tglx@...utronix.de>, "Ingo
 Molnar" <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Dave Hansen
	<dave.hansen@...ux.intel.com>, <x86@...nel.org>, "H. Peter Anvin"
	<hpa@...or.com>, "Xin Li (Intel)" <xin@...or.com>
CC: Jason Andryuk <jason.andryuk@....com>, Ingo Molnar <mingo@...nel.org>,
	<xen-devel@...ts.xenproject.org>, <linux-kernel@...r.kernel.org>
Subject: [PATCH] x86/xen: Fix __xen_hypercall_setfunc

Hypercall detection is failing with xen_hypercall_intel() chosen even on
an AMD processor.  Looking at the disassembly, the call to
xen_get_vendor() was removed.

The check for boot_cpu_has(X86_FEATURE_CPUID) was used as a proxy for
the x86_vendor having been set.  When
CONFIG_X86_REQUIRED_FEATURE_CPUID=y (the default value), DCE eliminates
the call to xen_get_vendor().  An uninitialized value 0 means
X86_VENDOR_INTEL, so the Intel function is always returned.

Remove the if and always call xen_get_vendor() to avoid this issue.

Fixes: 3d37d9396eb3 ("x86/cpufeatures: Add {REQUIRED,DISABLED} feature configs")
Suggested-by: Juergen Gross <jgross@...e.com>
Signed-off-by: Jason Andryuk <jason.andryuk@....com>
---
I think the Fixes is correct in that it default enables requiring the
feature.  There were a lot of other changes to how headers were
generated as well.
---
 arch/x86/xen/enlighten.c | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index 43dcd8c7badc..6a5cae915eb1 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -100,10 +100,6 @@ noinstr void *__xen_hypercall_setfunc(void)
 	void (*func)(void);
 
 	/*
-	 * Xen is supported only on CPUs with CPUID, so testing for
-	 * X86_FEATURE_CPUID is a test for early_cpu_init() having been
-	 * run.
-	 *
 	 * Note that __xen_hypercall_setfunc() is noinstr only due to a nasty
 	 * dependency chain: it is being called via the xen_hypercall static
 	 * call when running as a PVH or HVM guest. Hypercalls need to be
@@ -115,8 +111,7 @@ noinstr void *__xen_hypercall_setfunc(void)
 	 */
 	instrumentation_begin();
 
-	if (!boot_cpu_has(X86_FEATURE_CPUID))
-		xen_get_vendor();
+	xen_get_vendor();
 
 	if ((boot_cpu_data.x86_vendor == X86_VENDOR_AMD ||
 	     boot_cpu_data.x86_vendor == X86_VENDOR_HYGON))
-- 
2.34.1