linux-kernel - [tip: x86/urgent] x86/xen: Fix __xen_hypercall

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Message-ID: <174436567811.31282.12400498126986636858.tip-bot2@tip-bot2>
Date: Fri, 11 Apr 2025 10:01:14 -0000
From: "tip-bot2 for Jason Andryuk" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Juergen Gross <jgross@...e.com>, Jason Andryuk <jason.andryuk@....com>,
 Ingo Molnar <mingo@...nel.org>, Boris Ostrovsky <boris.ostrovsky@...cle.com>,
 "H. Peter Anvin" <hpa@...or.com>, "Xin Li (Intel)" <xin@...or.com>,
 x86@...nel.org, linux-kernel@...r.kernel.org
Subject: [tip: x86/urgent] x86/xen: Fix __xen_hypercall_setfunc()

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     164a9f712fa53e4c92b2a435bb071a5be0c31dbc
Gitweb:        https://git.kernel.org/tip/164a9f712fa53e4c92b2a435bb071a5be0c31dbc
Author:        Jason Andryuk <jason.andryuk@....com>
AuthorDate:    Thu, 10 Apr 2025 15:31:05 -04:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Fri, 11 Apr 2025 11:39:50 +02:00

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>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Reviewed-by: Juergen Gross <jgross@...e.com>
Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: "Xin Li (Intel)" <xin@...or.com>
Link: https://lore.kernel.org/r/20250410193106.16353-1-jason.andryuk@amd.com
---
 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 1b7710b..53282dc 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -103,10 +103,6 @@ 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
@@ -118,8 +114,7 @@ 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))