[PATCH] x86/xen: Fix __xen_hypercall_setfunc

[PATCH] x86/xen: Fix __xen_hypercall_setfunc

[Jason Andryuk]

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@suse.com>
Signed-off-by: Jason Andryuk <jason.andryuk@amd.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

Re: [PATCH] x86/xen: Fix __xen_hypercall_setfunc

[Juergen Gross]

[-- Attachment #1.1.1: Type: text/plain, Size: 862 bytes --]

On 10.04.25 21:31, Jason Andryuk wrote:
> 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@suse.com>
> Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>

Reviewed-by: Juergen Gross <jgross@suse.com>


Juergen

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Re: [PATCH] x86/xen: Fix __xen_hypercall_setfunc

[Ingo Molnar]

* Juergen Gross <jgross@suse.com> wrote:

> On 10.04.25 21:31, Jason Andryuk wrote:
> > 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@suse.com>
> > Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
> 
> Reviewed-by: Juergen Gross <jgross@suse.com>

Wanna merge this via the Xen tree, or should it go to x86/urgent?

The bug was *caused* by the x86 tree so we'd be glad to merge,
but your call.

If you'll merge it:

  Acked-by: Ingo Molnar <mingo@kernel.org>

Thanks,

	Ingo

Re: [PATCH] x86/xen: Fix __xen_hypercall_setfunc

[Jürgen Groß]

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On 11.04.25 11:27, Ingo Molnar wrote:
> 
> * Juergen Gross <jgross@suse.com> wrote:
> 
>> On 10.04.25 21:31, Jason Andryuk wrote:
>>> 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@suse.com>
>>> Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
>>
>> Reviewed-by: Juergen Gross <jgross@suse.com>
> 
> Wanna merge this via the Xen tree, or should it go to x86/urgent?
> 
> The bug was *caused* by the x86 tree so we'd be glad to merge,
> but your call.

x86/urgent is fine for me.


Juergen

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Re: [PATCH] x86/xen: Fix __xen_hypercall_setfunc

[Ingo Molnar]

* Jürgen Groß <jgross@suse.com> wrote:

> On 11.04.25 11:27, Ingo Molnar wrote:
> > 
> > * Juergen Gross <jgross@suse.com> wrote:
> > 
> > > On 10.04.25 21:31, Jason Andryuk wrote:
> > > > 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@suse.com>
> > > > Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
> > > 
> > > Reviewed-by: Juergen Gross <jgross@suse.com>
> > 
> > Wanna merge this via the Xen tree, or should it go to x86/urgent?
> > 
> > The bug was *caused* by the x86 tree so we'd be glad to merge,
> > but your call.
> 
> x86/urgent is fine for me.

Applied, thanks!

	Ingo

[tip: x86/urgent] x86/xen: Fix __xen_hypercall_setfunc()

[tip-bot2 for Jason Andryuk]

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@amd.com>
AuthorDate:    Thu, 10 Apr 2025 15:31:05 -04:00
Committer:     Ingo Molnar <mingo@kernel.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@suse.com>
Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Juergen Gross <jgross@suse.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: "Xin Li (Intel)" <xin@zytor.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))