linux-kernel - RE: [PATCH v3 0/6] Hyper-V: Implement hypervisor core collection

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Message-ID:
 <SN6PR02MB4157C70EBD25315F098DB3A1D4F7A@SN6PR02MB4157.namprd02.prod.outlook.com>
Date: Sat, 18 Oct 2025 02:54:11 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Mukesh R <mrathor@...ux.microsoft.com>, Wei Liu <wei.liu@...nel.org>
CC: "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"kys@...rosoft.com" <kys@...rosoft.com>, "haiyangz@...rosoft.com"
	<haiyangz@...rosoft.com>, "decui@...rosoft.com" <decui@...rosoft.com>,
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	<mingo@...hat.com>, "bp@...en8.de" <bp@...en8.de>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>, "x86@...nel.org"
	<x86@...nel.org>, "hpa@...or.com" <hpa@...or.com>, "arnd@...db.de"
	<arnd@...db.de>
Subject: RE: [PATCH v3 0/6] Hyper-V: Implement hypervisor core collection

From: Mukesh R <mrathor@...ux.microsoft.com> Sent: Friday, October 17, 2025 4:58 PM
> 
> On 10/17/25 15:57, Wei Liu wrote:
> > On Fri, Oct 17, 2025 at 10:33:00PM +0000, Wei Liu wrote:
> >> On Mon, Oct 06, 2025 at 03:42:02PM -0700, Mukesh Rathor wrote:
> >> [...]
> >>> Mukesh Rathor (6):
> >>>   x86/hyperv: Rename guest crash shutdown function
> >>>   hyperv: Add two new hypercall numbers to guest ABI public header
> >>>   hyperv: Add definitions for hypervisor crash dump support
> >>>   x86/hyperv: Add trampoline asm code to transition from hypervisor
> >>>   x86/hyperv: Implement hypervisor RAM collection into vmcore
> >>>   x86/hyperv: Enable build of hypervisor crashdump collection files
> >>>
> >>
> >> Applied to hyperv-next. Thanks.
> >
> > This breaks i386 build.
> >
> > /work/linux-on-hyperv/linux.git/arch/x86/hyperv/hv_init.c: In function ?hyperv_init?:
> > /work/linux-on-hyperv/linux.git/arch/x86/hyperv/hv_init.c:557:17: error: implicit declaration of function ?hv_root_crash_init? [-Werror=implicit-function-declaration]
> >   557 |                 hv_root_crash_init();
> >       |                 ^~~~~~~~~~~~~~~~~~
> >
> > That's because CONFIG_MSHV_ROOT is only available on x86_64. And the
> > crash feature is guarded by CONFIG_MSHV_ROOT.
> >
> > Applying the following diff fixes the build.
> 
> 
> Thanks. A bit surprising tho that CONFIG_MSHV_ROOT doesn't have
> hard dependency on x86_64. It should, no?

CONFIG_MSHV_ROOT *does* have a hard dependency on X86_64.

But the problem is actually more pervasive than just 32-bit builds. Because
of the hard dependency, 32-bit builds imply CONFIG_MSHV_ROOT=n, which is
the real problem. In arch/x86/include/asm/mshyperv.h the declaration for
hv_root_crash_init() is available only when CONFIG_MSHV_ROOT is defined
(m or y). There's a stub hv_root_crash_init() if CONFIG_MSHV_ROOT is defined
and CONFIG_CRASH_DUMP=n, but not if CONFIG_MSHV_ROOT=n. The solution
is to add a stub when CONFIG_MSHV_ROOT=n, as below:

diff --git a/arch/x86/include/asm/mshyperv.h b/arch/x86/include/asm/mshyperv.h
index 76582affefa8..a5b258d268ed 100644
--- a/arch/x86/include/asm/mshyperv.h
+++ b/arch/x86/include/asm/mshyperv.h
@@ -248,6 +248,8 @@ void hv_crash_asm_end(void);
 static inline void hv_root_crash_init(void) {}
 #endif  /* CONFIG_CRASH_DUMP */

+#else   /* CONFIG_MSHV_ROOT */
+static inline void hv_root_crash_init(void) {}
 #endif  /* CONFIG_MSHV_ROOT */

 #else /* CONFIG_HYPERV */

Annoyingly, this solution duplicates the hv_root_crash_init() stub.  So
an alternate approach that changes a few more lines of code is this:

diff --git a/arch/x86/include/asm/mshyperv.h b/arch/x86/include/asm/mshyperv.h
index 76582affefa8..1342d55c2545 100644
--- a/arch/x86/include/asm/mshyperv.h
+++ b/arch/x86/include/asm/mshyperv.h
@@ -237,18 +237,14 @@ static __always_inline u64 hv_raw_get_msr(unsigned int reg)
 }
 int hv_apicid_to_vp_index(u32 apic_id);

-#if IS_ENABLED(CONFIG_MSHV_ROOT)
-
-#ifdef CONFIG_CRASH_DUMP
+#if IS_ENABLED(CONFIG_MSHV_ROOT) && IS_ENABLED(CONFIG_CRASH_DUMP)
 void hv_root_crash_init(void);
 void hv_crash_asm32(void);
 void hv_crash_asm64(void);
 void hv_crash_asm_end(void);
-#else   /* CONFIG_CRASH_DUMP */
+#else   /* CONFIG_MSHV_ROOT && CONFIG_CRASH_DUMP */
 static inline void hv_root_crash_init(void) {}
-#endif  /* CONFIG_CRASH_DUMP */
-
-#endif  /* CONFIG_MSHV_ROOT */
+#endif  /* CONFIG_MSHV_ROOT && CONFIG_CRASH_DUMP */

 #else /* CONFIG_HYPERV */
 static inline void hyperv_init(void) {}

Michael