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

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Message-ID: <CAHd7WqzLaUXX_O6vw9YRUnPWNyTZgCANK+ZDVMYkTM4ozcZxGg@mail.gmail.com>
Date: Mon, 20 Oct 2025 13:48:44 -0700
From: Wei Liu <wei.liu@...nel.org>
To: Mukesh R <mrathor@...ux.microsoft.com>
Cc: Michael Kelley <mhklinux@...look.com>, Wei Liu <wei.liu@...nel.org>, 
	"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>, 
	"tglx@...utronix.de" <tglx@...utronix.de>, "mingo@...hat.com" <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

On Mon, 20 Oct 2025 at 12:05, Mukesh R <mrathor@...ux.microsoft.com> wrote:
>
> On 10/17/25 19:54, Michael Kelley wrote:
> > 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
>
> Thanks. Yeah, either of the above two is ok. The latter does not
> duplicate, so may be tiny bit better. Wei will pick one and apply
> directly.
>

I fixed hyperv-next using the second option.

Wei