linux-kernel - Re: [PATCH v2 1/1] x86/reboot: KVM: Guard nmi_shootdown_cpus_on

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Message-ID: <ZwbazZpTSAn9aAC7@google.com>
Date: Wed, 9 Oct 2024 12:34:37 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc: linux-kernel@...r.kernel.org, llvm@...ts.linux.dev, 
	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>, Nathan Chancellor <nathan@...nel.org>, 
	Nick Desaulniers <ndesaulniers@...gle.com>, Bill Wendling <morbo@...gle.com>, 
	Justin Stitt <justinstitt@...gle.com>
Subject: Re: [PATCH v2 1/1] x86/reboot: KVM: Guard nmi_shootdown_cpus_on_restart()
 with ifdeffery

On Tue, Oct 08, 2024, Andy Shevchenko wrote:
> The nmi_shootdown_cpus_on_restart() in some cases may be not used.
> This, in particular, prevents kernel builds with clang, `make W=1`
> and CONFIG_WERROR=y:
> 
> arch/x86/kernel/reboot.c:957:20: error: unused function 'nmi_shootdown_cpus_on_restart' [-Werror,-Wunused-function]
>   957 | static inline void nmi_shootdown_cpus_on_restart(void)
>       |                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> Fix this by guarging the definitions with the respective KVM ifdeffery.
> 
> See also commit 6863f5643dd7 ("kbuild: allow Clang to find unused static
> inline functions for W=1 build").
> 
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> ---
> v1: suggested by Dave Hansen
> v2: rebased on top of the latest changes in the file
>  arch/x86/kernel/reboot.c | 22 +++++++++++++---------
>  1 file changed, 13 insertions(+), 9 deletions(-)


Heh, I tried fixing this too, and have patches to clean things up, but I ended up
not posting them because I decided the W=1 warning was less ugly than the resulting
code in nmi_shootdown_cpus().

If we're willing to take on a bit of weirdness in nmi_shootdown_cpus(), then much
of the #ifdeffery can go away.  Patches attached (lightly tested).

$ git diff --stat next..HEAD
 arch/x86/kernel/reboot.c | 26 ++++++++------------------
 1 file changed, 8 insertions(+), 18 deletions(-)

View attachment "0001-x86-reboot-Allow-blindly-calling-nmi_shootdown_cpus-.patch" of type "text/x-diff" (2085 bytes)

View attachment "0002-x86-reboot-Open-code-nmi_shootdown_cpus_on_restart-i.patch" of type "text/x-diff" (2118 bytes)

View attachment "0003-x86-reboot-Delete-CONFIG_SMP-n-stub-for-nmi_shootdow.patch" of type "text/x-diff" (1211 bytes)