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Subject: Re: [PATCH] x86/perf: Use static_call for x86_pmu.guest_get_msrs
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     kbuild-all@lists.01.org, clang-built-linux@googlegroups.com,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>, x86@kernel.org,
        linux-kernel@vger.kernel.org, Jason Baron <jbaron@akamai.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        kernel test robot <lkp@intel.com>
References: <20210118072151.44481-1-like.xu@linux.intel.com>
 <202101182008.jQybUDa0-lkp@intel.com>
 <a82754e7-9a2d-7ab4-466d-fc0d51a3b7f2@linux.intel.com>
 <YAWOhxi6Vxiq8JLR@hirez.programming.kicks-ass.net>
From:   Like Xu <like.xu@linux.intel.com>
Organization: Intel OTC
Message-ID: <7f31ae2f-25a1-a7fb-1598-a8f21f3bc447@linux.intel.com>
Date:   Fri, 22 Jan 2021 11:19:31 +0800
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On 2021/1/18 21:35, Peter Zijlstra wrote:
> On Mon, Jan 18, 2021 at 09:05:06PM +0800, Like Xu wrote:
>> I guess this fix will silence the compiler error,
>> and v2 will be sent after the local 0day test passes.
> 
> I think there's also a problem where you don't assign a function at all.

Thank you!

> 
> I think you want something like
> 
> 	if (!x86_pmu.guest_get_msr)
> 		x86_pmu.guest_get_msr = guest_get_msr_nop;
> 
> right before x86_pmu_static_call_update();
> 
> And then have it be something like:
> 
> static void *guest_et_msr_nop(int *nr)

The [-Werror=incompatible-pointer-types] will yell at "void *".

Please review the v2 version.

https://lore.kernel.org/lkml/20210122030324.2754492-1-like.xu@linux.intel.com/T/#u

> {
> 	*nr = 0;
> 	return NULL;
> }
> 
> and then you can reduce the entire thing to:
> 
> struct perf_guest_switch_msr *perf_guest_get_msrs(int *nr)
> {
> 	return static_call(x86_pmu_guest_get_msrs)(nr);
> }
>