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Message-ID: <5727a163-c72a-9632-b497-1011689aaa75@redhat.com>
Date:   Mon, 3 Jul 2023 11:08:00 -0400
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Subject: Re: [PATCH v4 1/4] x86/speculation: Add __update_spec_ctrl() helper
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To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Josh Poimboeuf <jpoimboe@kernel.org>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Jacob Pan <jacob.jun.pan@linux.intel.com>,
        Len Brown <lenb@kernel.org>, Jonathan Corbet <corbet@lwn.net>,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
        x86@kernel.org, linux-pm@vger.kernel.org,
        Robin Jarry <rjarry@redhat.com>, Joe Mario <jmario@redhat.com>
References: <20230628022554.1638318-1-longman@redhat.com>
 <20230628022554.1638318-2-longman@redhat.com>
 <20230703102815.GD4253@hirez.programming.kicks-ass.net>
From:   Waiman Long <longman@redhat.com>
In-Reply-To: <20230703102815.GD4253@hirez.programming.kicks-ass.net>
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On 7/3/23 06:28, Peter Zijlstra wrote:
> On Tue, Jun 27, 2023 at 10:25:51PM -0400, Waiman Long wrote:
>> Add a new __update_spec_ctrl() helper which is a variant of
>> update_spec_ctrl() that can be used in a noinstr function.
>>
>> Suggested-by: Peter Zijlstra <peterz@infradead.org>
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>>   arch/x86/include/asm/nospec-branch.h | 11 ++++++++++-
>>   1 file changed, 10 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
>> index 55388c9f7601..1d363fcea207 100644
>> --- a/arch/x86/include/asm/nospec-branch.h
>> +++ b/arch/x86/include/asm/nospec-branch.h
>> @@ -9,7 +9,7 @@
>>   
>>   #include <asm/alternative.h>
>>   #include <asm/cpufeatures.h>
>> -#include <asm/msr-index.h>
>> +#include <asm/msr.h>
>>   #include <asm/unwind_hints.h>
>>   #include <asm/percpu.h>
>>   #include <asm/current.h>
>> @@ -488,6 +488,15 @@ DECLARE_PER_CPU(u64, x86_spec_ctrl_current);
>>   extern void update_spec_ctrl_cond(u64 val);
>>   extern u64 spec_ctrl_current(void);
>>   
>> +/*
>> + * This can be used in noinstr function.
>> + */
>> +static __always_inline void __update_spec_ctrl(u64 val)
>> +{
>> +	__this_cpu_write(x86_spec_ctrl_current, val);
>> +	native_wrmsrl(MSR_IA32_SPEC_CTRL, val);
>> +}
> Should we also use this to implement update_spec_ctrl() ?

I also thought about that. However, I am a bit worry about losing some 
additional check done in this_cpu_write() and the paravirt support in 
wrmsrl(). I have no problem making update_spec_ctrl() an instrument-able 
wrapper on __update_spec_ctrl() if you think these are not valid concerns.

Cheers,
Longman