linux-kernel - Re: [PATCH v4 1/4] x86/speculation: Add __update_spec

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Date:   Mon, 3 Jul 2023 11:08:00 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Len Brown <lenb@...nel.org>, Jonathan Corbet <corbet@....net>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        x86@...nel.org, linux-pm@...r.kernel.org,
        Robin Jarry <rjarry@...hat.com>, Joe Mario <jmario@...hat.com>
Subject: Re: [PATCH v4 1/4] x86/speculation: Add __update_spec_ctrl() helper


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@...radead.org>
>> Signed-off-by: Waiman Long <longman@...hat.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