linux-kernel - Re: [PATCH v5 02/27] arm64: cpufeature: Use alternatives for VHE cpu

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Message-ID: <2e46f172-9d1c-6daf-f4bd-1bfd20dc5702@arm.com>
Date:   Wed, 12 Sep 2018 13:37:33 +0100
From:   Suzuki K Poulose <Suzuki.Poulose@....com>
To:     James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry@....com>,
        linux-arm-kernel@...ts.infradead.org
Cc:     linux-kernel@...r.kernel.org, daniel.thompson@...aro.org,
        joel@...lfernandes.org, marc.zyngier@....com, mark.rutland@....com,
        christoffer.dall@....com, catalin.marinas@....com,
        will.deacon@....com
Subject: Re: [PATCH v5 02/27] arm64: cpufeature: Use alternatives for VHE
 cpu_enable

On 12/09/18 11:28, James Morse wrote:
> Hi Julien,
> 
> On 28/08/18 16:51, Julien Thierry wrote:
>> The cpu_enable callback for VHE feature requires all alternatives to have
>> been applied. This prevents applying VHE alternative separately from the
>> rest.
>>
>> Use an alternative depending on VHE feature to know whether VHE
>> alternatives have already been applied.
> 
>> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
>> index 1e433ac..3bc1c8b 100644
>> --- a/arch/arm64/kernel/cpufeature.c
>> +++ b/arch/arm64/kernel/cpufeature.c
>> @@ -1022,8 +1024,15 @@ static void cpu_copy_el2regs(const struct arm64_cpu_capabilities *__unused)
>>   	 * that, freshly-onlined CPUs will set tpidr_el2, so we don't need to
>>   	 * do anything here.
>>   	 */
>> -	if (!alternatives_applied)
>> -		write_sysreg(read_sysreg(tpidr_el1), tpidr_el2);
>> +	asm volatile(ALTERNATIVE(
>> +		"mrs	%0, tpidr_el1\n"
>> +		"msr	tpidr_el2, %0",
>> +		"nop\n"
>> +		"nop",
>> +		ARM64_HAS_VIRT_HOST_EXTN)
>> +		: "+r" (tmp)
>> +		:
>> +		: "memory");
>>   }
>>   #endif
> 
> Catalin's preference was to keep this all in C:
> https://patchwork.kernel.org/patch/10007977/
> , for which we need to know if 'the' alternative has been applied.
> 
> I suspect there may be more registers in this list if we have to switch to
> another EL2 register using alternatives. (but I don't have an example).
> 
> Could we make 'alternatives_applied' a macro that takes the cap as an argument?

Another crude way of doing this would be, take the action in the matches() of
a boot capability, when scope contains SCOPE_BOOT_CPU (to make sure that we
don't do this always for other "matches()" call backs via "this_cpu_has_cap()")
and get rid of this "enable()". But that looks more hackish.

Suzuki