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Subject: Re: [PATCH v5 02/27] arm64: cpufeature: Use alternatives for VHE
 cpu_enable
To:     James Morse <james.morse@arm.com>,
        linux-arm-kernel@lists.infradead.org
Cc:     linux-kernel@vger.kernel.org, daniel.thompson@linaro.org,
        joel@joelfernandes.org, marc.zyngier@arm.com, mark.rutland@arm.com,
        christoffer.dall@arm.com, catalin.marinas@arm.com,
        will.deacon@arm.com, Suzuki K Poulose <suzuki.poulose@arm.com>
References: <1535471497-38854-1-git-send-email-julien.thierry@arm.com>
 <1535471497-38854-3-git-send-email-julien.thierry@arm.com>
 <1d425320-aed3-b949-1ec5-f378248aac4d@arm.com>
From:   Julien Thierry <julien.thierry@arm.com>
Message-ID: <85d74cec-2290-9e89-b155-f0f7c523b4a4@arm.com>
Date:   Wed, 12 Sep 2018 13:03:08 +0100
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Hi James,

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?
> 

I wanted to do this initially, the issue was that the alternatives 
framework works on regions to patch rather than caps to apply. So I 
found it a bit odd to associate the "code corresponding to cap was 
applied" with the alternative application.

Although in patch 3 with the feature mask I guess that at the end of the 
__apply_alternatives loop, if the cap was part of the mask passed, it 
can be considered as applied.
If the asm inline is problematic I can go that route.

Thanks,

-- 
Julien Thierry