Re: [PATCH v3 04/13] arm64: alternatives: use tpidr_el2 on VHE hosts

James Morse
2017-10-13T16:50:45Z

Hi Catalin,

On 13/10/17 16:31, Catalin Marinas wrote:
> On Fri, Sep 22, 2017 at 07:26:05PM +0100, James Morse wrote:
>> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
>> index cd52d365d1f0..8e4c7da2b126 100644
>> --- a/arch/arm64/kernel/cpufeature.c
>> +++ b/arch/arm64/kernel/cpufeature.c

>> @@ -1308,3 +1309,25 @@ static int __init enable_mrs_emulation(void)
>> }
>>
>> late_initcall(enable_mrs_emulation);
>> +
>> +int cpu_copy_el2regs(void *__unused)
>> +{
>> + int do_copyregs = 0;
>> +
>> + /*
>> + * Copy register values that aren't redirected by hardware.
>> + *
>> + * Before code patching, we only set tpidr_el1, all CPUs need to copy
>> + * this value to tpidr_el2 before we patch the code. Once we've done
>> + * that, freshly-onlined CPUs will set tpidr_el2, so we don't need to
>> + * do anything here.
>> + */
>> + asm volatile(ALTERNATIVE("mov %0, #1", "mov %0, #0",
>> + ARM64_HAS_VIRT_HOST_EXTN)
>> + : "=r" (do_copyregs) : : );
>
> Can you just do:
>
> if (cpu_have_const_cap(ARM64_HAS_VIRT_HOST_EXTN))
> write_sysreg(read_sysreg(tpidr_el1), tpidr_el2);
>
> At this point the capability bits should be set and the jump labels
> enabled.

These are enabled too early, we haven't done patching yet.

We need to copy tpidr_el1 -> tpidr_el2 on all CPUs that are online before code
patching.

After code patching new CPUs set tpidr_el2 when they come online, so we don't
need to do the copy... but this enable method is still called. There is nothing
for us to do, and tpidr_el1 now contains junk, so the copy

cpu_have_const_cap() is great for knowing if we have a feature, here we want to
know if we've done the patching for this feature.

I can wrap the ALTERNATIVE() into a helper, something like:
> arm64_alternatives_applied(ARM64_HAS_VIRT_HOST_EXTN)

which should make it clearer.

Christoffer had the same question at connect, so I evidently haven't found the
right way of describing this yet.

> Otherwise:
>
> Reviewed-by: Catalin Marinas <catalin.marinas-5wv7dgnIgG8@public.gmane.org>

Thanks for taking a look, I'll leave this RB until your happy with the
ALTERNATIVE() hackery above.

James