Re: [Xen-devel] [PATCH 06/12] xen/arm64: sysreg: Implement the 32-bit helpers using the 64-bit helpers

[Julien Grall <Julien.Grall@xxxxxxx>]

Hi,

On 27/03/2019 19:15, Andrew Cooper wrote:
> On 27/03/2019 18:45, Julien Grall wrote:
>> Clang is pickier than GCC for the register size in asm statement. It
>> expects the register size to match the value size.
>>
>> The instructions msr/mrs are expecting a 64-bit register. This means the
>> implementation of the 32-bit helpers is not correct. The easiest
>> solution is to implement the 32-bit helpers using the 64-bit helpers.
>>
>> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> 
> (I don't have an opinion on how to fix the issue, but)
> 
> Are SYSREGS actually always 64 bits even on arm32, and these helpers
> just a shorthand for the lower 32 bits, or is this an
> effectively-unnecessary constraint imposed by Clang?

This code is Arm64 specific. On Arm64, system registers are always 
64-bits, and therefore the instructions msr/mrs require a 64-bit register.

So the complain from Clang is valid here. It happens that some of the 
registers have the top 32-bit RES0 in current architecture. One could 
argue that this is not very future-proof and we should get rid of {READ, 
WRITE)_SYSREG32.

Unfortunately, the callers are expecting a 32-bit value. I need to 
investigate all the callers to ensure no one is transforming the value 
to 64-bit again. I don't really want to block clang support on that, so 
I have added an action for fixing this later on.

Cheers,

-- 
Julien Grall
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