Re: Status of some Arm features

[Pierrick Bouvier <pierrick.bouvier@linaro.org>]

On 11/19/24 02:09, Peter Maydell wrote:
> On Mon, 18 Nov 2024 at 23:33, Pierrick Bouvier
> <pierrick.bouvier@linaro.org> wrote:
>> I'm currently reviewing the QEMU Arm documentation, and I have a
>> question about the status of following features:
>>
>> 8.0:
>> - FEAT_DoubleLock, Double Lock
> 
> This is actually an "anti-feature" :-)  It is optional from v8.0
> and it must not be implemented from v9.0. We implement the handling
> of it based on the DOUBLELOCK fields in ID_AA64DFR0 and DBGDEVID
> (so it does the right thing on older named CPU types) and don't
> advertise it in "max".
>

Despite this singularity on versions implementation, should we list that 
in our documentation?

>> 8.2:
>> - FEAT_ASMv8p2, Armv8.2 changes to the A64 ISA (bfc and rev64 instructions)
> 
> This isn't a feature for CPU implementations; it's a feature for
> assemblers and disassemblers, which have to recognize BFC and
> REV64 mnemonics as being ways to write special-case flavours
> of the BFM and REV instructions.
> 

Reading the feature description [1] or the A-profile manual:
FEAT_ASMv8p2 introduces the BFC instruction to the A64 instruction set 
as an alias of BFM. It also requires that the BFC instruction and the 
A64 pseudo-instruction REV64 are implemented by assemblers.

I understand it's both introducing the BFC instructions *and also* 
ensure that BFC and REV64 are implemented by assemblers.
Is my interpretation wrong?

[1] 
https://developer.arm.com/documentation/109697/2024_09/Feature-descriptions/The-Armv8-2-architecture-extension

It seems to be the only feature (through all versions >= 8.0) that 
describes something this way, so I'm a bit puzzled about.

>> 8.4:
>> - FEAT_CNTSC, Generic Counter Scaling (hw/timer/sse-counter.c)
> 
> This is optional, and we don't implement it yet. (There's an
> open ticket for it in Linaro JIRA at
> https://linaro.atlassian.net/browse/QEMU-309 )
> 

Ok. For my personal knowledge, does the implementation in 
hw/timer/sse-counter.c is related to it?

> thanks
> -- PMM

Thanks,
Pierrick