Re: [RFC 5/9] target/arm: introduce CPU feature dependency mechanism

[Peng Liang]

On 8/13/2020 8:48 PM, Andrew Jones wrote:
> On Thu, Aug 13, 2020 at 06:26:53PM +0800, Peng Liang wrote:
>> Some CPU features are dependent on other CPU features.  For example,
>> ID_AA64PFR0_EL1.FP field and ID_AA64PFR0_EL1.AdvSIMD must have the same
>> value, which means FP and ADVSIMD are dependent on each other, FPHP and
>> ADVSIMDHP are dependent on each other.
>>
>> This commit introduces a mechanism for CPU feature dependency in
>> AArch64.  We build a directed graph from the CPU feature dependency
>> relationship, each edge from->to means the `to` CPU feature is dependent
>> on the `from` CPU feature.  And we will automatically enable/disable CPU
>> feature according to the directed graph.
>>
>> For example, a, b, and c CPU features are in relationship a->b->c, which
>> means c is dependent on b and b is dependent on a.  If c is enabled by
>> user, then a and b is enabled automatically.  And if a is disabled by
>> user, then b and c is disabled automatically.
> 
> And what if a is mutually exclusive with b? I.e. a and b can both be
> disabled, but only a or b may be enabled.
> 
> Thanks,
> drew
> 
> .
> 

Currently, a and b will be both enabled or disabled.  For example, a and b are
in relationship a->b, which means b is dependent on a.  If -cpu host,a=off,b=on,
then both a and b are enabled.  If -cpu host,b=on,a=off, then both a and b are
disabled.  Maybe we should report an error to user in this scenario?

Thanks,
Peng