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

From: Andrew Jones
Subject: Re: [RFC 5/9] target/arm: introduce CPU feature dependency mechanism
Date: Thu, 13 Aug 2020 14:48:21 +0200

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.


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