[PATCH 00/19] Fix and rewrite arm64 spectre mitigations

[Will Deacon]

On the off-chance that anybody's reading this...

On Fri, Sep 18, 2020 at 05:47:10PM +0100, Will Deacon wrote:
> The temptation was to remove the code entirely, but after putting in
> some effort to untangle it, we ended up knocking it into a much better
> shape. Although that doesn't change the fact that we can't test it very
> well, it certainly appears to behave better than the old code in situations
> such as:
> 
>   - Err... wanting mitigation on more than one CPU
> 
>   - Not changing the mitigation state at runtime (i.e. after userspace
>     has started running)
> 
>   - Gracefully handling failure to bring late CPUs online (previously
>     this would only happen _after_ updating the mitigation state!)
> 
>   - Clear separation between mitigation state (am I vulnerable?) and
>     policy (the user wants to go fast)
> 
>   - Removal of the hideously expensive "dynamic" Spectre-v2 mitigation
>     for KVM guests

^^^ This should be 'hideously expensive "dynamic" Spectre-v4 mitigation'.
The firmware call itself is helpfully named "workaround 2", so I always
get them mixed up.

Will