Re: [Qemu-devel] [RFC 26/38] cpu: protect tb_jmp_cache with seqlock

[Paolo Bonzini]

> The seqlock for tb_jmp_cache is necessary the moment that the
> array can be wiped out with a memset(), as shown above. That
> function (tb_flush_jmp_cache) is called by tlb_flush_page,
> which has many callers.
> 
> One could argue that we could enforce calling tlb_flush_page to be
> a) always done by the owner thread or b) done while all others CPUs
> are paused.
> 
> I argue that worrying about that is not worth it; let's protect
> the array with a seqlock, which on TSO is essentially free, and
> worry about more important things.

Got it, this makes sense.

Paolo

> Wrt the next two patches:
> 
> Patch 27 is an improvement in that each TB has its own valid flag,
> which makes sense because this should only affect TB's that are
> trying to chain to/from it, not all TBs.
> 
> Patch 28 uses the RCU QLIST which to me seems cleaner and less
> error-prone than open-coding an RCU LIST.
> 
> Thanks,
> 
>               Emilio
>