Re: [Qemu-devel] [PATCH 0/3] per-TLB lock

[Paolo Bonzini]

On 02/10/2018 23:29, Emilio G. Cota wrote:
> This series introduces a per-TLB lock. This removes existing UB
> (e.g. memset racing with cmpxchg on another thread while flushing),
> and in my opinion makes the TLB code simpler to understand.
> 
> I had a bit of trouble finding the best place to initialize the
> mutex, since it has to be called before tlb_flush, and tlb_flush
> is called quite early during cpu initialization. I settled on
> cpu_exec_realizefn, since then cpu->env_ptr has been set
> but tlb_flush hasn't yet been called.
> 
> Perf-wise this change does have a small impact (~2% slowdown for
> the aarch64 bootup+shutdown test; 1.2% comes from using atomic_read
> consistently), but I think this is a fair price for avoiding UB.

The UB is unlikely to be an issue in practice, but I like the
simplicity.  In retrospect it was premature optimization.

Paolo