Re: [Qemu-devel] best way to implement emulation of AArch64 tagged addresses

[Richard Henderson <rth@twiddle.net>]

On 04/04/2016 09:31 AM, Peter Maydell wrote:
> On 4 April 2016 at 17:28, Richard Henderson <rth@twiddle.net> wrote:
>> On 04/04/2016 08:51 AM, Peter Maydell wrote:
>>> In particular I think if you just do the relevant handling of the tag
>>> bits in target-arm's get_phys_addr() and its subroutines then this
>>> should work ok, with the exceptions that:
>>>    * the QEMU TLB code will think that [tag A + address X] and
>>>      [tag B + address X] are different virtual addresses and they will
>>>      miss each other in the TLB
>>
>>
>> Yep.  Not only miss, but actively contend with each other.
>
> Yes. Can we avoid that, or do we just have to live with it? I guess
> if the TCG fast path is doing a compare on full insn+tag then we
> pretty much have to live with it.

We have to live with it.  Implementing a more complex hashing algorithm in the 
fast path is probably a non-starter.

Hopefully if one is using multiple tags, they'll still be in the victim cache 
and so you won't have to fall back to the full tlb lookup.


r~