Re: [Qemu-devel] ideas for improving TLB performance (help with TCG backend wanted)

["Alex Bennée" <alex.bennee@linaro.org>]

Emilio G. Cota <cota@braap.org> writes:

> On Thu, Sep 20, 2018 at 01:19:51 +0100, Alex Bennée wrote:
>> If we are going to have an indirection then we can also drop the
>> requirement to scale the TLB according to the number of MMU indexes we
>> have to support. It's fairly wasteful when a bunch of them are almost
>> never used unless you are running stuff that uses them.
>
> So with dynamic TLB sizing, what you're suggesting here is to resize
> each MMU array independently (depending on their use rate) instead
> of using a single "TLB size" for all MMU indexes. Am I understanding
> your point correctly?

Not quite - I think it would overly complicate the lookup to have a
differently sized TLB lookup for each mmu index - even if their usage
patterns are different.

I just meant that if we already have the cost of an indirection we don't
have to ensure:

    CPUTLBEntry tlb_table[NB_MMU_MODES][CPU_TLB_SIZE];
    CPUIOTLBEntry iotlb[NB_MMU_MODES][CPU_TLB_SIZE];

restrict their sizes so any entry in the 2D array can be indexed
directly from env. Currently CPU_TLB_SIZE/CPU_TLB_BITS is restricted by
the number of NB_MMU_MODES we have to support. But if each can be
flushed and managed separately we can have:

    CPUTLBEntry *tlb_table[NB_MMU_MODES];

And size CPU_TLB_SIZE for the maximum offset we can mange in the lookup
code. This is mainly driven by the varying
TCG_TARGET_TLB_DISPLACEMENT_BITS each backend has available to it.

>
> Thanks,
>
> 		E.


--
Alex Bennée