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

[Emilio G. Cota]

On Tue, Oct 02, 2018 at 07:48:20 +0100, Alex Bennée wrote:
> 
> Emilio G. Cota <address@hidden> 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.

It just adds a load to get the mask, which will most likely be
in the L1. The value is not used after 3 instructions later, when
the L1 read will have completed.

> 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.

What I implemented is what you suggest, but with dynamic resizing based
on usage. I'm keeping the current CPU_TLB_SIZE as the minimum size, and
took Pranith's TCG_TARGET_TLB_MAX_INDEX_BITS definitions (from 2017)
to limit the max tlb size per mmu.

I'll prepare an RFC.

Thanks,

                Emilio