Re: [Qemu-devel] [PATCH] Huge TLB performance improvement

[Paul Brook <paul@codesourcery.com>]

> > Other targets have a hardware managed TLB. On a hardware managed TLB the
> > OS treats it as if it were infinite size, and invalidation only occurs
> > when a OS changes the mappings. On a software managed TLB "flushes" are
> > more likely to occur during normal operation as TLB slots are reused.
>
> The excessive flushing for mips happens because Qemu doesn't properly
> model the hardware's ASID handling.

Are you sure? IIUC changing the ASID causes a full qemu TLB flush. The code 
we're tweaking here is for single page flush.

Actually that gives me an idea. When a TLB entry with a different ASID gets 
evicted we currently flush that page. This should be a no-op because we 
already did a full flush when the ASID changed.

The other explanation is that the gest OS is manually doing a full TLB flush 
by manually evicting all the TLB entries. I'd hope that a sane guest OS would 
only do that as a last resort though.

Paul