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Re: [Qemu-devel] [PATCH v7 16/27] cputlb: add tlb_flush_by_mmuidx async

From: Richard Henderson
Subject: Re: [Qemu-devel] [PATCH v7 16/27] cputlb: add tlb_flush_by_mmuidx async routines
Date: Tue, 24 Jan 2017 12:44:33 -0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.6.0

On 01/24/2017 12:31 PM, Alex Bennée wrote:
>> Why don't we just pass in this bitmap in the first place?  It's much better
>> than having to use varargs in tlb_flush_by_mmuidx...
> We could. By not messing with the API it leaves the door open to having
> other non-MTTCG architectures that have lots of MMU indexes versus a
> hard limit based on page-size. That said I think the number of indexes
> also affects the size of the TLB so I guess the current design is
> limited for arbitrarily large sets if indexes?

We hard-limit at 12 indices, though even that is arguably too high.
I hope we never see more than PPC's current 8.

> Is ARM is the current outlier for this functionality? Apart from SPARC's
> two uses are we likely to see more architectures using this?

In theory, Alpha could use it to avoid ever flushing MMU_PHYS_IDX.  It appears
that there are a few others that could also avoid flushing a "mmu-disabled" 

I suspect that PPC could make good use of it as well.  That one's complicated
enough that it probably needs a good going over -- especially for the non-local


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