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Re: [Qemu-arm] [PATCH v3 0/7] Runtime pagesize computation

From: Peter Maydell
Subject: Re: [Qemu-arm] [PATCH v3 0/7] Runtime pagesize computation
Date: Tue, 11 Oct 2016 13:18:07 -0700

On 11 October 2016 at 12:20, Richard Henderson <address@hidden> wrote:
> On 10/11/2016 12:08 PM, Peter Maydell wrote:
>> I would ideally have liked to finalize things much later, but
>> this is in practice hugely difficult because so many things
>> (in particular all the address space/memory system code)
>> assume the target page size is known.

> Unfortunate.  I suppose that 4k is still better than 1k, but
> I was hoping to get 16k or 64k (or higher) when the OS is
> configured to use such.  I.e. totally dynamically configurable
> upon write to the appropriate cpu register.

I think that would run into problems with migration:
the migration stream all works in guest-pages of ram and
a mismatch means migration doesn't work.

> Given how the memory subsystem already dynamically reconfigures
> itself for changes in address_space topology, I assumed page size
> changes would be trivial and fall out naturally.

The trouble is that all the data structures work in terms
of page sizes (even though we support sub-page allocations
those are still done by carving up a page-size chunk).
It could probably be done but it looked like a gargantuan
task so I decided this was a better compromise.

-- PMM

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