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Re: [RFC PATCH 0/3] hw/pflash_cfi01: Reduce memory consumption when flas

From: David Edmondson
Subject: Re: [RFC PATCH 0/3] hw/pflash_cfi01: Reduce memory consumption when flash image is smaller than region
Date: Tue, 16 Feb 2021 15:22:04 +0000

On Tuesday, 2021-02-16 at 16:03:05 +01, Philippe Mathieu-Daudé wrote:

> I am not a block expert, but I wonder if something like this could
> be used:
> - create a raw (parent) block image of 64MiB
> - add a raw (child) block with your 768kB of VARS file
> - add a null-co (child) block of 63Mib + 256kiB
> - pass the parent block to the pflash device

I'm not clear how this would behave if there is a write to the device at
(say) 1MiB.

More philosophically, how should it behave?

My expectation was that if the machine says that there is 64MiB of
writable flash, we have to allow writes throughout the full 64MiB and
(significantly) persist them to the backing block device.

Just because the backing block device started out 768KiB big doesn't
mean that we should not write to the remaining extent if that's what the
VM attempts.

Would the above approach achieve that? (It doesn't sound like it.)

No visible means of support and you have not seen nothing yet.

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