Re: [Qemu-devel] [RFC PATCH] hw/arm/virt: use variable size of flash device to save memory

[Peter Maydell]

On Mon, 25 Mar 2019 at 12:53, Xiang Zheng <address@hidden> wrote:
>
> Currently we fill the VIRT_FLASH space with two 64MB NOR images when
> using persistent UEFI variables on QEMU. Actually we only use a very
> small part of the memory while the rest significant large part of
> memory is wasted.
>
> This patch creates and maps a variable size of flash device instead of
> a mandatory 64MB one to save memory.
>
> Signed-off-by: Xiang Zheng <address@hidden>
> ---
>
> This patch might be insufficient since it also needs to modify the flash size
> in ACPI and DTB.
>
> BTW, I don't understand why it requires the two NOR images to be exactly 64MB
> in size when using -pflash.

I don't think we should do this. The board should in general
create the same hardware visible to the guest, not change
it based on subtle things like the size of the image files.

The reason why the flash images must be 64MB in size
when using -pflash is that they are the backing store
for a writable device. Suppose you have 1MB of data in your
backing image that you pass to QEMU and then the guest writes
to the last block of the flash device. The new data
written by the guest to the end of the device has to be
stored somewhere, so the file has to be large enough
to cover the whole of the flash area.

thanks
-- PMM