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

[Laszlo Ersek]

On 03/25/19 14:11, Peter Maydell wrote:
> 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.

I consider this question/behavior board-specific.

For the "virt" board, I agree with you (Peter), simply because that's
how the "virt" board has always worked.

(See also the tangentially related series

  [Qemu-devel] [PATCH v8 0/2]
  pflash: Require backend size to match device, improve errors

at

  http://mid.mail-archive.com/address@hidden

. This series still lets each board decide the question for itself, but
it improves error detection and reporting.)

Thanks,
Laszlo

> 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
>