Re: [Qemu-devel] [PATCH v2] hw/block: report when pflash backing file isn't aligned

From: Laszlo Ersek <lersek@redhat.com>
To: Markus Armbruster <armbru@redhat.com>
Cc: stappers@stappers.nl, "Alex Bennée" <alex.bennee@linaro.org>,
	qemu-devel@nongnu.org
Subject: Re: [Qemu-devel] [PATCH v2] hw/block: report when pflash backing file isn't aligned
Date: Mon, 18 Feb 2019 12:56:37 +0100	[thread overview]
Message-ID: <36adf8c8-009e-dc91-475b-c3cbf34fbd21@redhat.com> (raw)
In-Reply-To: <87tvh4t1v7.fsf@dusky.pond.sub.org>

On 02/16/19 12:21, Markus Armbruster wrote:
> Laszlo Ersek <lersek@redhat.com> writes:
>> On 02/15/19 17:01, Markus Armbruster wrote:

>>> Using whatever size the image has is sloppy modelling.
>>
>> Maybe so, but it's also very convenient, and also quite important, right
>> now (given the multiple firmware image sizes used in the wild).
>>
>>> A machine may come in minor variations that aren't worth their own
>>> machine type.  One such variation could be a different flash chip size.
>>> Using the image size to select one from the set of fixed sizes is
>>> tolerable.
>>
>> The problem is that this requires coordination between QEMU and firmware
>> development.
>>
>> (Well, I have to agree that the present patch is *already* that kind of
>> coordination;
> 
> We've always had that kind of coordination.  It just happens to be less
> tight in the case of PC firmware in flash than in most other cases.
> 
>>               my point is that when I introduced the 4MB build for OVMF,
>> I didn't have to touch QEMU. In retrospect, I'm extremely thankful for
>> that, as the introduction of the 4MB build was difficult in itself.)
> 
> You don't actually need "flash size is whatever the image size is" for
> that.  "Use image size to select one from the set of fixed sizes" should
> suffice.
> 
> Actually, the PC machines currently comply, just with a rather large
> set: { n * 4KiB | 1 <= n <= 2048 }.
> 
> I very much doubt PC firmware sizes other than powers of two between
> 64KiB and 8MiB matter.  Have you ever seen real flash chips with sizes
> like 64140KiB?

Honestly, I wouldn't know. I haven't seen any physical flash chip (as
in, directing my gaze at it). I also don't know what the usual flash
chip sizes are on physical boards (e.g. I have no clue what my laptop uses).

I think a jump from 4MB to 8MB would be too large. It gives too much of
sudden convenience to firmware developers. I do agree "7MB" looks quite
lame. I really wonder about the flash sizes used on physical UEFI boards.

Thanks,
Laszlo