Re: [RFC PATCH 0/5] ARM: reduce the memory consumed when mapping UEFI flash images

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 11/16/20 2:48 PM, Markus Armbruster wrote:
> Philippe Mathieu-Daudé <philmd@redhat.com> writes:
> 
>> Hi David,
>>
>> On 11/16/20 11:42 AM, David Edmondson wrote:
>>> Currently ARM UEFI images are typically built as 2MB/768kB flash
>>> images for code and variables respectively. These images are both then
>>> padded out to 64MB before being loaded by QEMU.
>>>
>>> Because the images are 64MB each, QEMU allocates 128MB of memory to
>>> read them, and then proceeds to read all 128MB from disk (dirtying the
>>> memory). Of this 128MB less than 3MB is useful - the rest is zero
>>> padding.
>>
>> 2 years ago I commented the same problem, and suggested to access the
>> underlying storage by "block", as this is a "block storage".
>>
>> Back then the response was "do not try to fix something that works".
>> This is why we choose the big hammer "do not accept image size not
>> matching device size" way.
>>
>> While your series seems to help, it only postpone the same
>> implementation problem. If what you want is use the least memory
>> required, I still believe accessing the device by block is the
>> best approach.
> 
> "Do not try to fix something that works" is hard to disagree with.
> However, at least some users seem to disagree with "this works".  Enough
> to overcome the resistance to change?

Yeah, at least 3 big users so far:

- Huawei
https://www.mail-archive.com/qemu-devel@nongnu.org/msg607292.html
- Tencent
https://www.mail-archive.com/qemu-devel@nongnu.org/msg742066.html
- Oracle
(this series).

Then Huawei tried the MicroVM approach:
https://www.mail-archive.com/qemu-devel@nongnu.org/msg680103.html

I simply wanted to save David time by remembering this other approach,
since Peter already commented on David's one (see Huawei link).

Regards,

Phil.