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

[Philippe Mathieu-Daudé]

+Ard & Leif for EDK2.

On 11/19/20 12:45 PM, Alex Bennée wrote:
> Philippe Mathieu-Daudé <philmd@redhat.com> writes:
>> 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).
> 
> IIRC the two questions that came up were:
> 
>   - what would reading memory not covered by a file look like (0's or
>     something more like real HW, 7f?).

For NOR flashes erased bit is high, programmed bit is low, so: 0xff.

> 
>   - what happens when the guest writes beyond the bounds of a backing
>     file?

Report an hardware error, so guest firmware have a chance to do
do something (not sure what, beside rebooting...).

> 
> I'm guessing for these cloudy type applications no one cares about
> persistence of EFI variables? Maybe we just need a formulation for the
> second pflash which is explicit about writes being ephemeral while also
> being accepted?

Someone suggested adding a new machine type to QEMU to be able to use
smaller flash for cloud usage (but I don't remember who). Then EDK2
could be built with for this new flash size.

Regards,

Phil.