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

From: David Edmondson
Subject: Re: [RFC PATCH 0/5] ARM: reduce the memory consumed when mapping UEFI flash images
Date: Mon, 16 Nov 2020 13:43:18 +0000

On Monday, 2020-11-16 at 12:39:46 +01, Philippe Mathieu-Daudé wrote:

> 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.

I agree that would reduce the size further, but it feels like it may be
a case of diminishing returns.

Currently the consumption for a single guest is 128MB. This change will
bring it down under 3MB, with the block approach perhaps reducing that
to zero (there will be some buffer block usage presumably, and perhaps a
small cache would make sense, so it won't really be zero).

Scaling that up for 100 guests, we're going from 12.5GB now down to
under 300MB with the changes, and again something around zero with the
block based approach.

I guess that it would mean that the machine model wouldn't have to
change, as we could return zeros for reads outside the underlying device
extent. This seems like the most interesting part - are there likely to
be any consequential *benefits* from reducing the amount of guest
address space consumed by the flash regions?

> Regards,
> Phil.
>> On a machine with 100 VMs this wastes over 12GB of memory.
>> This set of patches aims to reclaim the wasted memory by allowing QEMU
>> to respect the size of the flash images and allocate only the
>> necessary memory. This will, of course, require that the flash build
>> process be modified to avoid padding the images to 64MB.
>> Because existing machine types expect the full 128MB reserved for
>> flash to be occupied, do so for machine types older than virt-5.2. The
>> changes are beneficial even in this case, because while the full 128MB
>> of memory is allocated, only that required to actually load the flash
>> images from disk is touched.
>> David Edmondson (5):
>>   hw/block: blk_check_size_and_read_all should report backend name
>>   hw/block: Flash images can be smaller than the device
>>   hw/arm: Convert assertions about flash image size to error_report
>>   hw/arm: Flash image mapping follows image size
>>   hw/arm: Only minimise flash size on older machines
>>  hw/arm/trace-events      |  2 +
>>  hw/arm/virt-acpi-build.c | 30 ++++++++------
>>  hw/arm/virt.c            | 86 +++++++++++++++++++++++++++++-----------
>>  hw/block/block.c         | 26 ++++++------
>>  include/hw/arm/virt.h    |  2 +
>>  5 files changed, 97 insertions(+), 49 deletions(-)

Please forgive me if I act a little strange, for I know not what I do.

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