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

From: Philippe Mathieu-Daudé
Subject: Re: [RFC PATCH 0/5] ARM: reduce the memory consumed when mapping UEFI flash images
Date: Mon, 16 Nov 2020 12:39:46 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.4.0

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.



> 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(-)

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