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

From: David Edmondson
Subject: [RFC PATCH 0/5] ARM: reduce the memory consumed when mapping UEFI flash images
Date: Mon, 16 Nov 2020 10:42:11 +0000

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

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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