[RFC PATCH 0/3] hw/block/pflash: Mmap read-only backend files with MAP_SHARED

[Philippe Mathieu-Daudé]

Hi,

This series aims to reduce the memory footprint of flash devices
when the backing file is read-only.

When a backing file is read-only, the model considers the flash
is in "protected" mode. No write are allowed, but the MMIO
state machine is still usable.

This series introduces a new memory region helper to mmap files
and use it with the pflash device (only with read-only backing
files).

The goal is to reduce QEMU's memory footprint when multiple VMs
are instantiated using the same read-only backing file, which is
the case with the CODE flash from OVMF and AAVMF.

Previous attempts:

- 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
https://www.mail-archive.com/qemu-devel@nongnu.org/msg760065.html

RFC because yet another approach to tackle this technical debt,
and very little tested.

Regards,

Phil.

Philippe Mathieu-Daudé (3):
  exec/memory: Introduce memory_region_init_rom_device_from_file()
  hw/block/pflash: Move code around
  hw/block/pflash: use memory_region_init_rom_device_from_file()

 include/exec/memory.h   | 85 +++++++++++++++++++++++++++++++++++
 hw/block/pflash_cfi01.c | 34 ++++++++------
 hw/block/pflash_cfi02.c | 30 ++++++++-----
 softmmu/memory.c        | 98 +++++++++++++++++++++++++++++++++++++++++
 4 files changed, 224 insertions(+), 23 deletions(-)

-- 
2.26.2