arm64: Increase the payload memory region size to 512MB
kvmtool uses the same memory map for 64bit and 32bit guests, where it
copies the kernel, the initrd and DTB in the bottom 256MB.
The restriction on placing everything in the bottom 256MB comes from the
aarch32 boot protocol, where the kernel, DTB and initrd must be placed in a
region covered by the low-memory mapping. The size of the lowmem region
varies based on the kernel-userspace split, which is a Kconfig option, and
on the vmalloc area size, which can be specified by the user on the kernel
command line. Hence kvmtool's choice of using the bottom 256MB as a
reasonable compromise which has worked well so far.
Sina has reported in private that they were unable to create a 64bit
virtual machine with a 351MB initrd, and that's due to the 256MB
restriction placed on the arm64 payload layout.
This restriction is not found in the arm64 boot protocol: booting.rst in
the Linux v6.12 source tree specifies that the kernel and initrd must be
placed in the same 32GB window. There is also a mention of kernels prior
to v4.2 requiring the DTB to be placed within a 512MB region starting at
the kernel image minus the kernel offset.
Increase the payload region size to 512MB for arm64, which will provide
maximum compatibility with Linux guests, while allowing for larger initrds
or kernel images. This means that the gap between the DTB (or initrd, if
present) and the kernel is larger now.
For 32 bit guests, the payload region size has been kept unchanged, because
it has proven adequate so far.
Reported-by: Abdollahi Sina <s.abdollahi22@imperial.ac.uk>
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Link: https://lore.kernel.org/r/20241128151246.10858-5-alexandru.elisei@arm.com
Signed-off-by: Will Deacon <will@kernel.org>
4 files changed
