a portable SG_NOEXEC segment attribute was added to allow
non-executable (physical) segments. which will set the
PTENOEXEC bits for putmmu().
in the future, this can be used to make non-executable
stack / bss segments.
the SG_DEVICE attribute was added to distinguish between
mmio regions and uncached memory. only matterns on arm64.
on arm, theres the issue that PTEUNCACHED would have
no bits set when using the hardware bit definitions.
this is the reason bcm, kw, teg2 and omap kernels use
arteficial PTE constants. on zynq, the XN bit was used
as a hack to give PTEUNCACHED a non-zero value and when
the bit is clear then cache attributes where added to
the pte.
to fix this, PTECACHED constant was added.
the portable mmu code in fault.c will now explicitely set
PTECACHED bits for cached memory and PTEUNCACHED for
uncached memory. that way the hardware bit definitions
can be used everywhere.
on the 2GB and 4GB raspberry pi 4 variants, there are two
memory regions for ram:
[0x00000000..0x3e600000)
[0x40000000..0xfc000000)
the framebuffer is somewhere at the end of the first
GB of memory.
to handle these, we append the region base and limit
of the second region to *maxmem= like:
*maxmem=0x3e600000 0x40000000 0xfc000000
the mmu code has been changed to have non-existing
ram unmapped and mmukmap() now uses small 64K pages
instead of 512GB pages to avoid aliasing (framebuffer).
the VIRTPCI mapping has been removed as we now have
a proper vmap() implementation which assigns vritual
addresses automatically.
this adds a 4GB KMAP window into the kernel address space
so we can access all physical ram on raspberry pi 4 for
user pages.
note that kernel memory above KZERO is still limited
to 1GB because of DMA restrictions.
