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IRC, freenode, #hurd, 2011-02-15
<braunr> etenil: originally, mach had its own virtual space (the kernel
space)
<braunr> etenil: in order to use linux 2.0 drivers, it now directly maps
physical memory, as linux does
<braunr> etenil: but there is nothing similar to kmap() or vmalloc() in
mach, so the kernel is limited to its 1 GiB
<braunr> (3 GiB userspace / 1 GiB kernelspace)
<braunr> that's the short version, there is a vmalloc() in mach, but this
trick made it behave almost like a kmalloc()
<antrik> braunr: the direct mapping is *only* for the benefit of Linux
drivers?...
<braunr> also, the configuration of segments limits the kernel space
<braunr> antrik: i'm not sure, as i said, this is the short version
<braunr> antrik: but there is a paper which describes the integration of
those drivers in mach
<etenil> you mean the linux 2.0 drivers?
<antrik> braunr: I read it once, but I don't remember anything about the
physical mapping in there...
<antrik> etenil: well, originally it was 1.3, but essentially that's the
same...
<braunr> i don't see any other reason why there would be a direct mapping
<braunr> except for performance (because you can use larger - even very
lage - pages without resetting the mmu often thanks to global pages, but
that didn't exist at the time)
IRC, freenode, #hurd, 2011-02-15
<antrik> however, the kernel won't work in 64 bit mode without some changes
to physical memory management
<braunr> and mmu management
<braunr> (but maybe that's what you meant by physical memory)
IRC, freenode, #hurd, 2011-02-16
<braunr> antrik: youpi added it for xen, yes
<braunr> antrik: but you're right, since mach uses a direct mapped kernel
space, the true problem is the lack of linux-like highmem support
<braunr> which isn't required if the kernel space is really virtual
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