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[[!tag open_issue_documentation]]
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
---
IRC, freenode, #hurd, 2011-06-09
<braunr> btw, how can gnumach use 1 GiB of RAM ? did you lower the
user/kernel boundary address ?
<youpi> I did
<braunr> 2G ?
<youpi> yes
<braunr> ok
<youpi> it doesn't make so much sense to let processes have 3G addressing
space when there can't be more that 1G physical memory
<braunr> that's sad for an operating system which does most things by
mapping memory eh
<youpi> well, if a process wants to map crazy things, 3G may be tight
already
<youpi> e.g. ext2fs
<braunr> yes
<youpi> so there's little point in supporting them
<braunr> we need hurd/amd64
<youpi> and there's quite some benefit in shrinking them to 2G
<youpi> yes
<youpi> actually even 2G may become a bit tight
<youpi> webkit linking needs about 1.5-2GiB
<youpi> things become really crazy
<braunr> wow
<braunr> i remember the linux support for 4G/4G split when there was enough
RAM to fill the kernel space with struct page entries
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