Re: How CPU(x86) resolve kernel address

[ebiederm@xmission.com (Eric W. Biederman)]

Sanket Rathi <sanket.rathi@cdac.ernet.in> writes:

> I read all about the memory management in linux. all thing are clear to me
> like there is 3GB space for user procee and 1GB for kernel and thats why
> kernel address always greater then 0xC0000000. But one thing is not clear
> that is for kernel address there is no page table, actually there is no
> need because this is one to one mapping to physical memory but who resolve
> kernel address to actual physical address how CPU(X86) perform this task
> because when we do DMA we have to give actual physical address by
> virt_to_phys() so what is the mechanism by which CPU translate kernel
> address into physical address ( Somewhere i heard that CPU ignore some of
> the upper bits of address if so then how much bits and why).

Ouch virt_to_phys ouch! ouch! ouch!  Don't do that.

At the very least use virt_to_bus.  And almost certainly use
pci_alloc_consistent.  On x86 the devices and the cpu happen to see
the same addresses for memory.  On other architecture this just
doesn't work.

read:
Documentation/DMA-mapping.txt
Documentation/IO-mapping.txt

As for the page tables yes the kernel uses them.  But because it has
a simple mapping from virtual to physical address it can do a lot of
optimizations that don't normally work.  Like using 4MB pages.

Note that in the vmalloc region we use normal sized pages, and that
vmalloc actually has a smaller address space than normal kernel
memory, and there is a minor performance hit for using it, as the
normal optimization do not apply.

Eric
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