Re: [PATCH v5 1/4] riscv: Move kernel mapping to vmalloc zone

Hi Benjamin,

Le 7/21/20 à 7:11 PM, Benjamin Herrenschmidt a écrit :

On Tue, 2020-07-21 at 14:36 -0400, Alex Ghiti wrote:

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I guess I don't understand why this is necessary at all.
Specifically: why
can't we just relocate the kernel within the linear map?  That would
let the
bootloader put the kernel wherever it wants, modulo the physical
memory size we
support.  We'd need to handle the regions that are coupled to the
kernel's
execution address, but we could just put them in an explicit memory
region
which is what we should probably be doing anyway.

Virtual relocation in the linear mapping requires to move the kernel
physically too. Zong implemented this physical move in its KASLR RFC
patchset, which is cumbersome since finding an available physical spot
is harder than just selecting a virtual range in the vmalloc range.

In addition, having the kernel mapping in the linear mapping prevents
the use of hugepage for the linear mapping resulting in performance loss
(at least for the GB that encompasses the kernel).

Why do you find this "ugly" ? The vmalloc region is just a bunch of
available virtual addresses to whatever purpose we want, and as noted by
Zong, arm64 uses the same scheme.

I don't get it :-)

At least on powerpc we move the kernel in the linear mapping and it
works fine with huge pages, what is your problem there ? You rely on
punching small-page size holes in there ?

ARCH_HAS_STRICT_KERNEL_RWX prevents the use of a hugepage for the kernel 
mapping in the direct mapping as it sets different permissions to 
different part of the kernel (data, text..etc).

At least in the old days, there were a number of assumptions that
the kernel text/data/bss resides in the linear mapping.

If you change that you need to ensure that it's still physically
contiguous and you'll have to tweak __va and __pa, which might induce
extra overhead.

Yes that's done in this patch and indeed there is an overhead to those 
functions.

Cheers,
Ben.
  

Thanks,

Alex