[PATCH v5 1/4] riscv: Move kernel mapping to vmalloc zone
benh at kernel.crashing.org
Tue Jul 21 19:11:02 EDT 2020
On Tue, 2020-07-21 at 14:36 -0400, Alex Ghiti wrote:
> > > 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 ?
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
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