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

[Alex Ghiti]

Hi Benjamin,

Le 7/21/20 Ã 7:11 PM, Benjamin Herrenschmidt a ÃcritÂ:
> 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 ?

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