Re: [PATCH] arm64: mm: set ZONE_DMA size based on early IORT scan

[Ard Biesheuvel <ardb@kernel.org>]

On Mon, 12 Oct 2020 at 17:50, Catalin Marinas <catalin.marinas@arm.com> wrote:
>
> On Mon, Oct 12, 2020 at 04:19:08PM +0200, Ard Biesheuvel wrote:
> > On Mon, 12 Oct 2020 at 13:24, Catalin Marinas <catalin.marinas@arm.com> wrote:
> > > On Mon, Oct 12, 2020 at 12:43:05PM +0200, Ard Biesheuvel wrote:
> > > > On Mon, 12 Oct 2020 at 11:30, Ard Biesheuvel <ardb@kernel.org> wrote:
> > > > > On Mon, 12 Oct 2020 at 11:28, Catalin Marinas <catalin.marinas@arm.com> wrote:
> > > > > > On Sat, Oct 10, 2020 at 11:31:53AM +0200, Ard Biesheuvel wrote:
> > > > > > > diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> > > > > > > index f0599ae73b8d..829fa63c3d72 100644
> > > > > > > --- a/arch/arm64/mm/init.c
> > > > > > > +++ b/arch/arm64/mm/init.c
> > > > > > > @@ -191,6 +191,14 @@ static void __init zone_sizes_init(unsigned long min, unsigned long max)
> > > > > > >       unsigned long max_zone_pfns[MAX_NR_ZONES]  = {0};
> > > > > > >
> > > > > > >  #ifdef CONFIG_ZONE_DMA
> > > > > > > +     if (IS_ENABLED(CONFIG_ACPI)) {
> > > > > > > +             extern unsigned int acpi_iort_get_zone_dma_size(void);
> > > > > >
> > > > > > Nitpick: can we add this prototype to include/linux/acpi_iort.h?
> > > > > >
> > > > > > > +
> > > > > > > +             zone_dma_bits = min(zone_dma_bits,
> > > > > > > +                                 acpi_iort_get_zone_dma_size());
> > > > > > > +             arm64_dma_phys_limit = max_zone_phys(zone_dma_bits);
> > > > > > > +     }
> > > > > > > +
> > > > > > >       max_zone_pfns[ZONE_DMA] = PFN_DOWN(arm64_dma_phys_limit);
> > > > > >
> > > > > > I think we should initialise zone_dma_bits slightly earlier via
> > > > > > arm64_memblock_init(). We'll eventually have reserve_crashkernel()
> > > > > > called before this and it will make use of arm64_dma_phys_limit for
> > > > > > "low" reservations:
> > > > > >
> > > > > > https://lore.kernel.org/linux-arm-kernel/20200907134745.25732-7-chenzhou10@huawei.com/
> > > > > >
> > > > >
> > > > > We don't have access to the ACPI tables yet at that point.
> > > >
> > > > Also, could someone give an executive summary of why it matters where
> > > > the crashkernel is loaded? As far as I can tell, reserve_crashkernel()
> > > > only allocates memory for the kernel's executable image itself, which
> > > > can usually be loaded anywhere in memory. I could see how a
> > > > crashkernel might need some DMA'able memory if it needs to use the
> > > > hardware, but I don't think that is what is going on here.
> > >
> > > I thought the crashkernel needs some additional reserved RAM as well to
> > > be able to run. It should not touch the original kernel's memory as it
> > > usually needs to dump it.
> >
> > Looking at the code, it is definitely allocating memory for the kernel
> > itself (as it refers to the 2 MB alignment requirement), and given
> > that we used to require the kernel to be at the base of the linear
> > region to even be able to access all of memory, I suspect that we
> > might be able to relax this requirement. Not sure what that means for
> > the userland tools, though.
>
> The 2MB is an interpretation of booting.txt that the DRAM must start at
> this alignment (not sure what we do these days, in lots of
> configurations we just use 4K pages for the linear map).
>

On 4k granule kernels, We still need 2 MB alignment today unless you
use a relocatable kernel. The reason is that virtual addresses are
assigned at link time, and we use section mappings to map the kernel.
If CONFIG_RELOCATABLE=y, the kernel can run happily at any 64k aligned
address (except for the 64k granule kernel with CONFIG_VMAP=y, which
needs 128k in this case)

So keeping a 2 MB alignment requirement in booting.txt still makes sense.

> However, the crashkernel=... range is meant for sufficiently large
> reservation to be able to run the kdump kernel, not just load the image.
>

Sure. But I was referring to the requirement that it is loaded low in
memory. Unless I am misunderstanding something, all we need for the
crashkernel to be able to operate is some ZONE_DMA memory in case it
is needed by the hardware, and beyond that, it could happily live
anywhere in memory.