Re: [PATCH v2 0/8] pdx: introduce a new compression algorithm

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, Jun 27, 2025 at 07:08:29PM -0700, Stefano Stabellini wrote:
> Hi Roger,
> 
> We have an ARM board with the following memory layout:
> 
> 0x0-0x80000000, 0, 2G
> 0x800000000,0x880000000, 32GB, 2G
> 0x50000000000-0x50080000000 5T, 2GB 
> 0x60000000000-0x60080000000 6T, 2GB 
> 0x70000000000-0x70080000000 7T, 2GB 

With the current PDX mask compression you could compress 4bits AFAICT.

> It looks like your PDX series is exactly what we need.  However, I tried
> to use it and it doesn't seem to be hooked properly on ARM yet. I spent
> some time trying to fix it but I was unsuccessful.

Hm, weird.  It shouldn't need any special hooking, unless assumptions
about the existing PDX mask compression have leaked into ARM code.

> As far as I can tell the following functions need to be adjusted but I
> am not sure the list is comprehensive:
> 
> xen/arch/arm/include/asm/mmu/mm.h:maddr_to_virt

At least for CONFIG_ARM_64 this seems to be implemented correctly, as
it's using maddr_to_directmapoff() which should have the correct
translation between paddr -> directmap virt.

Also given the memory map above the adjustments done in ARM to remove
any initial memory map offset should be no-ops, since I expect
base_mfn == 0 in setup_directmap_mappings() in that particular case,
and then directmap_mfn_start = directmap_base_pdx = 0 and
directmap_virt_start = DIRECTMAP_VIRT_START.  FWIW, if ARM uses offset
compression the special casing about removing the initial gap can be
removed, as the compression should already take care of that.

> xen/arch/arm/mmu/mm.c:setup_frametable_mappings
> xen/arch/arm/setup.c:init_pdx

I've attempted to adjust init_pdx() myself so it works with the new
generic PDX compression setup, it seemed to work fine on the CI, but I
don't have any real ARM machines to test myself.

Is there a way I could reproduce the issue(s) you are seeing with
QEMU?

I'm already working on v3, as this version implementation of
mfn_valid() is buggy.  Maybe that's what you are hitting?

Regards, Roger.