Re: [PATCH 0/3] Highmem support for 32-bit RISC-V

afzal mohammed
2020-05-04T09:10:23Z

[ +linux-arm-kernel

Context: This is regarding VMSPLIT_4G_4G support for 32-bit ARM as a
possible replacement to highmem. For that, initially, it is being
attempted to move static kernel mapping from lowmem to vmalloc space.

in next reply, i will remove everyone/list !ARM related ]

Hi,

On Sun, May 03, 2020 at 10:20:39PM +0200, Arnd Bergmann wrote:

> Which SoC platform are you running this on? Just making
> sure that this won't conflict with static mappings later.

Versatile Express V2P-CA15 on qemu, qemu options include --smp 2 &
2GB memory.

BTW, i could not convince myself why, except for DEBUG_LL, static io
mappings are used.

>
> One problem I see immediately in arm_memblock_init()

Earlier it went past arm_memblock_init(), issue was clearing the page
tables from VMALLOC_START in devicemaps_init() thr' paging_init(),
which was like cutting the sitting branch of the tree.

Now it is crashing at debug_ll_io_init() of devicemap_init(), and
printascii/earlycon was & is being used to debug :). Things are going
wrong when it tries to create mapping for debug_ll. It looks like a
conflict with static mapping, which you mentioned above, at the same
time i am not seeing kernel static mapping in the same virtual
address, need to dig deeper.

Also tried removing DEBUG_LL, there is a deafening silence in the
console ;)

> is that it uses
> __pa() to convert from virtual address in the linear map to physical,
> but now you actually pass an address that is in vmalloc rather than
> the linear map.

__virt_to_phys_nodebug() which does the actual work on __pa() invocation
has been modifed to handle that case (ideas lifted from ARM64's
implementation), though currently it is a hack as below (and applicable
only for ARM_PATCH_PHYS_VIRT disabled case), other hacks being
VMALLOC_OFFSET set to 0 and adjusting vmalloc size.

static inline phys_addr_t __virt_to_phys_nodebug(unsigned long x)
{
phys_addr_t __x = (phys_addr_t)x;

if (__x >= 0xf0000000)
return __x - KIMAGE_OFFSET + PHYS_OFFSET;
else
return __x - PAGE_OFFSET + PHYS_OFFSET;
}

Regards
afzal