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

From: afzal mohammed <afzal.mohd.ma@gmail.com>
To: Arnd Bergmann <arnd@arndb.de>
Cc: Russell King <linux@armlinux.org.uk>,
	Alan Kao <alankao@andestech.com>,
	Eric Lin <tesheng@andestech.com>, Gary Guo <gary@garyguo.net>,
	alex@ghiti.fr,
	David Abdurachmanov <david.abdurachmanov@gmail.com>,
	Anup Patel <Anup.Patel@wdc.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Steven Price <steven.price@arm.com>,
	atish.patra@wdc.com, yash.shah@sifive.com,
	Albert Ou <aou@eecs.berkeley.edu>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Greentime Hu <green.hu@gmail.com>,
	zong.li@sifive.com, Paul Walmsley <paul.walmsley@sifive.com>,
	Mike Rapoport <rppt@linux.ibm.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Borislav Petkov <bp@suse.de>,
	Logan Gunthorpe <logang@deltatee.com>,
	linux-riscv@lists.infradead.org,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 0/3] Highmem support for 32-bit RISC-V
Date: Mon, 4 May 2020 14:40:18 +0530	[thread overview]
Message-ID: <20200504091018.GA24897@afzalpc> (raw)
In-Reply-To: <CAK8P3a3OC5UO72rTDWi6+XgmExJmkATEjscq8hns8Bng06OpcQ@mail.gmail.com>

[ +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