Re: [PATCH 1/2] ARM: mmu64: allow to disable null pointer trap on zero page

[Sascha Hauer <s.hauer@pengutronix.de>]

On Wed, Oct 14, 2020 at 06:29:47PM +0200, Ahmad Fatoum wrote:
> 
> 
> On 10/14/20 5:08 PM, Michael Tretter wrote:
> > Barebox uses the zero page to trap NULL pointer dereferences. However,
> > if the SDRAM starts at address 0x0, this makes the first page of the
> > SDRAM inaccessible and makes it impossible to load images to offset 0x0
> > in the SDRAM.
> > 
> > Trapping NULL pointer dereferences on such systems is still desirable.
> > Therefore, add a function to disable the traps if accessing the zero
> > page is necessary and to re-enable the traps after the access is done.
> 
> Can't we map the (phy_addr_t)0 at some higher virtual address and
> change uimage to use phys_to_virt() ?
> 
> Something like 
> 
> static inline void *phys_to_virt(unsigned long phys)
> {
> 	if (!IS_ENABLED(CONFIG_ARM_MACH_CUSTOM_MAPPING) || !arm_mach_phys_to_virt)
> 		return (void *)phys;
> 	return arm_mach_phys_to_virt(phys);
> }

Please don't. A user of that function would have to implicitly know that
phys_to_virt() would have to be called exactly for the zero page. Also
how big is the area returned from phys_to_virt()? If it's up to the next
page boundary a user would have to align all of it's memcpys to page
boundaries for no gain.

I rather suggest to introduce an explicit

void *memcpy_to_zero(void *dst, void *src, size_t len)

The implementation could then be architecture specific and could choose
if it likes to just map the zero page to NULL or somewhere else.

Sascha

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