Re: [PATCH 2/2] uimage: disable zero page when loading to SDRAM at address 0x0

[Michael Tretter <m.tretter@pengutronix.de>]

On Wed, 14 Oct 2020 18:33:25 +0200, Ahmad Fatoum wrote:
> On 10/14/20 5:08 PM, Michael Tretter wrote:
> > If the SDRAM is mapped to address 0x0 and an image should be loaded to
> > to the SDRAM without offset, Barebox would normally trap the access as a
> > null pointer.
> > 
> > However, since Linux kernel commit cfa7ede20f13 ("arm64: set TEXT_OFFSET
> > to 0x0 in preparation for removing it entirely") no offset is the
> > default for arm64. Therefore, copying the image to 0x0 of the SDRAM is
> > necessary.
> > 
> > Disable the zero page trap for copying an image to address 0x0.
> > 
> > Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
> > ---
> >  common/uimage.c | 18 ++++++++++++++++--
> >  1 file changed, 16 insertions(+), 2 deletions(-)
> > 
> > diff --git a/common/uimage.c b/common/uimage.c
> > index a84b8fddc4e7..b1e9b402e98a 100644
> > --- a/common/uimage.c
> > +++ b/common/uimage.c
> > @@ -27,6 +27,7 @@
> >  #include <rtc.h>
> >  #include <filetype.h>
> >  #include <memory.h>
> > +#include <zero_page.h>
> >  
> >  static inline int uimage_is_multi_image(struct uimage_handle *handle)
> >  {
> > @@ -359,7 +360,13 @@ static int uimage_sdram_flush(void *buf, unsigned int len)
> >  		}
> >  	}
> >  
> > -	memcpy(uimage_buf + uimage_size, buf, len);
> > +	if (zero_page_contains((unsigned long)uimage_buf + uimage_size)) {
> > +		zero_page_disable();
> > +		memcpy(uimage_buf + uimage_size, buf, len);
> > +		zero_page_enable();
> 
> If this remains, please add a memcpy_notrap or something.

Should I check the destination before calling memcpy_notrap or should I always
call the memcpy_notrap if there is a possibility to copy to 0x0 and check for
the destination within the function?

I fear that having such a "simple" function would encourage to use it more
often. I would prefer to make the code to use it more clumsy and make it
(similar to data_abort_mask()) the responsibility of the caller to be aware
that bad things might happen when the zero_page is disabled.

> 
> > +	} else {
> > +		memcpy(uimage_buf + uimage_size, buf, len);
> > +	}
> >  
> >  	uimage_size += len;
> >  
> > @@ -388,7 +395,14 @@ struct resource *file_to_sdram(const char *filename, unsigned long adr)
> >  			goto out;
> >  		}
> >  
> > -		now = read_full(fd, (void *)(res->start + ofs), BUFSIZ);
> > +		if (zero_page_contains(res->start + ofs)) {
> > +			zero_page_disable();
> > +			now = read_full(fd, (void *)(res->start + ofs), BUFSIZ);
> > +			zero_page_enable();
> 
> And use that new memcpy_notrap here to copy from an intermediate buffer. You open quite a can
> of worms when you treat NULL as a valid address. Better have this contained in a single
> file instead of hoping the compiler doesn't do a NULL-can't-happen-here optimization
> in all that block/cdev/fs code that read_full may call into.

Could you explain, what kind of optimization you would expect?

Michael

> 
> > +		} else {
> > +			now = read_full(fd, (void *)(res->start + ofs), BUFSIZ);
> > +		}
> > +
> >  		if (now < 0) {
> >  			release_sdram_region(res);
> >  			res = NULL;
> > 

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