On Wednesday 24 July 2013 11:49 PM, Sricharan R wrote:

Hi Nicolas,

On Thursday 25 July 2013 01:51 AM, Nicolas Pitre wrote:

[..]

quoted

I don't think I follow you here.

Let's assume:

phys_addr_t __pv_offset = PHYS_START - VIRT_START;

If PA = 0x0-8000-0000 and VA = 0xc000-0000 then
__pv_offset = 0xffff-ffff-c000-0000.

If PA = 0x2-8000-0000 and VA = 0xc000-0000 then
__pv_offset = 0x1-c000-0000.

So the __virt_to_phys() assembly stub could look like:

static inline phys_addr_t __virt_to_phys(unsigned long x)
{
	phys_addr_t t;

	if if (sizeof(phys_addr_t) == 4) {
		__pv_stub(x, t, "add", __PV_BITS_31_24);
	} else {
		__pv_movhi_stub(t);
		__pv_add_carry_stub(x, t);
	}

	return t;
}

And...

#define __pv_movhi_stub(y) \
	__asm__("@ __pv_movhi_stub\n" \
	"1:	mov	%R0, %1\n" \
	"	.pushsection .pv_table,\"a\"\n" \
	"	.long	1b\n" \
	"	.popsection\n" \
	: "=r" (y) \
	: "I" (__PV_BITS_8_0))

#define __pv_add_carry_stub(x, y) \
	__asm__("@ __pv_add_carry_stub\n" \
	"1:	adds	%Q0, %1, %2\n" \
	"	adc	%R0, %R0, #0\n" \
	"	.pushsection .pv_table,\"a\"\n" \
	"	.long	1b\n" \
	"	.popsection\n" \
	: "+r" (y) \
	: "r" (x), "I" (__PV_BITS_31_24) \
	: "cc")

The stub bits such as __PV_BITS_8_0 can be augmented with more bits in 
the middle to determine the type of fixup needed.  The fixup code would 
determine the shift needed on the value, and whether or not the low or 
high word of __pv_offset should be used according to those bits.

Then, in the case where a mov is patched, you need to check if the high 
word of __pv_offset is 0xffffffff and if so the mov should be turned 
into a "mvn rn, #0".

And there you are with all possible cases handled.

Brilliant !!
We knew you will have some tricks and better way. We were
not convinced with the extra stub for 'mov' but also didn't
have idea to avoid it.

  Thanks and you have given the full details here.

  Sorry if i was not clear on my previous response.

 1)  When i said special case can be avoided, i meant that
      we need not differentiate the 0xfffffff case inside the
      __virt_to_phy macro, but can handle it at the time of patching.
      Your above code makes that clear.
 
 2) I would have ended creating separate tables for 'mov' and 'add'
      case. But again thanks to your above idea of augumenting the
      __PV_BITS, with which we can find out run time. And 'mvn' would
      be needed for moving '0xffffffff' . Now I can get rid
     of the separate section that i created for 'mov' in my previous
     version.

We also get rid of calling separate patching for modules as well
as late patching. Overall the patch-set becomes smaller and
simpler. Thanks for help.

Regards,
Santosh