xmon memory dump does not handle LE

Douglas Miller <dougmill@linux.vnet.ibm.com> writes:

> I was hoping this would be any easy fix, but now it looks like it will 
> be more difficult.
>
> The basic problem is that xmon memory commands like 'dd' do not properly 
> display the data on LE instances. This means that not only is it 
> difficult to read but one cannot copy-paste addresses from the output. 
> This severely encumbers debugging using xmon on LE systems.

What do you mean by "properly"?

I think what you're saying is that dumping bytes of memory on LE doesn't
give you nice u64 pointers, but that's not a bug, that's just how memory
is laid out on LE.

Also memory isn't always filled with u64 pointers, so just byte swapping
at that size is not correct. Sometimes you'll be looking at ints, in
which case you need to swap 4 bytes at a time.

What we need is dump commands that do a byte swap of a given size.
Balbir was working on this but I think he got diverted.

His first attempt was here: https://patchwork.ozlabs.org/patch/696348/
 
But as I said in my reply:

  So as discussed, lets add d1, d2, d4, d8, which dump 1/2/4/8 bytes at a
  time, in cpu endian, and which each value separated by space.


Here's a quick hack to get it going. Let me know if that works for you,
it's not pretty, but we could probably merge it with a bit of cleanup.

cheers

I'm referring to the three commands listed in the help:

d     dump bytes

df    dump float values

dd    dump double values

As it turns out, all three of these commands do exactly the same thing, 
and it's certainly not what I'd expect based on experience with other 
debuggers. Maybe the original intent was only to simply output bytes of 
memory, but to me the help implies otherwise and certainly something 
more is needed.

I'll take a look at Balbir's patch and see if I can help move it along.

Thanks,

Doug

On 02/02/2017 07:46 PM, Michael Ellerman wrote:
> Douglas Miller <dougmill@linux.vnet.ibm.com> writes:
>
>> I was hoping this would be any easy fix, but now it looks like it will
>> be more difficult.
>>
>> The basic problem is that xmon memory commands like 'dd' do not properly
>> display the data on LE instances. This means that not only is it
>> difficult to read but one cannot copy-paste addresses from the output.
>> This severely encumbers debugging using xmon on LE systems.
> What do you mean by "properly"?
>
> I think what you're saying is that dumping bytes of memory on LE doesn't
> give you nice u64 pointers, but that's not a bug, that's just how memory
> is laid out on LE.
>
> Also memory isn't always filled with u64 pointers, so just byte swapping
> at that size is not correct. Sometimes you'll be looking at ints, in
> which case you need to swap 4 bytes at a time.
>
> What we need is dump commands that do a byte swap of a given size.
> Balbir was working on this but I think he got diverted.
>
> His first attempt was here: https://patchwork.ozlabs.org/patch/696348/
>
> But as I said in my reply:
>
>    So as discussed, lets add d1, d2, d4, d8, which dump 1/2/4/8 bytes at a
>    time, in cpu endian, and which each value separated by space.
>
>
> Here's a quick hack to get it going. Let me know if that works for you,
> it's not pretty, but we could probably merge it with a bit of cleanup.
>
> cheers
>
>
> diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
> index a44b049b9cf6..3903af5fe276 100644
> --- a/arch/powerpc/xmon/xmon.c
> +++ b/arch/powerpc/xmon/xmon.c
> @@ -2334,10 +2334,43 @@ static void dump_pacas(void)
>   }
>   #endif
>
> +static void dump_by_size(unsigned long addr, long count, int size)
> +{
> +	unsigned char temp[16];
> +	int i, j;
> +	u64 val;
> +
> +	count = ALIGN(count, 16);
> +
> +	for (i = 0; i < count; i += 16, addr += 16) {
> +		printf(REG, addr);
> +
> +		if (mread(addr, temp, 16) != 16) {
> +			printf("Faulted reading %d bytes from 0x"REG"\n", 16, addr);
> +			return;
> +		}
> +
> +		for (j = 0; j < 16; j += size) {
> +			putchar(' ');
> +			switch (size) {
> +			case 1: val = temp[j]; break;
> +			case 2: val = *(u16 *)&temp[j]; break;
> +			case 4: val = *(u32 *)&temp[j]; break;
> +			case 8: val = *(u64 *)&temp[j]; break;
> +			default: val = 0;
> +			}
> +
> +			printf("%0*lx", size * 2, val);
> +		}
> +
> +		printf("\n");
> +	}
> +}
> +
>   static void
>   dump(void)
>   {
> -	int c;
> +	int size, c;
>
>   	c = inchar();
>
> @@ -2350,8 +2383,9 @@ dump(void)
>   	}
>   #endif
>
> -	if ((isxdigit(c) && c != 'f' && c != 'd') || c == '\n')
> +	if (c == '\n')
>   		termch = c;
> +
>   	scanhex((void *)&adrs);
>   	if (termch != '\n')
>   		termch = 0;
> @@ -2383,9 +2417,21 @@ dump(void)
>   			ndump = 64;
>   		else if (ndump > MAX_DUMP)
>   			ndump = MAX_DUMP;
> -		prdump(adrs, ndump);
> +
> +		size = 0;
> +		switch (c) {
> +		case '8': size += 4;
> +		case '4': size += 2;
> +		case '2': size += 1;
> +		case '1': size += 1;
> +			dump_by_size(adrs, ndump, size);
> +			break;
> +		default:
> +			prdump(adrs, ndump);
> +			last_cmd = "d\n";
> +		}
> +
>   		adrs += ndump;
> -		last_cmd = "d\n";
>   	}
>   }
>
>

Douglas Miller <dougmill@linux.vnet.ibm.com> writes:

> I'm referring to the three commands listed in the help:
>
> d     dump bytes
>
> df    dump float values
>
> dd    dump double values
>
> As it turns out, all three of these commands do exactly the same thing, 
> and it's certainly not what I'd expect based on experience with other 
> debuggers. Maybe the original intent was only to simply output bytes of 
> memory, but to me the help implies otherwise and certainly something 
> more is needed.

OK. I don't think df/dd actually exist in the code, so I think the help
is just out of date, by about 20 years.

'd' definitely works as intended and does what the help says as far as
I'm concerned. The output is pretty similar to hexdump -C for example.

Also xmon isn't a debugger it's a crash handler :)

> I'll take a look at Balbir's patch and see if I can help move it along.

Actually take a look at mine instead.

cheers

Hi Michael,

Yes, your patch seems a more complete solution. The idea of "d1", "d2", 
"d4", and "d8" commands is more what I needed and makes more sense to 
someone hitting xmon "cold". I'll work on getting your patch submitted.


Question on these sorts of patches (PPC only), do we submit initially to 
upstream, or here on this (PPC) mailing list? It would be nice to get 
this into the distros as soon as possible.

Thanks,

Doug


On 02/03/2017 04:31 AM, Michael Ellerman wrote:
> Douglas Miller <dougmill@linux.vnet.ibm.com> writes:
>
>> I'm referring to the three commands listed in the help:
>>
>> d     dump bytes
>>
>> df    dump float values
>>
>> dd    dump double values
>>
>> As it turns out, all three of these commands do exactly the same thing,
>> and it's certainly not what I'd expect based on experience with other
>> debuggers. Maybe the original intent was only to simply output bytes of
>> memory, but to me the help implies otherwise and certainly something
>> more is needed.
> OK. I don't think df/dd actually exist in the code, so I think the help
> is just out of date, by about 20 years.
>
> 'd' definitely works as intended and does what the help says as far as
> I'm concerned. The output is pretty similar to hexdump -C for example.
>
> Also xmon isn't a debugger it's a crash handler :)
>
>> I'll take a look at Balbir's patch and see if I can help move it along.
> Actually take a look at mine instead.
>
> cheers
>

Douglas Miller <dougmill@linux.vnet.ibm.com> writes:

> Hi Michael,
>
> Yes, your patch seems a more complete solution. The idea of "d1", "d2", 
> "d4", and "d8" commands is more what I needed and makes more sense to 
> someone hitting xmon "cold". I'll work on getting your patch submitted.
>
>
> Question on these sorts of patches (PPC only), do we submit initially to 
> upstream, or here on this (PPC) mailing list?

Yeah send them here. Patches that are only touch powerpc should come
here, from where I merge them and then send them to Linus.

Anything that touches powerpc and some other code should at least be
CC'ed here.

If you're ever in doubt where to send a patch you can use:

  $ ./scripts/get_maintainer.pl --nogit-fallback --no-rolestats -f arch/powerpc/xmon/xmon.c
  Benjamin Herrenschmidt <benh@kernel.crashing.org>
  Paul Mackerras <paulus@samba.org>
  Michael Ellerman <mpe@ellerman.id.au>
  linuxppc-dev@lists.ozlabs.org
  linux-kernel@vger.kernel.org

> It would be nice to get this into the distros as soon as possible.

This isn't a bug fix so the distros won't pick it up automatically.
You'll have to ask them specifically to take a backport.


cheers