* Borislav Petkov <b...@alien8.de> wrote:
> On Fri, Jul 08, 2016 at 11:46:53AM +0200, Ingo Molnar wrote:
> > I'm not sure I can parse that: how can a reported error have bits corrupted?
>
> No, it is about the actual bits in memory the ECC error is generated
> for. So, for example, if an ECC error reports that memory location X had
> some bit flips, the syndrome value which gets reported together with
> same ECC error shows which actual bits have flipped.
>
> Here's an example from the AMD BKDG, maybe that'll make it more clear:
>
> http://support.amd.com/TechDocs/42301_15h_Mod_00h-0Fh_BKDG.pdf
>
> Go to page 246, there it says this:
>
> "For example, assume the ECC syndrome is 03EAh. First search row EAh
> for the complete syndrome. Since it is not found, search row 03h for
> the complete syndrome. It is found in column 9h, so symbol 9h has the
> error. Since the error bitmask indicates value 3h (0011b), bits 0 and 1
> within that symbol are corrupted. Symbol 9h maps to bits 72-79, so the
> corrupted bits are 72 and 73 of the line."
>
> So you basically search the table of x8 ECC correctable syndromes, first
> in row EAh (second syndrome byte) and if you don't find the complete
> syndrome there, you search row 03 for it.
>
> It is in column 9 and that means symbol 9. The symbols are 16 - one
> symbol for each byte in a 128bit DRAM word + 3 special symbols for the
> ECC bits.
>
> The row number 3h is also the error bitmask, so bits 0 and 1 are the
> ones which are corrupted.
>
> Which means, when you look at the value in DRAM at the address the error
> was reported, you need to go to symbol 9, that's 9*8 = 72 which means,
> bits 72-79 and the first 2 in that byte are bits 72 and 73.
>
> So if you want to correct them, you simply flip them as the syndrome
> tells you that those 2 are corrupted.
>
> Ok?
So is 'ECC syndrome' a fancy word and a complicated process for identifying
what
data got corrupted, in a more accurate fashion than what we had before?
Because previously we already had a memory address of the memory corruption,
right?
What is the typical 'scope' of that memory corruption address - a cache line, a
machine word, a byte or maybe a variable unit that is memory hardware dependent?
Thanks,
Ingo
