Re: [PATCH v2] powerpc/eeh: Fix EEH handling for hugepages in ioremap space.

["Oliver O'Halloran" <oohall@gmail.com>]

On Mon, Apr 12, 2021 at 5:52 PM Mahesh Salgaonkar <mahesh@linux.ibm.com> wrote:
>
> During the EEH MMIO error checking, the current implementation fails to map
> the (virtual) MMIO address back to the pci device on radix with hugepage
> mappings for I/O. This results into failure to dispatch EEH event with no
> recovery even when EEH capability has been enabled on the device.
>
> eeh_check_failure(token)                # token = virtual MMIO address
>   addr = eeh_token_to_phys(token);
>   edev = eeh_addr_cache_get_dev(addr);
>   if (!edev)
>         return 0;
>   eeh_dev_check_failure(edev);  <= Dispatch the EEH event
>
> In case of hugepage mappings, eeh_token_to_phys() has a bug in virt -> phys
> translation that results in wrong physical address, which is then passed to
> eeh_addr_cache_get_dev() to match it against cached pci I/O address ranges
> to get to a PCI device. Hence, it fails to find a match and the EEH event
> never gets dispatched leaving the device in failed state.
>
> The commit 33439620680be ("powerpc/eeh: Handle hugepages in ioremap space")
> introduced following logic to translate virt to phys for hugepage mappings:
>
> eeh_token_to_phys():
> +       pa = pte_pfn(*ptep);
> +
> +       /* On radix we can do hugepage mappings for io, so handle that */
> +       if (hugepage_shift) {
> +               pa <<= hugepage_shift;                  <= This is wrong
> +               pa |= token & ((1ul << hugepage_shift) - 1);
> +       }

I think I vaguely remember thinking "is this right?" at the time.
Apparently not!

Reviewed-by: Oliver O'Halloran <oohall@gmail.com>


It would probably be a good idea to add a debugfs interface to help
with testing the address translation. Maybe something like:

echo <mmio addr> > /sys/kernel/debug/powerpc/eeh_addr_check

Then in the kernel:

struct resource *r = lookup_resource(mmio_addr);
void *virt = ioremap_resource(r);
ret = eeh_check_failure(virt);
iounmap(virt)

return ret;

A little tedious, but then you can write a selftest :)

Oliver