> Hello all,
>
> kernel/src/plat/pc99/machine/intel-vtd.c::vtd_get_n_paging() confused me a
> lot. Wish to get any help.
>
> This func intends to calculate paging pages needed for all RMRRs. The
> following code 'filter the identical regions by pci bus id', and use them to
> figure out the final result.
> ```
> for (word_t i = 1; i < rmrr_list->num; i++) {
> if (vtd_get_root_index(rmrr_list->entries[i].device) !=
> vtd_get_root_index(filtered.entries[filtered.num - 1].device) &&
> rmrr_list->entries[i].base != filtered.entries[filtered.num -
> 1].base &&
> rmrr_list->entries[i].limit != filtered.entries[filtered.num -
> 1].limit) {
> filtered.entries[filtered.num] = rmrr_list->entries[i];
> filtered.num++;
> }
> }
> ```
> But vtd_map_reserved_page() says, every different devices(with diff bus-dev-
> func ID) have their own paging structures. Then why do filter here like this
> , yet the logic looks so confusing.
>
> And on line 313, it counts pages according to the specified level address
> bits.
> ```
> size += get_n_paging(region, 32 - (VTD_PT_INDEX_BITS * i +
> seL4_PageBits));
> ```
> So what does 32 mean? Maybe a typo?
>
> Thanks,
> laokz
Hi laokz,
`vtd_get_n_paging()` tries to pre-calculate how many page tables will be needed
to set up the initial VTD mappings. The value that it returns gets used to
preallocate the memory required and then later on in the boot process the page
tables are allocated using `it_alloc_paging`. Part of the logic in
vtd_get_n_paging appears to try and calculate how many page tables are required
for creating the reserved memory mapping regions. As you identify, the logic
doesn't seem to be correct in several places. Currently, the final result is
that it overcalculates the amount of page tables required by quite a lot, so
the consequence for the overall system is some memory gets leaked during boot
due to the overallocation. I'm working on a patch and I'll notify you when
it's up on GitHub. Thanks for tracking this down!
Kent
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