Re: A problem of Intel IOMMU hardware ？

[Lu Baolu]

Hi Nadav,

On 3/19/21 12:46 AM, Nadav Amit wrote:
> So here is my guess:
>
> Intel probably used as a basis for the IOTLB an implementation of
> some other (regular) TLB design.
>
> Intel SDM says regarding TLBs (4.10.4.2 “Recommended Invalidation”):
>
> "Software wishing to prevent this uncertainty should not write to
> a paging-structure entry in a way that would change, for any linear
> address, both the page size and either the page frame, access rights,
> or other attributes.”
>
>
> Now the aforementioned uncertainty is a bit different (multiple
> *valid*  translations of a single address). Yet, perhaps this is
> yet another thing that might happen.
>
>  From a brief look on the handling of MMU (not IOMMU) hugepages
> in Linux, indeed the PMD is first cleared and flushed before a
> new valid PMD is set. This is possible for MMUs since they
> allow the software to handle spurious page-faults gracefully.
> This is not the case for the IOMMU though (without PRI).
>
> Not sure this explains everything though. If that is the problem,
> then during a mapping that changes page-sizes, a TLB flush is
> needed, similarly to the one Longpeng did manually.

I have been working with Longpeng on this issue these days. It turned
out that your guess is right. The PMD is first cleared but not flushed
before a new valid one is set. The previous entry might be cached in the
paging structure caches hence leads to disaster.

In __domain_mapping():

2352                                 /*
2353                                  * Ensure that old small page 
tables are
2354                                  * removed to make room for 
superpage(s).
2355                                  * We're adding new large pages, so 
make sure
2356                                  * we don't remove their parent tables.
2357                                  */
2358                                 dma_pte_free_pagetable(domain, 
iov_pfn, end_pfn,
2359 
largepage_lvl + 1);

I guess adding a cache flush operation after PMD switching should solve
the problem.

I am still not clear about this comment:

"
 This is possible for MMUs since they allow the software to handle
 spurious page-faults gracefully. This is not the case for the IOMMU
 though (without PRI).
"

Can you please shed more light on this?

Best regards,
baolu