Re: [PATCH v3 10/10] iommu/vt-d: Disallow nesting on domains with read-only mappings

From: Baolu Lu
Date: Fri May 26 2023 - 00:29:13 EST

On 5/24/23 3:44 PM, Tian, Kevin wrote:
From: Liu, Yi L <yi.l.liu@xxxxxxxxx>
Sent: Thursday, May 11, 2023 10:51 PM

From: Lu Baolu <>

When remapping hardware is configured by system software in scalable
as Nested (PGTT=011b) and with PWSNP field Set in the PASID-table-entry,
it may Set Accessed bit and Dirty bit (and Extended Access bit if enabled)
in first-stage page-table entries even when second-stage mappings indicate
that corresponding first-stage page-table is Read-Only.

As the result, contents of pages designated by VMM as Read-Only can be
modified by IOMMU via PML5E (PML4E for 4-level tables) access as part of
address translation process due to DMAs issued by Guest.

Disallow the nested translation when there are read-only pages in the
corresponding second-stage mappings. And, no read-only pages are allowed
to be configured in the second-stage table of a nested translation.
For the latter, an alternative is to disallow read-only mappings in
any stage-2 domain as long as it's ever been used as a parent. In this
way, we can simply replace the user counter with a flag.

In concept if the user understands this errata and does expect to
enable nested translation it should never install any RO mapping
in stage-2 in the entire VM life cycle."

IMHO the alternative is reasonable and simpler. If the user decides to
enable nesting it should keep the nesting-friendly configuration static
since whether nesting is enabled on a device is according to viommu
configuration (i.e. whether the guest attaches the device to identity
domain or non-identity domain) and it's not good to break the nesting
setup just because the host inadvertently adds a RO mapping to s2 in
the middle between guest is detached/put back to identity domain
and then re-attach to an unmanaged domain.

Fair enough.

+ if (!(prot & DMA_PTE_WRITE) && !domain->read_only_mapped) {
+ spin_lock_irqsave(&domain->lock, flags);
+ if (domain->nested_users > 0) {
+ spin_unlock_irqrestore(&domain->lock, flags);
+ return -EINVAL;
+ }

this is worth a one-off warning. Same in the other path.


Best regards,