Re: [PATCH v5] IOMMU: make DMA containment of quarantined devices optional

[Paul Durrant <xadimgnik@gmail.com>]

On 26/05/2021 09:19, Jan Beulich wrote:
> IOMMU: make DMA containment of quarantined devices optional
> 
> Containing still in flight DMA was introduced to work around certain
> devices / systems hanging hard upon hitting a "not-present" IOMMU fault.
> Passing through (such) devices (on such systems) is inherently insecure
> (as guests could easily arrange for IOMMU faults of any kind to occur).
> Defaulting to a mode where admins may not even become aware of issues
> with devices can be considered undesirable. Therefore convert this mode
> of operation to an optional one, not one enabled by default.
> 
> This involves resurrecting code commit ea38867831da ("x86 / iommu: set
> up a scratch page in the quarantine domain") did remove, in a slightly
> extended and abstracted fashion. Here, instead of reintroducing a pretty
> pointless use of "goto" in domain_context_unmap(), and instead of making
> the function (at least temporarily) inconsistent, take the opportunity
> and replace the other similarly pointless "goto" as well.
> 
> In order to key the re-instated bypasses off of there (not) being a root
> page table this further requires moving the allocate_domain_resources()
> invocation from reassign_device() to amd_iommu_setup_domain_device() (or
> else reassign_device() would allocate a root page table anyway); this is
> benign to the second caller of the latter function.
> 
> In VT-d's domain_context_unmap(), instead of adding yet another
> "goto out" when all that's wanted is a "return", eliminate the "out"
> label at the same time.
> 
> Take the opportunity and also limit the control to builds supporting
> PCI.
> 
> Signed-off-by: Jan Beulich <jbeulich@suse.com>

Reviewed-by: Paul Durrant <paul@xen.org>