Re: [PATCH] xen/arm: fix gnttab_need_iommu_mapping

[Julien Grall]

On 08/02/2021 18:06, Rahul Singh wrote:
On 6 Feb 2021, at 12:38 am, Stefano Stabellini <sstabell...@kernel.org> wrote:

Commit 91d4eca7add broke gnttab_need_iommu_mapping on ARM.
The offending chunk is:

#define gnttab_need_iommu_mapping(d)                    \
-    (is_domain_direct_mapped(d) && need_iommu(d))
+    (is_domain_direct_mapped(d) && need_iommu_pt_sync(d))

On ARM we need gnttab_need_iommu_mapping to be true for dom0 when it is
directly mapped, like the old check did, but the new check is always
false.

In fact, need_iommu_pt_sync is defined as dom_iommu(d)->need_sync and
need_sync is set as:

    if ( !is_hardware_domain(d) || iommu_hwdom_strict )
        hd->need_sync = !iommu_use_hap_pt(d);

iommu_hwdom_strict is actually supposed to be ignored on ARM, see the
definition in docs/misc/xen-command-line.pandoc:

    This option is hardwired to true for x86 PVH dom0's (as RAM belonging to
    other domains in the system don't live in a compatible address space), and
    is ignored for ARM.

But aside from that, the issue is that iommu_use_hap_pt(d) is true,
hence, hd->need_sync is false, and gnttab_need_iommu_mapping(d) is false
too.

As a consequence, when using PV network from a domU on a system where
IOMMU is on from Dom0, I get:

(XEN) smmu: /smmu@fd800000: Unhandled context fault: fsr=0x402, 
iova=0x8424cb148, fsynr=0xb0001, cb=0
[   68.290307] macb ff0e0000.ethernet eth0: DMA bus error: HRESP not OK

I also observed the IOMMU fault when DOMU guest is created and great table is 
used when IOMMU is enabled. I fixed the error in different way but I am not 
sure if you also observing the same error. I submitted the patch to 
pci-passthrough integration branch. Please have a look once if that make sense.

I belive this is the same error as Stefano has observed. However, your 
patch will unfortunately not work if you have a system with a mix of 
protected and non-protected DMA-capable devices.

Cheers,

--
Julien Grall