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Re: [PATCH] xen/arm: fix gnttab_need_iommu_mapping

On 08/02/2021 18:19, Rahul Singh wrote:
Hello Julien,

Hi Rahul,

On 8 Feb 2021, at 6:11 pm, Julien Grall <julien@xxxxxxx> wrote:

On 08/02/2021 18:06, Rahul Singh wrote:
On 6 Feb 2021, at 12:38 am, Stefano Stabellini <sstabellini@xxxxxxxxxx> 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

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

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.

Yes you are right thats what I though when I fixed the error but then I thought 
in different direction if IOMMU is enabled system wise every device should be 
protected by IOMMU.
I am not aware of any rule preventing a mix of protected and unprotected DMA-capable devices.

However, even if they are all protected by an IOMMU, some of the IOMMUs may have been disabled by the firmware tables for various reasons (e.g. performance, buggy SMMU...). For instance, this is the case on Juno where 2 out of 3 SMMUs are disabled in the Linux upstream DT.

As we don't know which device will use the grant for DMA, we always need to return the machine physical address.


Julien Grall



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