Re: [PATCH] xen/arm: fix gnttab_need_iommu_mapping

[Rahul Singh <Rahul.Singh@arm.com>]

Hello Julien,

> On 8 Feb 2021, at 6:11 pm, Julien Grall <julien@xen.org> wrote:
> 
> 
> 
> On 08/02/2021 18:06, Rahul Singh wrote:
>>> On 6 Feb 2021, at 12:38 am, Stefano Stabellini <sstabellini@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.

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. My understanding might be wrong.

Regards,
Rahul

> Cheers,
> 
> -- 
> Julien Grall