Re: [PATCH v4 1/2] xen/arm: fix arm_iommu_map_page after f9f6b22abf1d

[Stewart Hildebrand <stewart.hildebrand@xxxxxxx>]

On 7/24/25 04:07, Jan Beulich wrote:
> On 23.07.2025 20:13, Stewart Hildebrand wrote:
>> From: Stefano Stabellini <stefano.stabellini@xxxxxxx>
>>
>> Up until f9f6b22abf1d "xen/arm: Map ITS doorbell register to IOMMU page
>> tables" the only caller of iommu_map on ARM was grant_table.c which has
>> a specific usage model and restrictions as described by the in-code
>> comment in arm_iommu_map_page.
>>
>> f9f6b22abf1d introduced a second caller to iommu_map on ARM:
>> vgic_v3_its_init_virtual. This specific statement in the
>> f9f6b22abf1d commit message is partially wrong:
>>
>> "Note that the 1:1 check in arm_iommu_map_page remains for now, as
>> virtual ITSes are currently only created for hwdom where the doorbell
>> mapping is always 1:1."
>>
>> Leading to crashes any time the hardware domain is not direct-mapped
>> (e.g. cache coloring and non-Dom0 hardware domain):
>>
>> (XEN) Xen BUG at drivers/passthrough/arm/iommu_helpers.c:47
>> [...]
>> (XEN) Xen call trace:
>> (XEN)    [<00000a000024c758>] arm_iommu_map_page+0x80/0x90 (PC)
>> (XEN)    [<00000a000024c750>] arm_iommu_map_page+0x78/0x90 (LR)
>> (XEN)    [<00000a0000250884>] iommu_map+0xcc/0x29c
>> (XEN)    [<00000a0000288024>] vgic_v3_its_init_domain+0x18c/0x1e8
>> (XEN)    [<00000a0000285228>] vgic-v3.c#vgic_v3_domain_init+0x168/0x21c
>> (XEN)    [<00000a0000281dcc>] domain_vgic_init+0x14c/0x210
>> (XEN)    [<00000a00002705a4>] arch_domain_create+0x150/0x1f0
>> (XEN)    [<00000a00002055e8>] domain_create+0x47c/0x6c0
>> (XEN)    [<00000a00002cf090>] create_domUs+0x7f8/0x8cc
>> (XEN)    [<00000a00002eb588>] start_xen+0x8f4/0x998
>> (XEN)    [<00000a000020018c>] head.o#primary_switched+0x4/0x10
>>
>> Specifically, non-1:1 hardware domain exists with cache coloring
>> enabled. For that, is_domain_direct_mapped(d) is false but
>> domain_use_host_layout(d) is true.
>>
>> Change the is_domain_direct_mapped(d) checks in arm_iommu_map_page and
>> arm_iommu_unmap_page into domain_use_host_layout(d) checks.
>>
>> Move the in-code comment specific to the grant table to grant-table.c
>> and adjust to be architecture-neutral.
>>
>> Fixes: f9f6b22abf1d ("xen/arm: Map ITS doorbell register to IOMMU page 
>> tables")
>> Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxx>
>> Signed-off-by: Stewart Hildebrand <stewart.hildebrand@xxxxxxx>
>> ---
>> v3->v4:
>> * adjust comment to be architecture-neutral
> 
> Hmm, it's now arch-neutral, but still not quite correct.
> 
>> --- a/xen/common/grant_table.c
>> +++ b/xen/common/grant_table.c
>> @@ -1274,6 +1274,11 @@ map_grant_ref(
>>          }
>>  
>>          /*
>> +         * Grant mappings can be used for DMA requests. The dev_bus_addr
>> +         * returned by the hypercall is the MFN (not the GFN). For device
>> +         * protected by an IOMMU, Xen needs to add a 1:1 mapping in the 
>> domain
>> +         * p2m to allow DMA request to work.
>> +         *
>>           * We're not translated, so we know that dfns and mfns are
>>           * the same things, so the IOMMU entry is always 1-to-1.
>>           */
> 
> The original comment, for a reason, talks about DFN, not GFN. The relationship
> to P2M (where GFNs might indeed matter) also isn't quite clear to me:
> iommu_legacy_map() alters IOMMU mappings. Which may or may not be shared with
> CPU ones.

Ah, you're right, I assumed iommu page tables are always shared with
cpu... A bad assumption, sorry about that.

> 
> Fundamental question: What exactly is insufficient in the comment that's there
> already?

Nothing. It was nothing more than trying to find a new home for the
comment in xen/drivers/passthrough/arm/iommu_helpers.c, but perhaps it's
better to drop the comment altogether and leave xen/common/grant_table.c
unchanged.