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Re: [PATCH v2] xen/arm: fix gnttab_need_iommu_mapping

Hi Stefano,

On 11/02/2021 20:55, Stefano Stabellini wrote:
On Thu, 11 Feb 2021, Julien Grall wrote:
On 11/02/2021 13:20, Rahul Singh wrote:
On 10 Feb 2021, at 7:52 pm, Julien Grall <julien@xxxxxxx> wrote:
On 10/02/2021 18:08, Rahul Singh wrote:
Hello Julien,
On 10 Feb 2021, at 5:34 pm, Julien Grall <julien@xxxxxxx> wrote:
On 10/02/2021 15:06, Rahul Singh wrote:
On 9 Feb 2021, at 8:36 pm, Stefano Stabellini
<sstabellini@xxxxxxxxxx> wrote:

On Tue, 9 Feb 2021, Rahul Singh wrote:
On 8 Feb 2021, at 6:49 pm, 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 and IOMMU is enabled for the domain, 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_use_hap_pt(d) means that the page-table used by the
IOMMU is the
P2M. It is true on ARM. need_sync means that you have a
separate IOMMU
page-table and it needs to be updated for every change.
need_sync is set
to false on ARM. Hence, gnttab_need_iommu_mapping(d) is false
which is wrong.

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:

The fix is to go back to something along the lines of the old
implementation of gnttab_need_iommu_mapping.

Signed-off-by: Stefano Stabellini
Fixes: 91d4eca7add
Backport: 4.12+


Given the severity of the bug, I would like to request this
patch to be
backported to 4.12 too, even if 4.12 is security-fixes only
since Oct

For the 4.12 backport, we can use iommu_enabled() instead of
is_iommu_enabled() in the implementation of

Changes in v2:
- improve commit message
- add is_iommu_enabled(d) to the check
xen/include/asm-arm/grant_table.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xen/include/asm-arm/grant_table.h
index 6f585b1538..0ce77f9a1c 100644
--- a/xen/include/asm-arm/grant_table.h
+++ b/xen/include/asm-arm/grant_table.h
@@ -89,7 +89,7 @@ int replace_grant_host_mapping(unsigned long
gpaddr, mfn_t mfn,
     (((i) >= nr_status_frames(t)) ? INVALID_GFN :

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

#endif /* __ASM_GRANT_TABLE_H__ */

I tested the patch and while creating the guest I observed the
below warning from Linux for block device.

So you are creating a guest with "xl create" in dom0 and you see
warnings below printed by the Dom0 kernel? I imagine the domU has
virtual "disk" of some sort.
Yes you are right I am trying to create the guest with "xl create”
and before that, I created the logical volume and trying to attach
the logical volume
block to the domain with “xl block-attach”. I observed this error
with the "xl block-attach” command.
This issue occurs after applying this patch as what I observed this
patch introduce the calls to iommu_legacy_{, un}map() to map the
grant pages for
IOMMU that touches the page-tables. I am not sure but what I
observed is that something is written wrong when iomm_unmap calls
unmap the pages
because of that issue is observed.

Can you clarify what you mean by "written wrong"? What sort of error
do you see in the iommu_unmap()?
I might be wrong as per my understanding for ARM we are sharing the P2M
between CPU and IOMMU always and the map_grant_ref() function is written
in such a way that we have to call iommu_legacy_{, un}map() only if P2M
is not shared.

map_grant_ref() will call the IOMMU if gnttab_need_iommu_mapping() returns
true. I don't really see where this is assuming the P2M is not shared.

In fact, on x86, this will always be false for HVM domain (they support
both shared and separate page-tables).

As we are sharing the P2M when we call the iommu_map() function it will
overwrite the existing GFN -> MFN ( For DOM0 GFN is same as MFN) entry
and when we call iommu_unmap() it will unmap the  (GFN -> MFN ) entry
from the page-table.
AFAIK, there should be nothing mapped at that GFN because the page belongs
to the guest. At worse, we would overwrite a mapping that is the same.
  > Sorry I should have mention before backend/frontend is dom0 in this
case and GFN is mapped. I am trying to attach the block device to DOM0

Ah, your log makes a lot more sense now. Thank you for the clarification!

So yes, I agree that iommu_{,un}map() will do the wrong thing if the frontend
and backend in the same domain.

I don't know what the state in Linux, but from Xen PoV it should be possible
to have the backend/frontend in the same domain.

I think we want to ignore the IOMMU mapping request when the domain is the
same. Can you try this small untested patch:

Given that all the pages already owned by the domain should already be
in the shared pagetable between MMU and IOMMU, there is no need to
create a second mapping. In fact it is guaranteed to overlap with an
existing mapping.

It is **almost** guaranteed :). I can see a few reasons for this to not be valid:
   - Using the domain shared info in a grant
- With a good timing, it would be possible that a differente vCPU remove the mapping after the P2M walk

That said, I feel it is not an expected behavior for a domain guest. So it is not something we should care at least for now.

In theory, if guest_physmap_add_entry returned -EEXIST if a mapping
identical to the one we want to add is already in the pagetable, in this
instance we would see -EEXIST being returned.

While I agree that the GFN and MFN would be the same, there mapping still not be identical because the P2M type (and potentially the permission) will differ.

However, guest_physmap_add_entry() doesn't do such check today. It will just happily replace any mapping. It would be good to harden the code P2M as this is not the first time we see report of mapping overwritten.

I actually have a task in my todo list but I never got the chance to spend time on it.

Based on that, I cannot think of unwanted side-effects for this patch.
It looks OK to me.

Given that it solves a different issue, I think it should be a separate
patch from [1]. Julien, are you OK with that or would you rather merge
the two?

They are two distinct issues. In fact, the bug has always been present on Arm. I will send a separate patch.


Julien Grall



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