RE: [PATCH v6 01/21] backends/iommufd: Introduce the iommufd object

[Duan, Zhenzhong]

Hi Cédric,

>-----Original Message-----
>From: Cédric Le Goater <clg@redhat.com>
>Sent: Monday, November 20, 2023 4:25 PM
>Subject: Re: [PATCH v6 01/21] backends/iommufd: Introduce the iommufd object
>
>>>>> A similar issue with a fix submitted below, ccing related people.
>>>>> https://lists.gnu.org/archive/html/qemu-devel/2023-11/msg02937.html
>>>>> It looks the fix will not work for hotplug.
>>>>>
>>>>> Or below qemu cmdline may help:
>>>>> "-cpu host,host-phys-bits-limit=39"
>>>>
>>>> don't you have the same issue with legacy VFIO code, you should?
>>>
>>> I tend to be lazy and use seabios for guests on the command line.
>>> I do see the error with legacy VFIO and uefi.
>>>
>>> However, with the address space size work-around and iommufd, the
>>> error is different, an EFAULT now. Some page pinning issue it seems.
>>
>> Yes, this reminds me of iommufd not supporting p2p mapping yet.
>
>OK. Should we transform this error in a warning ? The code needs
>at least a comment.

Make sense, though I'm not clear if there is other corner case return EFAULT.
I plan below change in v7:

diff --git a/backends/iommufd.c b/backends/iommufd.c
index 53fdac4cc0..ba58a0eb0d 100644
--- a/backends/iommufd.c
+++ b/backends/iommufd.c
@@ -178,7 +178,13 @@ int iommufd_backend_map_dma(IOMMUFDBackend *be, uint32_t 
ioas_id, hwaddr iova,
                                   vaddr, readonly, ret);
     if (ret) {
         ret = -errno;
-        error_report("IOMMU_IOAS_MAP failed: %m");
+
+        /* TODO: Not support mapping hardware PCI BAR region for now. */
+        if (errno == EFAULT) {
+            warn_report("IOMMU_IOAS_MAP failed: %m, PCI BAR?");
+        } else {
+            error_report("IOMMU_IOAS_MAP failed: %m");
+        }
     }
     return ret;
 }

I failed to change vfio_container_dma_map print as warning because for legacy 
container, it's real errro.
So print after fix:

qemu-system-x86_64: warning: IOMMU_IOAS_MAP failed: Bad address, PCI BAR?
qemu-system-x86_64: vfio_container_dma_map(0x560cb6cb1620, 0xe000000021000, 
0x3000, 0x7f32ed55c000) = -14 (Bad address)

>
>> So EFAULT is expected. Maybe I should add a comment in docs/devel/vfio-
>iommufd.rst
>
>Yes. It would be good to have a list of gaps and effects in the
>documentation. See Jason's presentation at LPC.
>
>
>https://lpc.events/event/17/contributions/1418/attachments/1297/2607/LPC202
>3_iommufd.pdf

I see, PCI Peer to Peer and POWER/SPAPR are related to qemu iommufd 
implementation.
For POWER/SPAPR, we have "Supported platform" section.
Below are other gaps I can think of for now:

Gaps:
1. dirty page sync, WIP (Joao)
2. p2p dma not supported yet.
3. fd passing with mdev not support ram discard(vfio-pci) as no way to know 
it's a mdev from a fd.

Thanks
Zhenzhong