[PATCH v2 0/2] iommufd: give hints to user about kernel support

[PATCH v2 0/2] iommufd: give hints to user about kernel support

[Pierrick Bouvier]

Using iommufd requires two different kernel config:
CONFIG_IOMMUFD and CONFIG_VFIO_DEVICE_CDEV
https://docs.kernel.org/driver-api/vfio.html#vfio-device-cdev

It can be complex to identify what is missing when trying to use it. This series
gives hints to user about it.

Suggested on:
https://lore.kernel.org/qemu-devel/0cc139d6-9289-44a0-9122-d701426dae0c@linaro.org/

Tested using a cross compiled QEMU and a custom kernel + debian rootfs [1].

[1] https://github.com/p-b-o/qemu-linux-stack/tree/device_passthrough

v2
--

- Move /dev/iommu to iommufd_backend_complete, so we can report the error
  instead of aborting.

Pierrick Bouvier (2):
  backends/iommufd.c: report error when /dev/iommu is not available
  hw/vfio/iommufd.c: report hint to user when vfio-dev/vfio*/dev is
    missing

 backends/iommufd.c | 6 ++++++
 hw/vfio/iommufd.c  | 5 ++++-
 2 files changed, 10 insertions(+), 1 deletion(-)

-- 
2.47.3

[PATCH v2 1/2] backends/iommufd.c: report error when /dev/iommu is not available

[Pierrick Bouvier]

In case current kernel does not support /dev/iommu, qemu will probably
fail first because /sys/bus/pci/devices/*/vfio-dev/ is not present,
since QEMU opens it before /dev/iommu.

Instead, report an error directly when completing an iommufd object, to
inform user that kernel does not support it, with a hint about missing
CONFIG_IOMMUFD. We can't do this from initialize as there is no way to
return an error, and we don't want to abort at this step.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
---
 backends/iommufd.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/backends/iommufd.c b/backends/iommufd.c
index acfab907c03..2c3ca23ca0f 100644
--- a/backends/iommufd.c
+++ b/backends/iommufd.c
@@ -72,6 +72,12 @@ static bool iommufd_backend_can_be_deleted(UserCreatable *uc)
 
 static void iommufd_backend_complete(UserCreatable *uc, Error **errp)
 {
+    if (!g_file_test("/dev/iommu", G_FILE_TEST_EXISTS)) {
+        error_setg(errp, "/dev/iommu does not exist"
+                         " (is your kernel config missing CONFIG_IOMMUFD?)");
+        return;
+    }
+
     IOMMUFDBackend *be = IOMMUFD_BACKEND(uc);
     const char *name = iommufd_fd_name(be);
 
-- 
2.47.3

[PATCH v2 2/2] hw/vfio/iommufd.c: report hint to user when vfio-dev/vfio*/dev is missing

[Pierrick Bouvier]

Give a hint about missing kernel config CONFIG_VFIO_DEVICE_CDEV.

Reviewed-by: Yi Liu <yi.l.liu@intel.com>
Reviewed-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
---
 hw/vfio/iommufd.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/hw/vfio/iommufd.c b/hw/vfio/iommufd.c
index 131612eb836..3e33dfbb356 100644
--- a/hw/vfio/iommufd.c
+++ b/hw/vfio/iommufd.c
@@ -274,7 +274,10 @@ static int iommufd_cdev_getfd(const char *sysfs_path, Error **errp)
     }
 
     if (!g_file_get_contents(vfio_dev_path, &contents, &length, NULL)) {
-        error_setg(errp, "failed to load \"%s\"", vfio_dev_path);
+        error_setg(errp,
+                   "failed to load \"%s\""
+                   " (is your kernel config missing CONFIG_VFIO_DEVICE_CDEV?)",
+                   vfio_dev_path);
         goto out_close_dir;
     }
 
-- 
2.47.3

Re: [PATCH v2 1/2] backends/iommufd.c: report error when /dev/iommu is not available

[Cédric Le Goater]

On 3/18/26 19:06, Pierrick Bouvier wrote:
> In case current kernel does not support /dev/iommu, qemu will probably
> fail first because /sys/bus/pci/devices/*/vfio-dev/ is not present,
> since QEMU opens it before /dev/iommu.
> 
> Instead, report an error directly when completing an iommufd object, to
> inform user that kernel does not support it, with a hint about missing
> CONFIG_IOMMUFD. We can't do this from initialize as there is no way to
> return an error, and we don't want to abort at this step.
> 
> Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
> Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
> ---
>   backends/iommufd.c | 6 ++++++
>   1 file changed, 6 insertions(+)
> 
> diff --git a/backends/iommufd.c b/backends/iommufd.c
> index acfab907c03..2c3ca23ca0f 100644
> --- a/backends/iommufd.c
> +++ b/backends/iommufd.c
> @@ -72,6 +72,12 @@ static bool iommufd_backend_can_be_deleted(UserCreatable *uc)
>   
>   static void iommufd_backend_complete(UserCreatable *uc, Error **errp)
>   {
> +    if (!g_file_test("/dev/iommu", G_FILE_TEST_EXISTS)) {
> +        error_setg(errp, "/dev/iommu does not exist"
> +                         " (is your kernel config missing CONFIG_IOMMUFD?)");
> +        return;
> +    }
> +
>       IOMMUFDBackend *be = IOMMUFD_BACKEND(uc);
>       const char *name = iommufd_fd_name(be);
>   

Reviewed-by: Cédric Le Goater <clg@redhat.com>

Thanks,

C.

RE: [PATCH v2 1/2] backends/iommufd.c: report error when /dev/iommu is not available

[Duan, Zhenzhong]

>-----Original Message-----
>From: Pierrick Bouvier <pierrick.bouvier@linaro.org>
>Subject: [PATCH v2 1/2] backends/iommufd.c: report error when /dev/iommu is
>not available
>
>In case current kernel does not support /dev/iommu, qemu will probably
>fail first because /sys/bus/pci/devices/*/vfio-dev/ is not present,
>since QEMU opens it before /dev/iommu.
>
>Instead, report an error directly when completing an iommufd object, to
>inform user that kernel does not support it, with a hint about missing
>CONFIG_IOMMUFD. We can't do this from initialize as there is no way to
>return an error, and we don't want to abort at this step.
>
>Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
>Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
>---
> backends/iommufd.c | 6 ++++++
> 1 file changed, 6 insertions(+)
>
>diff --git a/backends/iommufd.c b/backends/iommufd.c
>index acfab907c03..2c3ca23ca0f 100644
>--- a/backends/iommufd.c
>+++ b/backends/iommufd.c
>@@ -72,6 +72,12 @@ static bool
>iommufd_backend_can_be_deleted(UserCreatable *uc)
>
> static void iommufd_backend_complete(UserCreatable *uc, Error **errp)
> {
>+    if (!g_file_test("/dev/iommu", G_FILE_TEST_EXISTS)) {
>+        error_setg(errp, "/dev/iommu does not exist"
>+                         " (is your kernel config missing CONFIG_IOMMUFD?)");
>+        return;
>+    }
>+
>     IOMMUFDBackend *be = IOMMUFD_BACKEND(uc);
>     const char *name = iommufd_fd_name(be);

I think more accurately it could be:

@@ -82,7 +82,12 @@ static void iommufd_backend_complete(UserCreatable *uc, Error **errp)
         } else {
             cpr_save_fd(name, 0, be->fd);
         }
+    } else if (!g_file_test("/dev/iommu", G_FILE_TEST_EXISTS)) {
+        error_setg(errp, "/dev/iommu does not exist"
+                         " (is your kernel config missing CONFIG_IOMMUFD?)");
+        return;
     }
+
 }

Thanks
Zhenzhong

Re: [PATCH v2 1/2] backends/iommufd.c: report error when /dev/iommu is not available

[Pierrick Bouvier]

On 3/19/26 1:35 AM, Duan, Zhenzhong wrote:
> 
> 
>> -----Original Message-----
>> From: Pierrick Bouvier <pierrick.bouvier@linaro.org>
>> Subject: [PATCH v2 1/2] backends/iommufd.c: report error when /dev/iommu is
>> not available
>>
>> In case current kernel does not support /dev/iommu, qemu will probably
>> fail first because /sys/bus/pci/devices/*/vfio-dev/ is not present,
>> since QEMU opens it before /dev/iommu.
>>
>> Instead, report an error directly when completing an iommufd object, to
>> inform user that kernel does not support it, with a hint about missing
>> CONFIG_IOMMUFD. We can't do this from initialize as there is no way to
>> return an error, and we don't want to abort at this step.
>>
>> Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
>> Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
>> ---
>> backends/iommufd.c | 6 ++++++
>> 1 file changed, 6 insertions(+)
>>
>> diff --git a/backends/iommufd.c b/backends/iommufd.c
>> index acfab907c03..2c3ca23ca0f 100644
>> --- a/backends/iommufd.c
>> +++ b/backends/iommufd.c
>> @@ -72,6 +72,12 @@ static bool
>> iommufd_backend_can_be_deleted(UserCreatable *uc)
>>
>> static void iommufd_backend_complete(UserCreatable *uc, Error **errp)
>> {
>> +    if (!g_file_test("/dev/iommu", G_FILE_TEST_EXISTS)) {
>> +        error_setg(errp, "/dev/iommu does not exist"
>> +                         " (is your kernel config missing CONFIG_IOMMUFD?)");
>> +        return;
>> +    }
>> +
>>      IOMMUFDBackend *be = IOMMUFD_BACKEND(uc);
>>      const char *name = iommufd_fd_name(be);
> 
> I think more accurately it could be:
> 
> @@ -82,7 +82,12 @@ static void iommufd_backend_complete(UserCreatable *uc, Error **errp)
>           } else {
>               cpr_save_fd(name, 0, be->fd);
>           }
> +    } else if (!g_file_test("/dev/iommu", G_FILE_TEST_EXISTS)) {
> +        error_setg(errp, "/dev/iommu does not exist"
> +                         " (is your kernel config missing CONFIG_IOMMUFD?)");
> +        return;
>       }
> +
>   }
> 
> Thanks
> Zhenzhong

Good for me, will send v3.

Regards,
Pierrick

Re: [PATCH v2 0/2] iommufd: give hints to user about kernel support

[Pierrick Bouvier]

On 3/18/26 11:06 AM, Pierrick Bouvier wrote:
> Using iommufd requires two different kernel config:
> CONFIG_IOMMUFD and CONFIG_VFIO_DEVICE_CDEV
> https://docs.kernel.org/driver-api/vfio.html#vfio-device-cdev
> 
> It can be complex to identify what is missing when trying to use it. This series
> gives hints to user about it.
> 
> Suggested on:
> https://lore.kernel.org/qemu-devel/0cc139d6-9289-44a0-9122-d701426dae0c@linaro.org/
> 
> Tested using a cross compiled QEMU and a custom kernel + debian rootfs [1].
> 
> [1] https://github.com/p-b-o/qemu-linux-stack/tree/device_passthrough
> 
> v2
> --
> 
> - Move /dev/iommu to iommufd_backend_complete, so we can report the error
>    instead of aborting.
> 
> Pierrick Bouvier (2):
>    backends/iommufd.c: report error when /dev/iommu is not available
>    hw/vfio/iommufd.c: report hint to user when vfio-dev/vfio*/dev is
>      missing
> 
>   backends/iommufd.c | 6 ++++++
>   hw/vfio/iommufd.c  | 5 ++++-
>   2 files changed, 10 insertions(+), 1 deletion(-)
> 

sent v3:
https://lore.kernel.org/qemu-devel/20260319205942.367705-1-pierrick.bouvier@linaro.org/T/#t

Regards,
Pierrick