Re: [PATCH] pcie_sriov: register user created virtual function before realizing it

["Michael S. Tsirkin" <mst@redhat.com>]

On Sat, Sep 05, 2026 at 02:29:24PM +0200, Daniel Paziyski wrote:
> There are two ways of creating virtual functions: by using the sriov-pf device
> parameter (this way, creating a user created VF and binding it to a PF), or
> by using a device specific parameter, where the device manually creates a given
> amount of VFs.
> 
> When a PCI device is realized, the device specific realize function is called
> first, and then the pcie_sriov_register_device function is called, which checks
> whether the device that has been created is a user created VF, and if it is the
> case and the device allows this kind of VFs, it inserts it into a hashmap, with
> the key being the ID of the PF, and the items being arrays of VFs.
> 
> User created VFs are instantiated independently, and later, when the PF calls
> pcie_sriov_pf_init_from_user_created_vfs, it discovers its VFs from the hashmap
> mentioned previously, and sets up in their PCIDevice.ex.sriov_pf structure
> the pointer to the PF. This means that, during realization, VFs have no access
> to the PF.
> 
> Device created VFs are instantiated during or after PF realization using the
> pcie_sriov_pf_init function, which sets up for the VFs the pointer to the PF
> before their realization, so when they're realized, they can access the PF
> with no issues.
> 
> The problem here is that pcie_sriov_register_device is called after the
> realization, and not before. This means that when a user created VF is created
> for a device which does not support user created VFs, but supports device
> created VFs, the realize function will notice that a VF is being created, and
> may try to access the PF, causing a null pointer dereference fault.
> 
> Fix this by placing the user created VF check and registering before the
> realization. This way, incorrectly created user created VFs will be noticed,
> and device creation will be aborted.
> 
> Additionally, remove from the pcie_sriov_register_device function the top
> check. It seems that this check is done to error out if the PF failed for some
> reason to initialize its list of user created VFs. However, in such situations,
> the pcie_sriov_pf_init_from_user_created_vfs function will error during
> realization, and it will be caught before the check is done at all. Moreover,
> since now pcie_sriov_register_device is called before realization, the check
> will always fail for PFs with user created VFs, because the list will be
> populated during realization.
> 
> The rest of the function though, correctly errors out if a user created VF
> is created for an unsupported device type, if a VF is created for a non-PCIe
> device, or if the PF is already instantiated, with now the advantage being
> that the check is done before the VF instantiation.
> 
> Moreover, move the unregistering of a device's acpi-index and the removal
> of a VF from its PF list from pci_qdev_unrealize to do_pci_unregister_device.
> We do the former so that the acpi-index is unclaimed on user created VF
> creation error, and the latter to not have extra code on the device specific
> realization error path.
> 
> When instantiating a user created VF for a NVME controller:
> 
> Command line:
> 
>     qemu-system-x86_64 -device nvme-subsys,id=subsys0 \
>     -device nvme,id=vctrl0,sriov-pf=ctrl0,subsys=subsys0 \
>     -device nvme,id=ctrl0,subsys=subsys0,serial=s
> 
> ASAN splat:
> 
> ../hw/nvme/ctrl.c:9613:28: runtime error: member access within null pointer of type 'struct NvmeCtrl'
> AddressSanitizer:DEADLYSIGNAL
> =================================================================
> ==91050==ERROR: AddressSanitizer: SEGV on unknown address 0x000000001cf0 (pc 0x7f79a8573dcd bp 0x7ffd622c1bc0 sp 0x7ffd622c1b68 T0)
> ==91050==The signal is caused by a READ memory access.
>     #0 0x7f79a8573dcd  (/usr/lib/libc.so.6+0x173dcd) (BuildId: 1fa174a830cef40a5b2388add4318ee2795f573e)
>     #1 0x5644c6c0266e in nvme_realize ../hw/nvme/ctrl.c:9613
>     #2 0x5644c6c67213 in pci_qdev_realize ../hw/pci/pci.c:2316
>     #3 0x5644c7a2cfc1 in device_set_realized ../hw/core/qdev.c:514
>     #4 0x5644c7a4f767 in property_set_bool ../qom/object.c:2484
>     #5 0x5644c7a48d0b in object_property_set ../qom/object.c:1548
>     #6 0x5644c7a568a5 in object_property_set_qobject ../qom/qom-qobject.c:28
>     #7 0x5644c7a49385 in object_property_set_bool ../qom/object.c:1618
>     #8 0x5644c7a2aeb0 in qdev_realize ../hw/core/qdev.c:277
>     #9 0x5644c735f29f in qdev_device_add_from_qdict ../system/qdev-monitor.c:740
>     #10 0x5644c735f3ab in qdev_device_add ../system/qdev-monitor.c:758
>     #11 0x5644c72b1901 in device_init_func ../system/vl.c:1217
>     #12 0x5644c829d4a3 in qemu_opts_foreach ../util/qemu-option.c:1148
>     #13 0x5644c72bc33e in qemu_create_cli_devices ../system/vl.c:2762
>     #14 0x5644c72bcaa1 in qmp_x_exit_preconfig ../system/vl.c:2822
>     #15 0x5644c72c3241 in qemu_init ../system/vl.c:3862
>     #16 0x5644c801abf8 in main ../system/main.c:71
>     #17 0x7f79a8427780  (/usr/lib/libc.so.6+0x27780) (BuildId: 1fa174a830cef40a5b2388add4318ee2795f573e)
>     #18 0x7f79a84278b8 in __libc_start_main (/usr/lib/libc.so.6+0x278b8) (BuildId: 1fa174a830cef40a5b2388add4318ee2795f573e)
>     #19 0x5644c5f0a1f4 in _start (BuildId: 8483f952216d9e345c3744300d0aefa38feb50d7)
> 
> ==91050==Register values:
> rax = 0x00007e49988640f0  rbx = 0x00007e49988640f0  rcx = 0x00000fc9b3104828  rdx = 0x0000000000000058
> rdi = 0x00007e49988640f0  rsi = 0x0000000000001cf0  rbp = 0x00007ffd622c1bc0  rsp = 0x00007ffd622c1b68
>  r8 = 0x00000fc9b3104829   r9 = 0x00000fc9b3104828  r10 = 0x00000fc9b310481e  r11 = 0x00000fc9b310481e
> r12 = 0x0000000000001cf0  r13 = 0x00000f6f32e78578  r14 = 0x00000000ffffffff  r15 = 0x00007ffd622c1c10
> AddressSanitizer can not provide additional info.
> SUMMARY: AddressSanitizer: SEGV (/usr/lib/libc.so.6+0x173dcd) (BuildId: 1fa174a830cef40a5b2388add4318ee2795f573e)
> ==91050==ABORTING
> 
> Cc: qemu-stable@nongnu.org
> Fixes: 19e55471d4e8 ("pcie_sriov: Allow user to create SR-IOV device")
> Signed-off-by: Daniel Paziyski <danielpaziyski@gmail.com>

/scm/qemu/build/qemu-system-x86_64 -nodefaults -machine q35 -device
e1000e,addr=0.0 -device e1000e,addr=0.0

crashes with this:

- With the patch: SIGSEGV (exit 139), backtrace at hw/pci/pci.c:2334 in pci_qdev_realize.
- Without the patch: exit 1: PCI: slot 0 function 0 not available for e1000e, in use by mch,id=(null).



> ---
> 
> This patch is the same as
> https://lore.kernel.org/qemu-devel/20260901153721.10292-1-danielpaziyski@gmail.com/
> , which improves the original patch by releasing the acpi-index on failure, as
> pointed out by Michael S. Tsirkin <mst@redhat.com> on the replies of
> https://lore.kernel.org/qemu-devel/20260805124519.30054-4-danielpaziyski@gmail.com/
> ("[PATCH 3/3] pcie_sriov: register user created virtual function before realizing it")
> posted on the series
> https://lore.kernel.org/qemu-devel/20260805124519.30054-1-danielpaziyski@gmail.com/
> ("[PATCH 0/3] NVMe hotunplug and PCI SR-IOV fixes").
> 
> I am sending this as a patch separate from the series, since imo it was a mistake
> for me to mix unrelated patches on the same series.
> 
>  hw/pci/pci.c        | 42 ++++++++++++++++++++++++------------------
>  hw/pci/pcie_sriov.c |  6 ------
>  2 files changed, 24 insertions(+), 24 deletions(-)
> 
> diff --git a/hw/pci/pci.c b/hw/pci/pci.c
> index c15f2b9f08..06c432292f 100644
> --- a/hw/pci/pci.c
> +++ b/hw/pci/pci.c
> @@ -1223,6 +1223,19 @@ static void pci_config_free(PCIDevice *pci_dev)
>      g_free(pci_dev->used);
>  }
>  
> +static void pci_unregister_acpi_index(PCIDevice *pci_dev)
> +{
> +    if (pci_dev->acpi_index) {
> +        GSequence *used_indexes = pci_acpi_index_list();
> +
> +        g_sequence_remove(g_sequence_lookup(used_indexes,
> +                          GINT_TO_POINTER(pci_dev->acpi_index),
> +                          g_cmp_uint32, NULL));
> +
> +        pci_dev->acpi_index = 0;
> +    }
> +}
> +
>  static void do_pci_unregister_device(PCIDevice *pci_dev)
>  {
>      pci_get_bus(pci_dev)->devices[pci_dev->devfn] = NULL;
> @@ -1236,6 +1249,9 @@ static void do_pci_unregister_device(PCIDevice *pci_dev)
>                                      &pci_dev->bus_master_enable_region);
>      }
>      address_space_destroy(&pci_dev->bus_master_as);
> +
> +    pcie_sriov_unregister_device(pci_dev);
> +    pci_unregister_acpi_index(pci_dev);
>  }
>  
>  /* Extract PCIReqIDCache into BDF format */
> @@ -1487,7 +1503,6 @@ static void pci_qdev_unrealize(DeviceState *dev)
>  
>      pci_unregister_io_regions(pci_dev);
>      pci_del_option_rom(pci_dev);
> -    pcie_sriov_unregister_device(pci_dev);
>  
>      if (pc->exit) {
>          pc->exit(pci_dev);
> @@ -1497,17 +1512,6 @@ static void pci_qdev_unrealize(DeviceState *dev)
>      do_pci_unregister_device(pci_dev);
>  
>      pci_dev->msi_trigger = NULL;
> -
> -    /*
> -     * clean up acpi-index so it could reused by another device
> -     */
> -    if (pci_dev->acpi_index) {
> -        GSequence *used_indexes = pci_acpi_index_list();
> -
> -        g_sequence_remove(g_sequence_lookup(used_indexes,
> -                          GINT_TO_POINTER(pci_dev->acpi_index),
> -                          g_cmp_uint32, NULL));
> -    }
>  }
>  
>  void pci_register_bar(PCIDevice *pci_dev, int region_num,
> @@ -2318,8 +2322,15 @@ static void pci_qdev_realize(DeviceState *qdev, Error **errp)
>      pci_dev = do_pci_register_device(pci_dev,
>                                       object_get_typename(OBJECT(qdev)),
>                                       pci_dev->devfn, errp);
> -    if (pci_dev == NULL)
> +    if (pci_dev == NULL) {
> +        pci_unregister_acpi_index(pci_dev);

So this passes NULL to pci_unregister_acpi_index.



>          return;
> +    }
> +
> +    if (!pcie_sriov_register_device(pci_dev, errp)) {
> +        do_pci_unregister_device(pci_dev);
> +        return;
> +    }
>  
>      if (pc->realize) {
>          pc->realize(pci_dev, &local_err);
> @@ -2330,11 +2341,6 @@ static void pci_qdev_realize(DeviceState *qdev, Error **errp)
>          }
>      }
>  
> -    if (!pcie_sriov_register_device(pci_dev, errp)) {
> -        pci_qdev_unrealize(DEVICE(pci_dev));
> -        return;
> -    }
> -
>      /*
>       * A PCIe Downstream Port that do not have ARI Forwarding enabled must
>       * associate only Device 0 with the device attached to the bus
> diff --git a/hw/pci/pcie_sriov.c b/hw/pci/pcie_sriov.c
> index c41ac95bee..69930c7b8c 100644
> --- a/hw/pci/pcie_sriov.c
> +++ b/hw/pci/pcie_sriov.c
> @@ -357,12 +357,6 @@ int16_t pcie_sriov_pf_init_from_user_created_vfs(PCIDevice *dev,
>  
>  bool pcie_sriov_register_device(PCIDevice *dev, Error **errp)
>  {
> -    if (!dev->exp.sriov_pf.vf && dev->qdev.id &&
> -        pfs && g_hash_table_contains(pfs, dev->qdev.id)) {
> -        error_setg(errp, "attaching user-created SR-IOV VF unsupported");
> -        return false;
> -    }
> -
>      if (dev->sriov_pf) {
>          PCIDevice *pci_pf;
>          GPtrArray *pf;
> -- 
> 2.55.0