Re: [PATCH 04/15] pcie: Add callback preceding SR-IOV VFs update

[Lukasz Maniak <lukasz.maniak@linux.intel.com>]

On Tue, Oct 12, 2021 at 03:25:12AM -0400, Michael S. Tsirkin wrote:
> On Thu, Oct 07, 2021 at 06:23:55PM +0200, Lukasz Maniak wrote:
> > PCIe devices implementing SR-IOV may need to perform certain actions
> > before the VFs are unrealized or vice versa.
> > 
> > Signed-off-by: Lukasz Maniak <lukasz.maniak@linux.intel.com>
> 
> Callbacks are annoying and easy to misuse though.
> VFs are enabled through a config cycle, we generally just
> have devices invoke the capability handler.
> E.g.
> 
> static void pci_bridge_dev_write_config(PCIDevice *d,
>                                         uint32_t address, uint32_t val, int len)
> {
>     pci_bridge_write_config(d, address, val, len);
>     if (msi_present(d)) {
>         msi_write_config(d, address, val, len);
>     }
> }
> 
> this makes it easy to do whatever you want before/after
> the write. You can also add a helper to check
> that SRIOV is being enabled/disabled if necessary.
> 
> > ---
> >  docs/pcie_sriov.txt         |  2 +-
> >  hw/pci/pcie_sriov.c         | 14 +++++++++++++-
> >  include/hw/pci/pcie_sriov.h |  8 +++++++-
> >  3 files changed, 21 insertions(+), 3 deletions(-)
> > 
> > diff --git a/docs/pcie_sriov.txt b/docs/pcie_sriov.txt
> > index f5e891e1d4..63ca1a7b8e 100644
> > --- a/docs/pcie_sriov.txt
> > +++ b/docs/pcie_sriov.txt
> > @@ -57,7 +57,7 @@ setting up a BAR for a VF.
> >        /* Add and initialize the SR/IOV capability */
> >        pcie_sriov_pf_init(d, 0x200, "your_virtual_dev",
> >                         vf_devid, initial_vfs, total_vfs,
> > -                       fun_offset, stride);
> > +                       fun_offset, stride, pre_vfs_update_cb);
> >  
> >        /* Set up individual VF BARs (parameters as for normal BARs) */
> >        pcie_sriov_pf_init_vf_bar( ... )
> > diff --git a/hw/pci/pcie_sriov.c b/hw/pci/pcie_sriov.c
> > index 501a1ff433..cac2aee061 100644
> > --- a/hw/pci/pcie_sriov.c
> > +++ b/hw/pci/pcie_sriov.c
> > @@ -30,7 +30,8 @@ static void unregister_vfs(PCIDevice *dev);
> >  void pcie_sriov_pf_init(PCIDevice *dev, uint16_t offset,
> >                          const char *vfname, uint16_t vf_dev_id,
> >                          uint16_t init_vfs, uint16_t total_vfs,
> > -                        uint16_t vf_offset, uint16_t vf_stride)
> > +                        uint16_t vf_offset, uint16_t vf_stride,
> > +                        SriovVfsUpdate pre_vfs_update)
> >  {
> >      uint8_t *cfg = dev->config + offset;
> >      uint8_t *wmask;
> > @@ -41,6 +42,7 @@ void pcie_sriov_pf_init(PCIDevice *dev, uint16_t offset,
> >      dev->exp.sriov_pf.num_vfs = 0;
> >      dev->exp.sriov_pf.vfname = g_strdup(vfname);
> >      dev->exp.sriov_pf.vf = NULL;
> > +    dev->exp.sriov_pf.pre_vfs_update = pre_vfs_update;
> >  
> >      pci_set_word(cfg + PCI_SRIOV_VF_OFFSET, vf_offset);
> >      pci_set_word(cfg + PCI_SRIOV_VF_STRIDE, vf_stride);
> > @@ -180,6 +182,11 @@ static void register_vfs(PCIDevice *dev)
> >      assert(dev->exp.sriov_pf.vf);
> >  
> >      trace_sriov_register_vfs(SRIOV_ID(dev), num_vfs);
> > +
> > +    if (dev->exp.sriov_pf.pre_vfs_update) {
> > +        dev->exp.sriov_pf.pre_vfs_update(dev, dev->exp.sriov_pf.num_vfs, num_vfs);
> > +    }
> > +
> >      for (i = 0; i < num_vfs; i++) {
> >          dev->exp.sriov_pf.vf[i] = register_vf(dev, devfn, dev->exp.sriov_pf.vfname, i);
> >          if (!dev->exp.sriov_pf.vf[i]) {
> > @@ -198,6 +205,11 @@ static void unregister_vfs(PCIDevice *dev)
> >      uint16_t i;
> >  
> >      trace_sriov_unregister_vfs(SRIOV_ID(dev), num_vfs);
> > +
> > +    if (dev->exp.sriov_pf.pre_vfs_update) {
> > +        dev->exp.sriov_pf.pre_vfs_update(dev, dev->exp.sriov_pf.num_vfs, 0);
> > +    }
> > +
> >      for (i = 0; i < num_vfs; i++) {
> >          PCIDevice *vf = dev->exp.sriov_pf.vf[i];
> >          object_property_set_bool(OBJECT(vf), "realized", false, &local_err);
> > diff --git a/include/hw/pci/pcie_sriov.h b/include/hw/pci/pcie_sriov.h
> > index 0974f00054..9ab48b79c0 100644
> > --- a/include/hw/pci/pcie_sriov.h
> > +++ b/include/hw/pci/pcie_sriov.h
> > @@ -13,11 +13,16 @@
> >  #ifndef QEMU_PCIE_SRIOV_H
> >  #define QEMU_PCIE_SRIOV_H
> >  
> > +typedef void (*SriovVfsUpdate)(PCIDevice *dev, uint16_t prev_num_vfs,
> > +                               uint16_t num_vfs);
> > +
> >  struct PCIESriovPF {
> >      uint16_t num_vfs;           /* Number of virtual functions created */
> >      uint8_t vf_bar_type[PCI_NUM_REGIONS];  /* Store type for each VF bar */
> >      const char *vfname;         /* Reference to the device type used for the VFs */
> >      PCIDevice **vf;             /* Pointer to an array of num_vfs VF devices */
> > +
> > +    SriovVfsUpdate pre_vfs_update;  /* Callback preceding VFs count change */
> >  };
> >  
> >  struct PCIESriovVF {
> > @@ -28,7 +33,8 @@ struct PCIESriovVF {
> >  void pcie_sriov_pf_init(PCIDevice *dev, uint16_t offset,
> >                          const char *vfname, uint16_t vf_dev_id,
> >                          uint16_t init_vfs, uint16_t total_vfs,
> > -                        uint16_t vf_offset, uint16_t vf_stride);
> > +                        uint16_t vf_offset, uint16_t vf_stride,
> > +                        SriovVfsUpdate pre_vfs_update);
> >  void pcie_sriov_pf_exit(PCIDevice *dev);
> >  
> >  /* Set up a VF bar in the SR/IOV bar area */
> > -- 
> > 2.25.1
>

Hi Michael,

A custom config_write callback was the first approach we used.
However, once implemented, we realized it looks the same as the
pcie_sriov_config_write function. To avoid code duplication and
interfering with the internal SR-IOV structures for purposes of NVMe,
we opted for this callback prior to the VFs update.
After all, we have callbacks in both approaches, config_write and the
added pre_vfs_update, so both are prone to misuse.

But I agree it may not be a good moment yet to add a new API
specifically for SR-IOV functionality, as NVMe will be the first device
to use it.

CCing Knut, perhaps as the author of SR-IOV you have some thoughts on
how the device notification of an upcoming VFs update would be handled.

Thanks,
Lukasz