Re: [PATCH v5 2/5] drm/xe/vf: Document SRIOV VF restore flow

[Michal Wajdeczko <michal.wajdeczko@intel.com>]

On 29.10.2024 20:39, Tomasz Lis wrote:
> This adds a documentation chapter, containing high level flow
> of VF restore procedure.
> 
> Signed-off-by: Tomasz Lis <tomasz.lis@intel.com>
> ---
>  drivers/gpu/drm/xe/xe_sriov_vf.h | 93 ++++++++++++++++++++++++++++++++
>  1 file changed, 93 insertions(+)
> 
> diff --git a/drivers/gpu/drm/xe/xe_sriov_vf.h b/drivers/gpu/drm/xe/xe_sriov_vf.h
> index 7b8622cff2b7..22683dd643f8 100644
> --- a/drivers/gpu/drm/xe/xe_sriov_vf.h
> +++ b/drivers/gpu/drm/xe/xe_sriov_vf.h
> @@ -8,6 +8,99 @@
>  
>  struct xe_device;
>  
> +/**
> + * DOC: VF restore procedure in PF KMD and VF KMD
> + *
> + * Restoring previously saved state of a VF is one of core features of
> + * SR-IOV. All major VM Management applications allow saving and restoring
> + * the VM state, and doing that to a VM which uses SRIOV VF as one of
> + * the accessible devices requires support from KMD on both PF and VF side.
> + * VMM initiates all required operations through VFIO module, which then
> + * translates them into PF KMD calls. This description will focus on these
> + * calls, leaving out the module which initiates these steps (VFIO).
> + *
> + * In order to start the restore procedure, GuC needs to keep the VF in
> + * proper state. The PF driver can ensure VF entered this state by
> + * performing Function Level Reset on said VF. The FLR procedure ends with
> + * GuC sending message `GUC_PF_NOTIFY_VF_FLR_DONE`, and then the actual
> + * restore can begin.

hmm, I would do some rewording here, as it's not that VF_FLR_DONE
message is a prerequisite for the restore flow, it's that VF must be in
VF_READY state, which will be once the VF is successfully provisioned.

The FLR is required only if the VF was previously used.

> + *
> + * During VF Restore, state of several resources is restored. These may
> + * include local memory content (system memory is restored by VMM itself),
> + * values of MMIO registers, stateless compression metadata and others.
> + * The final resource which also needs restoring is state of the VF
> + * submission maintained within GuC. For that, `GUC_PF_OPCODE_VF_RESTORE`
> + * message is used, with reference to the state blob to be consumed by
> + * GuC.
> + *
> + * Next, when VFIO is asked to set the VM into running state, the PF driver
> + * sends `GUC_PF_TRIGGER_VF_RESUME` to GuC. When sent after restore, this
> + * changes VF state within GuC to `VF_RESFIX_BLOCKED` rather than the
> + * usual `VF_RUNNING`. At this point GuC triggers an interrupt to inform
> + * the VF KMD within the VM that it was migrated.
> + *
> + * As soon as Virtual GPU of the VM starts, the VF driver within receives
> + * the MIGRATED interrupt and schedules post-migration recovery worker.
> + * That worker queries GuC for new provisioning (using MMIO communication),
> + * and applies fixups to any non-virtualized resources used by the VF.
> + *
> + * When the VF driver is ready to continue operation on the newly connected
> + * hardware, it sends `VF2GUC_NOTIFY_RESFIX_DONE` which causes it to
> + * enter the long awaited `VF_RUNNING` state, and therefore start handling
> + * CTB messages and scheduling workloads from the VF::
> + *

nit: maybe this would look little nicer with leading lines like:

> + *      PF                             GuC                              VF
           |                               |
> + *     [ ]                             [ ]                              |
> + *     [ ] GUC_PF_NOTIFY_VF_FLR_DONE   [ ]                              |

nit: this label should be aligned to the right, closer to GuC

> + *     [ ] <---------------------------[ ]                              |

but IMO the whole diagram should start here, as it's about the restore
flow, not the FLR flow (which is just a prerequisite, like a VF
re-provisioning, that also must happen before the restore)

if you really want to include prerequisite steps in diagram then maybe
do it like this:

          PF                             GuC
           |                               |
          [ ] VF provisioning/FLR          |
          [ ] --------------------------> [ ]
          [ ]                             [ ]--- VF_READY
          [ ]                             [ ]<---|
           |                               |


> + *     [ ]                              |                               |
> + *     [ ] PF loads resources from the  |                               |
> + *     [ ]------- saved image supplied  |                               |
> + *     [ ]      |                       |                               |
> + *     [ ] <-----                       |                               |
> + *     [ ]                              |                               |
> + *     [ ] GUC_PF_OPCODE_VF_RESTORE     |                               |
> + *     [ ]---------------------------> [ ]                              |
> + *     [ ]                             [ ]  GuC loads contexts and CTB  |
> + *     [ ]                             [ ]------- state from image      |

maybe also say that GuC moves VF to VF_RESFIX_PAUSED state?

> + *     [ ]                             [ ]      |                       |
> + *     [ ] success                     [ ] <-----                       |
> + *     [ ] <---------------------------[ ]                              |
> + *     [ ]                              |                               |
> + *     [ ] GUC_PF_TRIGGER_VF_RESUME     |                               |
> + *     [ ]---------------------------> [ ]                              |
> + *     [ ]                             [ ]  GuC sets new VF state to    |
> + *     [ ]                             [ ]------- VF_RESFIX_BLOCKED     |
> + *     [ ]                             [ ]      |                       |
> + *     [ ]                             [ ] <-----                       |
> + *     [ ]                             [ ]                              |
> + *     [ ]                             [ ] GUC_INTR_SW_INT_0            |
> + *     [ ] success                     [ ]---------------------------> [ ]
> + *     [ ] <---------------------------[ ]                             [ ]
> + *      |                               |      VF2GUC_QUERY_SINGLE_KLV [ ]
> + *      |                              [ ] <---------------------------[ ]
> + *      |                              [ ]                             [ ]
> + *      |                              [ ]        new VF provisioning  [ ]
> + *      |                              [ ]---------------------------> [ ]
> + *      |                               |                              [ ]
> + *      |                               |       VF driver applies post [ ]
> + *      |                               |      migration fixups -------[ ]
> + *      |                               |                       |      [ ]
> + *      |                               |                       -----> [ ]
> + *      |                               |                              [ ]
> + *      |                               |    VF2GUC_NOTIFY_RESFIX_DONE [ ]
> + *      |                              [ ] <---------------------------[ ]
> + *      |                              [ ]                             [ ]
> + *      |                              [ ]  GuC sets new VF state to   [ ]
> + *      |                              [ ]------- VF_READY             [ ]

this should say VF_RUNNING state

> + *      |                              [ ]      |                      [ ]
> + *      |                              [ ] <-----                      [ ]
> + *      |                              [ ]                     success [ ]
> + *      |                              [ ]---------------------------> [ ]
> + *      |                               |                               |
> + *      |                               |                               |
> + */
> +
>  void xe_sriov_vf_init_early(struct xe_device *xe);
>  void xe_sriov_vf_start_migration_recovery(struct xe_device *xe);
>