Re: [RFC PATCH 16/21] vfio/pci: Save and restore the PCI state of the VFIO device

[Vipin Sharma <vipinsh@google.com>]

On 2025-10-18 09:25:30, Lukas Wunner wrote:
> On Fri, Oct 17, 2025 at 05:07:08PM -0700, Vipin Sharma wrote:
> > Save and restore the PCI state of the VFIO device which in the normal
> > flow is recorded by VFIO when the device FD is opened for the first time
> > and then reapplied to PCI device when the last opened device FD is
> > closed.
> > 
> > Introduce "_ser" version of the struct pci_saved_state{} and struct
> > pci_cap_saved_data{} to serialized saved PCI state for liveupdate. Store
> > PCI state in VFIO in a separate folio as the size is indeterministic at
> > build time to reserve space in struct vfio_pci_core_device_ser{}.
> 
> Unfortunately this commit message is of the type "summarize the code
> changes without explaining the reason for these changes".
> 
> Comparing the pci_saved_state_ser and pci_cap_saved_data_ser structs
> which you're introducing here with the existing pci_saved_state and
> pci_cap_saved_data structs, the only difference seems to be that
> you're adding __packed to your new structs.  Is that all?  Is that
> the only reason why these structs need to be duplicated?  Maybe
> it would make more sense to add __packed to the existing structs,
> though the gain seems minimal.
> 

It allows (in future) to build more validation and compatibility between
layout changes of struct across kernel version. We can add more fields
in the *_ser version which can act as metadata to support in
deserialization.

I do agree in the current form (with the assumption of no layout
changes) we can get away with using the existing structs. I also think
this should be taken care by PCI series instead of VFIO series.

Lets see what others also think, I am open to not adding these *_ser
structs if we should wait for a proper support for struct serialization
and work under assumption that these won't change.