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

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Message-ID: <aPNA6q-i2GWTl0-A@wunner.de>
Date: Sat, 18 Oct 2025 09:25:30 +0200
From: Lukas Wunner <lukas@...ner.de>
To: Vipin Sharma <vipinsh@...gle.com>
Cc: bhelgaas@...gle.com, alex.williamson@...hat.com,
	pasha.tatashin@...een.com, dmatlack@...gle.com, jgg@...pe.ca,
	graf@...zon.com, pratyush@...nel.org, gregkh@...uxfoundation.org,
	chrisl@...nel.org, rppt@...nel.org, skhawaja@...gle.com,
	parav@...dia.com, saeedm@...dia.com, kevin.tian@...el.com,
	jrhilke@...gle.com, david@...hat.com, jgowans@...zon.com,
	dwmw2@...radead.org, epetron@...zon.de, junaids@...gle.com,
	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
	kvm@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [RFC PATCH 16/21] vfio/pci: Save and restore the PCI state of
 the VFIO device

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.

Thanks,

Lukas