Re: [PATCH v4 15/19] PCI: liveupdate: Add feature header to struct pci_ser

["David Matlack" <dmatlack@google.com>]

On Tue Nov 10, 2026 at 5:01 AM UTC, Tarun Sahu wrote:
> PCI_SER_REQUIRED_FLAGS_SUPPORTED. Remove dev_ser_size and
> PCI_DEV_SER_MIN_SIZE so restoration derives the struct pci_dev_ser stride
> from the KHO block header entry_size. Drop __packed from struct pci_ser

Taking the stride from the block header is good, but dropping
PCI_DEV_SER_MIN_SIZE removes the only check that incoming entries are
large enough to hold the fields this kernel reads.
kho_block_set_restore() only rejects an entry_size of 0, so
pci_flb_retrieve() will now read past the end of entries smaller than
offsetofend(struct pci_dev_ser, refcount) instead of panicking. Please
keep an equivalent check against the incoming entry size.

> from the KHO block header entry_size. Drop __packed from struct pci_ser
> and struct pci_dev_ser, adding an explicit u32 reserved field to struct
> pci_ser for 8-byte alignment, and bump PCI_LUO_FLB_COMPATIBLE from pci to

Please move this to patch 1, which drops __packed from the other ABI
structs.

> pci_ser for 8-byte alignment, and bump PCI_LUO_FLB_COMPATIBLE from pci to
> pci-v2.

The PCI Live Update ABI hasn't been in a release yet, so maybe we don't
need a version bump?

[...]

> - *  * The size of each struct pci_dev_ser is recorded in
> - *    ``pci_ser.dev_ser_size`` and is never smaller than
> - *    ``PCI_DEV_SER_MIN_SIZE``. The next kernel ignores any fields beyond the
> - *    ones it knows about, and must handle entries that are smaller than its
> - *    own struct pci_dev_ser.
> + * Preserved devices are serialized as struct pci_dev_ser entries stored in
> + * a KHO block set. The block header records the entry size, allowing the
> + * next kernel to step through entries using the source kernel's stride
> + * even if fields are added in later versions.

Please keep the rules for extending struct pci_dev_ser. I'd say
something like: fields appended to struct pci_dev_ser must be gated by
a feature in struct pci_ser's header, entries are never smaller than
PCI_DEV_SER_MIN_SIZE, and the next kernel must not read fields beyond
the incoming entry size.

[...]

>  struct pci_flb_outgoing {
> -	struct pci_ser *ser;
> +	DECLARE_KHO_SER_HDR(pci_ser, *fhdr);
>  	struct kho_block_set block_set;
>  };

I'd keep the ser pointer here (and in struct pci_flb_incoming) and add
the header pointer alongside it. The header is only needed to free the
allocation and for args->data, and keeping ser avoids sprinkling
kho_ser_payload(outgoing->fhdr, pci_ser)-> throughout the file.

Also, please call it header rather than fhdr, here and throughout the
series. fhdr isn't obvious to someone reading this code for the first
time, whereas header is very obvious and clear.

[...]

> + * @reserved: Reserved for alignment.

Please document that this must be zero, so it can be used by a future
feature.