Re: [PATCH][next] PCI: hv: Avoid multiple -Wflex-array-member-not-at-end warnings

Wei Liu
2025-05-13T16:45:09Z

On Tue, May 13, 2025 at 02:07:45AM +0000, Michael Kelley wrote:
> From: Michael Kelley <mhklinux@outlook.com> Sent: Sunday, April 27, 2025 8:22 AM
> >
> > From: Gustavo A. R. Silva <gustavoars@kernel.org> Sent: Friday, April 25, 2025 9:48
> > AM
> > >
> > > -Wflex-array-member-not-at-end was introduced in GCC-14, and we are
> > > getting ready to enable it, globally.
> > >
> > > Use the `DEFINE_RAW_FLEX()` helper for a few on-stack definitions
> > > of a flexible structure where the size of the flexible-array member
> > > is known at compile-time, and refactor the rest of the code,
> > > accordingly.
> > >
> > > So, with these changes, fix the following warnings:
> > >
> > > drivers/pci/controller/pci-hyperv.c:3809:35: warning: structure containing a flexible
> > > array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> > > drivers/pci/controller/pci-hyperv.c:2831:35: warning: structure containing a flexible
> > > array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> > > drivers/pci/controller/pci-hyperv.c:2468:35: warning: structure containing a flexible
> > > array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> > > drivers/pci/controller/pci-hyperv.c:1830:35: warning: structure containing a flexible
> > > array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> > > drivers/pci/controller/pci-hyperv.c:1593:35: warning: structure containing a flexible
> > > array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> > > drivers/pci/controller/pci-hyperv.c:1504:35: warning: structure containing a flexible
> > > array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> > > drivers/pci/controller/pci-hyperv.c:1424:35: warning: structure containing a flexible
> > > array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> >
> > I'm supportive of cleaning up these warnings. I've worked with the pci-hyperv.c
> > code a fair amount over the years, but never had looked closely at the on-stack
> > structs that are causing the warnings. The current code is a bit unusual and
> > perhaps unnecessarily obtuse.
> >
> > Rather than the approach you've taken below, I tried removing the flex array
> > entirely from struct pci_packet. In all cases except one, it was used only to
> > locate the end of struct pci_packet, which is the beginning of the follow-on
> > message. Locating that follow-on message can easily be done by just referencing
> > the "buf" field in the on-stack structs, or as (pkt + 1) in the dynamically allocated
> > case. In both cases, there's no need for the flex array. In the one exception, a
> > couple of minor tweaks avoids the need for the flex array as well.
> >
> > So here's an alternate approach to solving the problem. This approach is
> > 14 insertions and 15 deletions, so it's a lot less change than your approach.
> > I still don't understand why the on-stack struct are declared as (for example):
> >
> > struct {
> > struct pci_packet pkt;
> > char buf[sizeof(struct pci_read_block)];
> > } pkt;
> >
> > instead of just:
> >
> > struct {
> > struct pci_packet pkt;
> > struct pci_read_block msg;
> > } pkt;
> >
> > but that's a topic for another time. Anyway, here's my proposed diff, which I've
> > compiled and smoke-tested in a VM in the Azure cloud:
> >
>
> Gustavo -- Are you waiting for me to submit a patch with my alternate proposal?
> I had not seen any follow up, so wanted to make sure we have clarity on who
> has the next action. Thx.

Michael, I prefer your approach. Please send a patch.

Thanks,
Wei.

>
> Michael