Re: [PATCH] arm: Silence gcc warnings about arch ABI drift

Russell King (Oracle)
2024-02-19T09:27:00Z

On Mon, Feb 19, 2024 at 07:21:11AM +0100, Arnd Bergmann wrote:
> I think these should be addressed in bcachefs instead.
> While it's not the fault of bcachefs that the calling
> convention changed between gcc versions, have a look at
> the actual structure layout:
>
> struct bch_val {
> __u64 __nothing[0];
> };
> struct bpos {
> /*
> * Word order matches machine byte order - btree code treats a bpos as a
> * single large integer, for search/comparison purposes
> *
> * Note that wherever a bpos is embedded in another on disk data
> * structure, it has to be byte swabbed when reading in metadata that
> * wasn't written in native endian order:
> */
> #if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
> __u32 snapshot;
> __u64 offset;
> __u64 inode;
> #elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
> __u64 inode;
> __u64 offset; /* Points to end of extent - sectors */
> __u32 snapshot;
> #else
> #error edit for your odd byteorder.
> #endif
> } __packed
> struct bch_backpointer {
> struct bch_val v;
> __u8 btree_id;
> __u8 level;
> __u8 data_type;
> __u64 bucket_offset:40;
> __u32 bucket_len;
> struct bpos pos;
> } __packed __aligned(8);
>
> This is not something that should ever be passed by value
> into a function.

+1 - bcachefs definitely needs fixing. Passing all that as an argument
not only means that it has to be read into registers, but also when
accessing members, it has to be extracted from those registers as well.

Passing that by argument is utterly insane.

--
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 80Mbps down 10Mbps up. Decent connectivity at last!