Re: [PATCH net-next v8 03/10] net/smc: unify the structs of accept or confirm message for v1 and v2
From: Wen Gu
Date: Wed Dec 20 2023 - 07:17:25 EST
On 2023/12/20 19:37, Alexandra Winter wrote:
On 19.12.23 15:26, Wen Gu wrote:
struct smc_clc_msg_accept_confirm { /* clc accept / confirm message */
- struct smc_clc_msg_hdr hdr;
- union {
- struct smcr_clc_msg_accept_confirm r0; /* SMC-R */
- struct { /* SMC-D */
- struct smcd_clc_msg_accept_confirm_common d0;
- u32 reserved5[3];
- };
- };
-} __packed; /* format defined in RFC7609 */
-
-struct smc_clc_msg_accept_confirm_v2 { /* clc accept / confirm message */
struct smc_clc_msg_hdr hdr;
union {
struct { /* SMC-R */
struct smcr_clc_msg_accept_confirm r0;
- u8 eid[SMC_MAX_EID_LEN];
- u8 reserved6[8];
- } r1;
+ struct { /* v2 only */
+ u8 eid[SMC_MAX_EID_LEN];
+ u8 reserved6[8];
+ } __packed r1;
+ };
struct { /* SMC-D */
struct smcd_clc_msg_accept_confirm_common d0;
- __be16 chid;
- u8 eid[SMC_MAX_EID_LEN];
- u8 reserved5[8];
- } d1;
+ struct { /* v2 only, but 12 bytes reserved in v1 */
+ __be16 chid;
+ u8 eid[SMC_MAX_EID_LEN];
+ u8 reserved5[8];
+ } __packed d1;
+ };
};
};
I still think the __packed at the outmost level is the safest place.
Like you have it now the compiler could place unused memory between
ro and r1 or between d0 and d1.
Afaik compilers don't do that, if the blocks are word-aligned, but
there is no guarantee.
Up to you. My R-b still applies.
Sandy
Thank you, Sandy.
IIUC, if only outmost level has __packed, it won't work for the inner block.
e.g.
If __packed is added at d1 and r1:
struct smc_clc_msg_accept_confirm { /* clc accept / confirm message */
struct smc_clc_msg_hdr hdr;
union {
struct { /* SMC-R */
struct smcr_clc_msg_accept_confirm r0;
struct { /* v2 only */
u8 eid[SMC_MAX_EID_LEN];
u8 reserved6[8];
} __packed r1;
};
struct { /* SMC-D */
struct smcd_clc_msg_accept_confirm_common d0;
struct { /* v2 only, but 12 bytes reserved in v1 */
__be16 chid;
u8 eid[SMC_MAX_EID_LEN];
u64 gid_ext;
} __packed d1;
};
};
};
According to pahole, it will be:
struct smc_clc_msg_accept_confirm {
struct smc_clc_msg_hdr hdr; /* 0 8 */
union {
struct {
struct smcr_clc_msg_accept_confirm r0; /* 8 56 */
/* --- cacheline 1 boundary (64 bytes) --- */
struct {
u8 eid[32]; /* 64 32 */
u8 reserved6[8]; /* 96 8 */
} r1; /* 64 40 */
}; /* 8 96 */
struct {
struct smcd_clc_msg_accept_confirm_common d0; /* 8 24 */
struct {
__be16 chid; /* 32 2 */
u8 eid[32]; /* 34 32 */
/* --- cacheline 1 boundary (64 bytes) was 2 bytes ago --- */
u64 gid_ext; /* 66 8 */
} __attribute__((__packed__)) d1; /* 32 42 */
} __attribute__((__packed__)); /* 8 66 */
}; /* 8 96 */
/* size: 104, cachelines: 2, members: 2 */
/* last cacheline: 40 bytes */
};
If __packed is added at outmost level:
struct smc_clc_msg_accept_confirm { /* clc accept / confirm message */
struct smc_clc_msg_hdr hdr;
union {
struct { /* SMC-R */
struct smcr_clc_msg_accept_confirm r0;
struct { /* v2 only */
u8 eid[SMC_MAX_EID_LEN];
u8 reserved6[8];
} r1;
};
struct { /* SMC-D */
struct smcd_clc_msg_accept_confirm_common d0;
struct { /* v2 only, but 12 bytes reserved in v1 */
__be16 chid;
u8 eid[SMC_MAX_EID_LEN];
u64 gid_ext;
} d1;
};
};
} __packed;
According to pahole, it will be:
struct smc_clc_msg_accept_confirm {
struct smc_clc_msg_hdr hdr; /* 0 8 */
union {
struct {
struct smcr_clc_msg_accept_confirm r0; /* 8 56 */
/* --- cacheline 1 boundary (64 bytes) --- */
struct {
u8 eid[32]; /* 64 32 */
u8 reserved6[8]; /* 96 8 */
} r1; /* 64 40 */
}; /* 8 96 */
struct {
struct smcd_clc_msg_accept_confirm_common d0; /* 8 24 */
struct {
__be16 chid; /* 32 2 */
u8 eid[32]; /* 34 32 */
/* XXX 6 bytes hole, try to pack */
/* --- cacheline 1 boundary (64 bytes) was 8 bytes ago --- */
u64 gid_ext; /* 72 8 */
} d1; /* 32 48 */ <- doesn't work for inner d1.
}; /* 8 72 */
}; /* 8 96 */
/* size: 104, cachelines: 2, members: 2 */
/* last cacheline: 40 bytes */
};
I also considered add them all:
struct smc_clc_msg_accept_confirm { /* clc accept / confirm message */
struct smc_clc_msg_hdr hdr;
union {
struct { /* SMC-R */
struct smcr_clc_msg_accept_confirm r0;
struct { /* v2 only */
u8 eid[SMC_MAX_EID_LEN];
u8 reserved6[8];
} __packed r1;
} __packed;
struct { /* SMC-D */
struct smcd_clc_msg_accept_confirm_common d0;
struct { /* v2 only, but 12 bytes reserved in v1 */
__be16 chid;
u8 eid[SMC_MAX_EID_LEN];
u64 gid_ext;
} __packed d1;
} __packed;
};
} __packed;
but a little bit strange since for only d1 needs to packed, so I kept it as it is now.
Thanks,
Wen Gu