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Date: Mon, 20 Sep 2021 09:42:22 +0200
From: Rasmus Villemoes <linux@...musvillemoes.dk>
To: "Luck, Tony" <tony.luck@...el.com>, Borislav Petkov <bp@...en8.de>
Cc: Yazen Ghannam <Yazen.Ghannam@....com>, X86 ML <x86@...nel.org>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4] x86/mce: Get rid of machine_check_vector
On 20/09/2021 06.57, Luck, Tony wrote:
> On Fri, Sep 17, 2021 at 12:53:53PM +0200, Borislav Petkov wrote:
>> @@ -126,7 +123,9 @@ struct mca_config {
>> ser : 1,
>> recovery : 1,
>> bios_cmci_threshold : 1,
>> - __reserved : 59;
>> + /* Proper #MC exception handler is set */
>> + initialized : 1,
>> + __reserved : 58;
>
> Does this __reserved field do anything useful? It seems to
> just be an annoyance that must be updated each time a new
> bit is added. Surely the compiler will see that these bitfields
> are in a "u64" and do the math and skip to the right boundary
> without this.
Not at all. And it also seems that the current layout is not at all what
may have been intended (the bit fields do not start at an 8-byte boundary).
$ cat a.c
#include <string.h>
#include <stdint.h>
struct s1 {
char x;
uint64_t a:1,
b:1,
c:1,
d:61;
char y;
};
struct s2 {
char x;
uint64_t a:1,
b:1,
c:1;
char y;
};
struct s3 {
uint64_t x;
uint64_t a:1,
b:1,
c:1;
char y;
};
// some dummy functions to make the structs appear used and make gcc
// actually emit debug info
void f1(struct s1 *s) { memset(s, 0xff, sizeof(*s)); }
void f2(struct s2 *s) { memset(s, 0xff, sizeof(*s)); }
void f3(struct s3 *s) { memset(s, 0xff, sizeof(*s)); }
$ gcc -o a.o -c a.c -O2 -g
$ pahole a.o
struct s1 {
char x; /* 0 1 */
/* Bitfield combined with previous fields */
uint64_t a:1; /* 0: 8 8 */
uint64_t b:1; /* 0: 9 8 */
uint64_t c:1; /* 0:10 8 */
/* XXX 53 bits hole, try to pack */
/* Force alignment to the next boundary: */
uint64_t :0;
uint64_t d:61; /* 8: 0 8 */
/* XXX 3 bits hole, try to pack */
char y; /* 16 1 */
/* size: 24, cachelines: 1, members: 6 */
/* sum members: 2 */
/* sum bitfield members: 64 bits, bit holes: 2, sum bit holes: 56
bits */
/* padding: 7 */
/* last cacheline: 24 bytes */
};
struct s2 {
char x; /* 0 1 */
/* Bitfield combined with previous fields */
uint64_t a:1; /* 0: 8 8 */
uint64_t b:1; /* 0: 9 8 */
uint64_t c:1; /* 0:10 8 */
/* XXX 5 bits hole, try to pack */
/* Bitfield combined with next fields */
char y; /* 2 1 */
/* size: 8, cachelines: 1, members: 5 */
/* sum members: 2 */
/* sum bitfield members: 3 bits, bit holes: 1, sum bit holes: 5 bits */
/* padding: 5 */
/* last cacheline: 8 bytes */
};
struct s3 {
uint64_t x; /* 0 8 */
uint64_t a:1; /* 8: 0 8 */
uint64_t b:1; /* 8: 1 8 */
uint64_t c:1; /* 8: 2 8 */
/* XXX 5 bits hole, try to pack */
/* Bitfield combined with next fields */
char y; /* 9 1 */
/* size: 16, cachelines: 1, members: 5 */
/* sum members: 9 */
/* sum bitfield members: 3 bits, bit holes: 1, sum bit holes: 5 bits */
/* padding: 6 */
/* last cacheline: 16 bytes */
};
And, since in the concrete case mca_config just has four bool members
before the bitfields, we see that the 1-bit bitfields are put within the
first 8 bytes of the struct, while the __reserved field gets an entire
u64 all to itself:
struct mca_config {
_Bool dont_log_ce; /* 0 1 */
_Bool cmci_disabled; /* 1 1 */
_Bool ignore_ce; /* 2 1 */
_Bool print_all; /* 3 1 */
/* Bitfield combined with previous fields */
long long unsigned int lmce_disabled:1; /* 0:32 8 */
long long unsigned int disabled:1; /* 0:33 8 */
long long unsigned int ser:1; /* 0:34 8 */
long long unsigned int recovery:1; /* 0:35 8 */
long long unsigned int bios_cmci_threshold:1; /* 0:36 8 */
/* XXX 27 bits hole, try to pack */
/* Force alignment to the next boundary: */
long long unsigned int :0;
long long unsigned int __reserved:59; /* 8: 0 8 */
/* XXX 5 bits hole, try to pack */
signed char bootlog; /* 16 1 */
/* XXX 3 bytes hole, try to pack */
int tolerant; /* 20 4 */
int monarch_timeout; /* 24 4 */
int panic_timeout; /* 28 4 */
unsigned int rip_msr; /* 32 4 */
/* size: 40, cachelines: 1, members: 15 */
/* sum members: 21, holes: 1, sum holes: 3 */
/* sum bitfield members: 64 bits, bit holes: 2, sum bit holes: 32
bits */
/* padding: 4 */
/* last cacheline: 40 bytes */
};
But why the messy mix between 1-bit bitfields and _Bools in the first place?
Rasmus
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