From: Christophe Leroy <christophe.leroy@csgroup.eu>
To: Guenter Roeck <linux@roeck-us.net>,
Charlie Jenkins <charlie@rivosinc.com>,
David Laight <David.Laight@aculab.com>
Cc: 'Russell King' <linux@armlinux.org.uk>,
Palmer Dabbelt <palmer@dabbelt.com>,
Andrew Morton <akpm@linux-foundation.org>,
Helge Deller <deller@gmx.de>,
"James E.J. Bottomley" <James.Bottomley@hansenpartnership.com>,
Parisc List <linux-parisc@vger.kernel.org>,
Arnd Bergmann <arnd@arndb.de>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Palmer Dabbelt <palmer@rivosinc.com>,
Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH v10] lib: checksum: Use aligned accesses for ip_fast_csum and csum_ipv6_magic tests
Date: Tue, 27 Feb 2024 06:47:38 +0000 [thread overview]
Message-ID: <9b4ce664-3ddb-4789-9d5d-8824f9089c48@csgroup.eu> (raw)
In-Reply-To: <cdd09f7a-83b2-41ba-a32c-9886dd79c43e@roeck-us.net>
Le 27/02/2024 à 00:48, Guenter Roeck a écrit :
> On 2/26/24 15:17, Charlie Jenkins wrote:
>> On Mon, Feb 26, 2024 at 10:33:56PM +0000, David Laight wrote:
>>> ...
>>>> I think you misunderstand. "NET_IP_ALIGN offset is what the kernel
>>>> defines to be supported" is a gross misinterpretation. It is not
>>>> "defined to be supported" at all. It is the _preferred_ alignment
>>>> nothing more, nothing less.
>>
>> This distinction is arbitrary in practice, but I am open to being proven
>> wrong if you have data to back up this statement. If the driver chooses
>> to not follow this, then the driver might not work. ARM defines the
>> NET_IP_ALIGN to be 2 to pad out the header to be on the supported
>> alignment. If the driver chooses to pad with one byte instead of 2
>> bytes, the driver may fail to work as the CPU may stall after the
>> misaligned access.
>>
>>>
>>> I'm sure I've seen code that would realign IP headers to a 4 byte
>>> boundary before processing them - but that might not have been in
>>> Linux.
>>>
>>> I'm also sure there are cpu which will fault double length misaligned
>>> memory transfers - which might be used to marginally speed up code.
>>> Assuming more than 4 byte alignment for the IP header is likely
>>> 'wishful thinking'.
>>>
>>> There is plenty of ethernet hardware that can only write frames
>>> to even boundaries and plenty of cpu that fault misaligned accesses.
>>> There are even cases of both on the same silicon die.
>>>
>>> You also pretty much never want a fault handler to fixup misaligned
>>> ethernet frames (or really anything else for that matter).
>>> It is always going to be better to check in the code itself.
>>>
>>> x86 has just made people 'sloppy' :-)
>>>
>>> David
>>>
>>> -
>>> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes,
>>> MK1 1PT, UK
>>> Registration No: 1397386 (Wales)
>>>
>>
>> If somebody has a solution they deem to be better, I am happy to change
>> this test case. Otherwise, I would appreciate a maintainer resolving
>> this discussion and apply this fix.
>>
> Agreed.
>
> I do have a couple of patches which add explicit unaligned tests as well as
> corner case tests (which are intended to trigger as many carry overflows
> as possible). Once I get those working reliably, I'll be happy to submit
> them as additional tests.
>
The functions definitely have to work at least with and without VLAN,
which means the alignment cannot be greater than 4 bytes. That's also
the outcome of the discussion.
Therefore, we can easily fix the tests with for instance the following
changes. For the IPv6 test I switched proto and csum to keep csum
aligned. (In addition expected values need to be recalculated for the
IPv6 case).
diff --git a/lib/checksum_kunit.c b/lib/checksum_kunit.c
index bf70850035c7..26b0dbc5b8fd 100644
--- a/lib/checksum_kunit.c
+++ b/lib/checksum_kunit.c
@@ -581,7 +581,7 @@ static void test_ip_fast_csum(struct kunit *test)
u16 expected;
for (int len = IPv4_MIN_WORDS; len < IPv4_MAX_WORDS; len++) {
- for (int index = 0; index < NUM_IP_FAST_CSUM_TESTS; index++) {
+ for (int index = 0; index < NUM_IP_FAST_CSUM_TESTS; index += 4) {
csum_result = ip_fast_csum(random_buf + index, len);
expected =
expected_fast_csum[(len - IPv4_MIN_WORDS) *
@@ -603,12 +603,10 @@ static void test_csum_ipv6_magic(struct kunit *test)
const int daddr_offset = sizeof(struct in6_addr);
const int len_offset = sizeof(struct in6_addr) + sizeof(struct in6_addr);
- const int proto_offset = sizeof(struct in6_addr) + sizeof(struct
in6_addr) +
- sizeof(int);
- const int csum_offset = sizeof(struct in6_addr) + sizeof(struct
in6_addr) +
- sizeof(int) + sizeof(char);
+ const int csum_offset = len_offset + sizeof(int);
+ const int proto_offset = csum_offset + sizeof(int);
- for (int i = 0; i < NUM_IPv6_TESTS; i++) {
+ for (int i = 0; i < NUM_IPv6_TESTS; i += 4) {
saddr = (const struct in6_addr *)(random_buf + i);
daddr = (const struct in6_addr *)(random_buf + i +
daddr_offset);
---
We could do even better by taking into account
CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS and do +1 when it is selected and
+4 when it is not selected.
Christophe
next prev parent reply other threads:[~2024-02-27 6:47 UTC|newest]
Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-02-23 22:11 [PATCH v10] lib: checksum: Use aligned accesses for ip_fast_csum and csum_ipv6_magic tests Charlie Jenkins
2024-02-25 15:58 ` Guenter Roeck
2024-02-26 11:34 ` Christophe Leroy
2024-02-26 11:47 ` Russell King (Oracle)
2024-02-26 11:57 ` Christophe Leroy
2024-02-26 12:03 ` Russell King (Oracle)
2024-02-26 16:44 ` Guenter Roeck
2024-02-26 17:50 ` Russell King (Oracle)
2024-02-26 18:35 ` Charlie Jenkins
2024-02-26 19:06 ` Russell King (Oracle)
2024-02-26 19:19 ` Charlie Jenkins
2024-02-26 22:33 ` David Laight
2024-02-26 23:17 ` Charlie Jenkins
2024-02-26 23:48 ` Guenter Roeck
2024-02-27 6:47 ` Christophe Leroy [this message]
2024-02-27 10:28 ` Russell King (Oracle)
2024-02-27 11:32 ` Christophe Leroy
2024-02-27 17:54 ` Charlie Jenkins
2024-02-27 18:11 ` Christophe Leroy
2024-02-27 18:21 ` Charlie Jenkins
2024-02-27 18:35 ` Christophe Leroy
2024-02-27 19:04 ` Charlie Jenkins
2024-02-27 19:31 ` Guenter Roeck
2024-02-27 22:44 ` David Laight
2024-02-28 5:19 ` Guenter Roeck
2024-02-28 0:24 ` Charlie Jenkins
2024-02-28 0:21 ` Charlie Jenkins
2024-02-28 7:25 ` Christophe Leroy
2024-02-28 7:59 ` Guenter Roeck
2024-02-28 10:15 ` Geert Uytterhoeven
2024-02-28 15:40 ` Guenter Roeck
2024-02-29 8:07 ` David Gow
2024-02-29 19:38 ` Charlie Jenkins
2024-02-29 20:22 ` Guenter Roeck
2024-03-01 7:00 ` Christophe Leroy
2024-03-01 6:46 ` Christophe Leroy
2024-03-01 16:24 ` Guenter Roeck
2024-03-01 20:47 ` Guenter Roeck
2024-02-27 11:17 ` Geert Uytterhoeven
2024-02-27 17:55 ` Charlie Jenkins
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