Message-ID: <0c61ff65-fdc7-43a3-a62b-75e0d76b95fd@nvidia.com>
Date: Tue, 16 Sep 2025 12:47:17 +0300
From: Patrisious Haddad <phaddad@...dia.com>
To: Arnd Bergmann <arnd@...db.de>, Nathan Chancellor <nathan@...nel.org>,
 Jason Gunthorpe <jgg@...dia.com>
Cc: Tariq Toukan <tariqt@...dia.com>,
 Catalin Marinas <catalin.marinas@....com>, Eric Dumazet
 <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
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 <saeedm@...dia.com>, Leon Romanovsky <leon@...nel.org>,
 Mark Bloch <mbloch@...dia.com>, Sabrina Dubroca <sd@...asysnail.net>,
 Netdev <netdev@...r.kernel.org>, linux-rdma@...r.kernel.org,
 linux-kernel@...r.kernel.org, Gal Pressman <gal@...dia.com>,
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 linux-arm-kernel@...ts.infradead.org, Mark Rutland <mark.rutland@....com>,
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 Niklas Schnelle <schnelle@...ux.ibm.com>, Jijie Shao <shaojijie@...wei.com>
Subject: Re: [PATCH net-next V2] net/mlx5: Improve write-combining test
 reliability for ARM64 Grace CPUs


On 9/16/2025 11:58 AM, Arnd Bergmann wrote:
> External email: Use caution opening links or attachments
>
>
> On Tue, Sep 16, 2025, at 10:39, Patrisious Haddad wrote:
>> On 9/16/2025 2:15 AM, Nathan Chancellor wrote:
>>> External email: Use caution opening links or attachments
>> ifeq ($(ARCH),arm64)
>>           CFLAGS_lib/neon_iowrite64_copy.o += -ffreestanding
>>           CFLAGS_REMOVE_lib/neon_iowrite64_copy.o += -mgeneral-regs-only
>> endif
>>
>> Which is actually equivalent to the diff you sent, Thanks for the
>> heads-up will fix and resend.
>>
> I think it's better to handle this inside of the inline asm itself
> by adding
>
>        ".arch_extension simd;\n\t"
>
> at the start of it.

I don't get it why and how is that better - than using the correct CC 
flags for neon, also are you suggesting this in addition or instead of 
the CC flags fix natan sent above ?

Using the correct CC flags by itself is sufficient and correct - and I 
don't see other neon users using the ".arch_extension simd" you 
mentioned , so why do you think it is needed here ?

and I'm assuming you meant to add it like so? (incase it is really 
needed - I'm still not convinced it is ...)

diff --git 
a/drivers/net/ethernet/mellanox/mlx5/core/lib/wc_neon_iowrite64_copy.c 
b/drivers/net/ethernet/mellanox/mlx5/core/lib/wc_neon_iowrite64_copy.c
index 8c07d2040607..cde3d11909a8 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/lib/wc_neon_iowrite64_copy.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/lib/wc_neon_iowrite64_copy.c
@@ -6,7 +6,8 @@
  void mlx5_wc_neon_iowrite64_copy(void __iomem *to, const void *from)
  {
         asm volatile
-       ("ld1 {v0.16b, v1.16b, v2.16b, v3.16b}, [%0]\n\t"
+       (".arch_extension simd;\n\t"
+       "ld1 {v0.16b, v1.16b, v2.16b, v3.16b}, [%0]"
         "st1 {v0.16b, v1.16b, v2.16b, v3.16b}, [%1]"
         :
         : "r"(from), "r"(to)


Patrisious.

>
>       Arnd

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