Re: [PATCH v3 6/6] IB/mlx5: Use __iowrite64_copy() for write combining stores

Leon Romanovsky
2024-04-16T08:30:02Z

On Thu, Apr 11, 2024 at 01:46:19PM -0300, Jason Gunthorpe wrote:
> mlx5 has a built in self-test at driver startup to evaluate if the
> platform supports write combining to generate a 64 byte PCIe TLP or
> not. This has proven necessary because a lot of common scenarios end up
> with broken write combining (especially inside virtual machines) and there
> is other way to learn this information.
>
> This self test has been consistently failing on new ARM64 CPU
> designs (specifically with NVIDIA Grace's implementation of Neoverse
> V2). The C loop around writeq() generates some pretty terrible ARM64
> assembly, but historically this has worked on a lot of existing ARM64 CPUs
> till now.
>
> We see it succeed about 1 time in 10,000 on the worst effected
> systems. The CPU architects speculate that the load instructions
> interspersed with the stores makes the WC buffers statistically flush too
> often and thus the generation of large TLPs becomes infrequent. This makes
> the boot up test unreliable in that it indicates no write-combining,
> however userspace would be fine since it uses a ST4 instruction.
>
> Further, S390 has similar issues where only the special zpci_memcpy_toio()
> will actually generate large TLPs, and the open coded loop does not
> trigger it at all.
>
> Fix both ARM64 and S390 by switching to __iowrite64_copy() which now
> provides architecture specific variants that have a high change of
> generating a large TLP with write combining. x86 continues to use a
> similar writeq loop in the generate __iowrite64_copy().
>
> Fixes: 11f552e21755 ("IB/mlx5: Test write combining support")
> Tested-by: Niklas Schnelle <schnelle@linux.ibm.com>
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> ---
> drivers/infiniband/hw/mlx5/mem.c | 8 +++-----
> 1 file changed, 3 insertions(+), 5 deletions(-)
>

Thanks,
Acked-by: Leon Romanovsky <leonro@nvidia.com>