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Date:   Mon, 1 Feb 2021 13:15:35 -0500
From:   Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
To:     Ben Widawsky <ben.widawsky@intel.com>
Cc:     linux-cxl@vger.kernel.org, linux-acpi@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-nvdimm@lists.01.org,
        linux-pci@vger.kernel.org, Bjorn Helgaas <helgaas@kernel.org>,
        Chris Browy <cbrowy@avery-design.com>,
        Christoph Hellwig <hch@infradead.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Ira Weiny <ira.weiny@intel.com>,
        Jon Masters <jcm@jonmasters.org>,
        Jonathan Cameron <Jonathan.Cameron@huawei.com>,
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        Randy Dunlap <rdunlap@infradead.org>,
        Vishal Verma <vishal.l.verma@intel.com>,
        daniel.lll@alibaba-inc.com,
        "John Groves (jgroves)" <jgroves@micron.com>,
        "Kelley, Sean V" <sean.v.kelley@intel.com>
Subject: Re: [PATCH 07/14] cxl/mem: Add send command
Message-ID: <20210201181535.GI197521@fedora>
References: <20210130002438.1872527-1-ben.widawsky@intel.com>
 <20210130002438.1872527-8-ben.widawsky@intel.com>
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> +/**
> + * struct cxl_send_command - Send a command to a memory device.
> + * @id: The command to send to the memory device. This must be one of the
> + *	commands returned by the query command.
> + * @flags: Flags for the command (input).
> + * @rsvd: Must be zero.
> + * @retval: Return value from the memory device (output).
> + * @size_in: Size of the payload to provide to the device (input).
> + * @size_out: Size of the payload received from the device (input/output). This
> + *	      field is filled in by userspace to let the driver know how much
> + *	      space was allocated for output. It is populated by the driver to
> + *	      let userspace know how large the output payload actually was.
> + * @in_payload: Pointer to memory for payload input (little endian order).
> + * @out_payload: Pointer to memory for payload output (little endian order).
> + *
> + * Mechanism for userspace to send a command to the hardware for processing. The
> + * driver will do basic validation on the command sizes. In some cases even the
> + * payload may be introspected. Userspace is required to allocate large
> + * enough buffers for size_out which can be variable length in certain
> + * situations.
> + */
I think (and this would help if you ran `pahole` on this structure) has
some gaps in it:

> +struct cxl_send_command {
> +	__u32 id;
> +	__u32 flags;
> +	__u32 rsvd;
> +	__u32 retval;
> +
> +	struct {
> +		__s32 size_in;

Here..Maybe just add:

__u32 rsv_2;
> +		__u64 in_payload;
> +	};
> +
> +	struct {
> +		__s32 size_out;

And here. Maybe just add:
__u32 rsv_2;
> +		__u64 out_payload;
> +	};
> +};

Perhaps to prepare for the future where this may need to be expanded, you
could add a size at the start of the structure, and
maybe what version of structure it is?

Maybe for all the new structs you are adding?