linux-kernel - Re: [PATCH v3 4/8] iommu/amd: Add debugfs support to dump IOMMU command buffer

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Message-ID: <014da701-dc74-4082-98b0-378da8ebbfd0@amd.com>
Date: Thu, 13 Mar 2025 16:08:30 +0530
From: Vasant Hegde <vasant.hegde@....com>
To: Dheeraj Kumar Srivastava <dheerajkumar.srivastava@....com>,
 joro@...tes.org, suravee.suthikulpanit@....com, will@...nel.org,
 robin.murphy@....com, linux-kernel@...r.kernel.org, iommu@...ts.linux.dev
Subject: Re: [PATCH v3 4/8] iommu/amd: Add debugfs support to dump IOMMU
 command buffer

Dheeraj,


On 2/6/2025 11:29 AM, Dheeraj Kumar Srivastava wrote:
> IOMMU driver sends command to IOMMU hardware via command buffer. In cases
> where IOMMU hardware fails to process commands in command buffer, dumping
> it is a valuable input to debug the issue.
> 
> IOMMU hardware processes command buffer entry at offset equals to the head
> pointer. Dumping just the entry at the head pointer may not always be
> useful. The current head may not be pointing to the entry of the command
> buffer which is causing the issue. IOMMU Hardware may have processed the
> entry and updated the head pointer. So dumping the entire command buffer
> gives a broad understanding of what hardware was/is doing. The command
> buffer dump will have all entries from start to end of the command buffer.
> Along with that, it will have a head and tail command buffer pointer
> register dump to facilitate where the IOMMU driver and hardware are in
> the command buffer for injecting and processing the entries respectively.
> 
> Command buffer is a per IOMMU data structure. So dumping on per IOMMU
> basis.
> eg. To get command buffer dump for iommu<x>
>     #cat /sys/kernel/debug/iommu/amd/iommu<x>/cmdbuf
> 
> Signed-off-by: Dheeraj Kumar Srivastava <dheerajkumar.srivastava@....com>
> ---
>  drivers/iommu/amd/amd_iommu_types.h |  7 +++++++
>  drivers/iommu/amd/debugfs.c         | 26 ++++++++++++++++++++++++++
>  drivers/iommu/amd/iommu.c           |  7 -------
>  3 files changed, 33 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/iommu/amd/amd_iommu_types.h b/drivers/iommu/amd/amd_iommu_types.h
> index 0bbda60d3cdc..c4ca6d9c6b51 100644
> --- a/drivers/iommu/amd/amd_iommu_types.h
> +++ b/drivers/iommu/amd/amd_iommu_types.h
> @@ -892,6 +892,13 @@ struct dev_table_entry {
>  	};
>  };
>  
> +/*
> + * Structure defining one entry in the command buffer
> + */
> +struct iommu_cmd {
> +	u32 data[4];
> +};
> +
>  /*
>   * Structure to sture persistent DTE flags from IVHD
>   */
> diff --git a/drivers/iommu/amd/debugfs.c b/drivers/iommu/amd/debugfs.c
> index dc5fdc905754..bf718e7258c5 100644
> --- a/drivers/iommu/amd/debugfs.c
> +++ b/drivers/iommu/amd/debugfs.c
> @@ -103,6 +103,30 @@ static int iommu_capability_show(struct seq_file *m, void *unused)
>  }
>  DEFINE_SHOW_STORE_ATTRIBUTE(iommu_capability);
>  
> +static int iommu_cmdbuf_show(struct seq_file *m, void *unused)
> +{
> +	struct amd_iommu *iommu = m->private;
> +	struct iommu_cmd *cmd;
> +	unsigned long flag;
> +	u32 head, tail;
> +	int i;
> +
> +	raw_spin_lock_irqsave(&iommu->lock, flag);
> +	head = readl(iommu->mmio_base + MMIO_CMD_HEAD_OFFSET);
> +	tail = readl(iommu->mmio_base + MMIO_CMD_TAIL_OFFSET);
> +	seq_printf(m, "CMD Buffer Head Offset:%d Tail Offset:%d\n",
> +		   (head >> 4) & 0x7fff, (tail >> 4) & 0x7fff);
> +	for (i = 0; i < CMD_BUFFER_ENTRIES; i++) {
> +		cmd = (struct iommu_cmd *)(iommu->cmd_buf + i * sizeof(*cmd));
> +		seq_printf(m, "%3d: %08x%08x%08x%08x\n", i, cmd->data[0],

Can you give space between words so that its easy to read?

-Vasant