Re: [RFC PATCH v4 5/6] drivers/migrate_offload: add DMA batch copy driver (dcbm)

[Garg, Shivank]

On 3/9/2026 11:34 PM, Gregory Price wrote:
> On Mon, Mar 09, 2026 at 12:07:31PM +0000, Shivank Garg wrote:
>> diff --git a/drivers/migrate_offload/Kconfig b/drivers/migrate_offload/Kconfig
>> new file mode 100644
>> index 000000000000..0bbaedbae4ad
>> --- /dev/null
>> +++ b/drivers/migrate_offload/Kconfig
>> @@ -0,0 +1,8 @@
>> +config DCBM_DMA
>> +	bool "DMA Core Batch Migrator"
> 
> Should this be tri-state or is built-in the only valid state?

Right, will fix this.

> 
>> +static int setup_sg_tables(struct dma_work *work, struct list_head **src_pos,
>> +		struct list_head **dst_pos, int nr)
>> +{
> ... snip ..
>> +	dev = dmaengine_get_dma_device(work->chan);
>> +	if (!dev) {
>> +		ret = -ENODEV;
>> +		goto err_free_dst_table;
>> +	}
>> +	ret = dma_map_sgtable(dev, work->src_sgt, DMA_TO_DEVICE,
>> +			DMA_ATTR_SKIP_CPU_SYNC | DMA_ATTR_NO_KERNEL_MAPPING);
>> +	if (ret)
>> +		goto err_free_dst_table;
>> +	ret = dma_map_sgtable(dev, work->dst_sgt, DMA_FROM_DEVICE,
>> +			DMA_ATTR_SKIP_CPU_SYNC | DMA_ATTR_NO_KERNEL_MAPPING);
>> +	if (ret)
>> +		goto err_unmap_src;
>> +
>> +	if (work->src_sgt->nents != work->dst_sgt->nents) {
>> +		ret = -EINVAL;
>> +		goto err_unmap_dst;
>> +	}
> 
> Fairly new to dma space, but I thought the dma stuff could merge pages
> on iommu systems. Wouldn't this check hit fairly often?
>

I tested on Zen3 system (with PTDMA) across different folio sizes and
didn't see this check hit in ~1000s of runs. I'll think more about this
problem and discuss with IOMMU team.

For now the focus of this series is the batch migration and core offload
infrastructure. This is a reference driver to test the offload plumbing
potential performance benefit. I'm happy to refine this once the design
settles.

Best regards,
Shivank