Re: [RFC PATCH] dma-iommu: Add iommu_dma_max_mapping_size()

[John Garry]

On 17/05/2022 09:38, Christoph Hellwig wrote:
> On Mon, May 16, 2022 at 09:06:01PM +0800, John Garry wrote:
> > For streaming DMA mappings involving an IOMMU and whose IOVA len regularly
> > exceeds the IOVA rcache upper limit (meaning that they are not cached),
> > performance can be reduced.
> >
> > Add the IOMMU callback for DMA mapping API dma_max_mapping_size(), which
> > allows the drivers to know the mapping limit and thus limit the requested
> > IOVA lengths.
> >
> > This resolves the performance issue originally reported in [0] for a SCSI
> > HBA driver which was regularly mapping SGLs which required IOVAs in
> > excess of the IOVA caching limit. In this case the block layer limits the
> > max sectors per request - as configured in __scsi_init_queue() - which
> > will limit the total SGL length the driver tries to map and in turn limits
> > IOVA lengths requested.

BTW, on a separate topic, I noticed that even with this change my ATA 
devices have max_hw_sectors_kb of 32767, as opposed to 128 for SAS 
devices. It seems that libata-scsi - specifically ata_scsi_dev_config() 
- doesn't honour the shost max_sectors limit. I guess that is not 
intentional.

> > [0] https://lore.kernel.org/linux-iommu/20210129092120.1482-1-thunder.leizhen@xxxxxxxxxx/
> >
> > Signed-off-by: John Garry <john.garry@xxxxxxxxxx>
> > ---
> > Sending as an RFC as iommu_dma_max_mapping_size() is a soft limit, and not
> > a hard limit which I expect is the semantics of dma_map_ops.max_mapping_size
> >
> > diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> > index 09f6e1c0f9c0..e2d5205cde37 100644
> > --- a/drivers/iommu/dma-iommu.c
> > +++ b/drivers/iommu/dma-iommu.c
> > @@ -1442,6 +1442,21 @@ static unsigned long iommu_dma_get_merge_boundary(struct device *dev)
> >   	return (1UL << __ffs(domain->pgsize_bitmap)) - 1;
> >   }
> >   
>
> > +	if (!domain)
> > +		return 0;
> > +
> > +	cookie = domain->iova_cookie;
> > +	if (!cookie || cookie->type != IOMMU_DMA_IOVA_COOKIE)
> > +		return 0;
>
> Can these conditions even be true here?

I don't think so. Paranoia on my part.

> > +static inline unsigned long iova_rcache_range(void)
> > +{
> > +	return 0;
> > +}
>
> Given that IOMMU_DMA select IOMMU_IOVA there is no need for this stub.

hmmm.. ok. Policy was to be stub everything but I think that it has changed.

> Otherwise this looks sensible to me.
>
> .

Great, thanks.