RE: [PATCH] dma-mapping: benchmark: check the validity of dma mask bits

["Song Bao Hua (Barry Song)" <song.bao.hua@hisilicon.com>]

> -----Original Message-----
> From: Robin Murphy [mailto:robin.murphy@arm.com]
> Sent: Saturday, December 19, 2020 7:10 AM
> To: Song Bao Hua (Barry Song) <song.bao.hua@hisilicon.com>; hch@lst.de;
> m.szyprowski@samsung.com
> Cc: iommu@lists.linux-foundation.org; Linuxarm <linuxarm@huawei.com>; Dan
> Carpenter <dan.carpenter@oracle.com>
> Subject: Re: [PATCH] dma-mapping: benchmark: check the validity of dma mask
> bits
> 
> On 2020-12-12 10:18, Barry Song wrote:
> > While dma_mask_bits is larger than 64, the bahvaiour is undefined. On the
> > other hand, dma_mask_bits which is smaller than 20 (1MB) makes no sense
> > in real hardware.
> >
> > Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
> > Signed-off-by: Barry Song <song.bao.hua@hisilicon.com>
> > ---
> >   kernel/dma/map_benchmark.c | 6 ++++++
> >   1 file changed, 6 insertions(+)
> >
> > diff --git a/kernel/dma/map_benchmark.c b/kernel/dma/map_benchmark.c
> > index b1496e744c68..19f661692073 100644
> > --- a/kernel/dma/map_benchmark.c
> > +++ b/kernel/dma/map_benchmark.c
> > @@ -214,6 +214,12 @@ static long map_benchmark_ioctl(struct file *file,
> unsigned int cmd,
> >   			return -EINVAL;
> >   		}
> >
> > +		if (map->bparam.dma_bits < 20 ||
> 
> FWIW I don't think we need to bother with a lower limit here - it's
> unsigned, and a pointlessly small value will fail gracefully when we
> come to actually set the mask anyway. We only need to protect kernel
> code from going wrong, not userspace from being stupid to its own detriment.

I am not sure if kernel driver can reject small dma mask bit if drivers
don't handle it properly.
As a month ago, when I was debugging dma map benchmark, I set a value
less than 32 to devices behind arm-smmu-v3, it could always succeed.
But dma_map_single() was always failing.
At that time, I didn't debug this issue. Not sure the latest status of
iommu driver.

drivers/iommu/intel/iommu.c used to have a dma_supported() to reject
small dma_mask:
static const struct dma_map_ops bounce_dma_ops = {
	...
	.dma_supported		= dma_direct_supported,
};


> 
> Robin.
> 
> > +		    map->bparam.dma_bits > 64) {
> > +			pr_err("invalid dma_bits\n");
> > +			return -EINVAL;
> > +		}
> > +
> >   		if (map->bparam.node != NUMA_NO_NODE &&
> >   		    !node_possible(map->bparam.node)) {
> >   			pr_err("invalid numa node\n");
> >

Thanks
Barry

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