Re: [PATCH v3 02/12] iommu: Add iommu_split_block interface

From: Lu Baolu <baolu.lu@linux.intel.com>
To: Keqian Zhu <zhukeqian1@huawei.com>,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	iommu@lists.linux-foundation.org,
	Robin Murphy <robin.murphy@arm.com>,
	Will Deacon <will@kernel.org>, Joerg Roedel <joro@8bytes.org>,
	Yi Sun <yi.y.sun@linux.intel.com>,
	Jean-Philippe Brucker <jean-philippe@linaro.org>,
	Jonathan Cameron <Jonathan.Cameron@huawei.com>,
	Tian Kevin <kevin.tian@intel.com>
Cc: jiangkunkun@huawei.com, Cornelia Huck <cohuck@redhat.com>,
	Kirti Wankhede <kwankhede@nvidia.com>,
	lushenming@huawei.com,
	Alex Williamson <alex.williamson@redhat.com>,
	wanghaibin.wang@huawei.com
Subject: Re: [PATCH v3 02/12] iommu: Add iommu_split_block interface
Date: Wed, 14 Apr 2021 15:14:32 +0800	[thread overview]
Message-ID: <fb350f27-be8b-80bf-1ce8-e7e8aba26f02@linux.intel.com> (raw)
In-Reply-To: <20210413085457.25400-3-zhukeqian1@huawei.com>

On 4/13/21 4:54 PM, Keqian Zhu wrote:
> Block(largepage) mapping is not a proper granule for dirty log tracking.
> Take an extreme example, if DMA writes one byte, under 1G mapping, the
> dirty amount reported is 1G, but under 4K mapping, the dirty amount is
> just 4K.
> 
> This adds a new interface named iommu_split_block in IOMMU base layer.
> A specific IOMMU driver can invoke it during start dirty log. If so, the
> driver also need to realize the split_block iommu ops.
> 
> We flush all iotlbs after the whole procedure is completed to ease the
> pressure of IOMMU, as we will hanle a huge range of mapping in general.
> 
> Signed-off-by: Keqian Zhu <zhukeqian1@huawei.com>
> Signed-off-by: Kunkun Jiang <jiangkunkun@huawei.com>
> ---
>   drivers/iommu/iommu.c | 41 +++++++++++++++++++++++++++++++++++++++++
>   include/linux/iommu.h | 11 +++++++++++
>   2 files changed, 52 insertions(+)
> 
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index 667b2d6d2fc0..bb413a927870 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -2721,6 +2721,47 @@ int iommu_domain_set_attr(struct iommu_domain *domain,
>   }
>   EXPORT_SYMBOL_GPL(iommu_domain_set_attr);
>   
> +int iommu_split_block(struct iommu_domain *domain, unsigned long iova,
> +		      size_t size)
> +{
> +	const struct iommu_ops *ops = domain->ops;
> +	unsigned int min_pagesz;
> +	size_t pgsize;
> +	bool flush = false;
> +	int ret = 0;
> +
> +	if (unlikely(!ops || !ops->split_block))
> +		return -ENODEV;
> +
> +	min_pagesz = 1 << __ffs(domain->pgsize_bitmap);
> +	if (!IS_ALIGNED(iova | size, min_pagesz)) {
> +		pr_err("unaligned: iova 0x%lx size 0x%zx min_pagesz 0x%x\n",
> +		       iova, size, min_pagesz);
> +		return -EINVAL;
> +	}
> +
> +	while (size) {
> +		flush = true;
> +
> +		pgsize = iommu_pgsize(domain, iova, size);
> +
> +		ret = ops->split_block(domain, iova, pgsize);
> +		if (ret)
> +			break;
> +
> +		pr_debug("split handled: iova 0x%lx size 0x%zx\n", iova, pgsize);
> +
> +		iova += pgsize;
> +		size -= pgsize;
> +	}
> +
> +	if (flush)
> +		iommu_flush_iotlb_all(domain);
> +
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(iommu_split_block);

Do you really have any consumers of this interface other than the dirty
bit tracking? If not, I don't suggest to make this as a generic IOMMU
interface.

There is an implicit requirement for such interfaces. The
iommu_map/unmap(iova, size) shouldn't be called at the same time.
Currently there's no such sanity check in the iommu core. A poorly
written driver could mess up the kernel by misusing this interface.

This also applies to iommu_merge_page().

Best regards,
baolu
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