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

From: Keqian Zhu <zhukeqian1@huawei.com>
To: Lu Baolu <baolu.lu@linux.intel.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: Alex Williamson <alex.williamson@redhat.com>,
	Cornelia Huck <cohuck@redhat.com>,
	Kirti Wankhede <kwankhede@nvidia.com>,
	<wanghaibin.wang@huawei.com>, <jiangkunkun@huawei.com>,
	<yuzenghui@huawei.com>, <lushenming@huawei.com>
Subject: Re: [PATCH v3 02/12] iommu: Add iommu_split_block interface
Date: Mon, 19 Apr 2021 17:32:08 +0800	[thread overview]
Message-ID: <491da550-dc54-42e6-ac91-13d411575fad@huawei.com> (raw)
In-Reply-To: <fb350f27-be8b-80bf-1ce8-e7e8aba26f02@linux.intel.com>

Hi Baolu,

On 2021/4/14 15:14, Lu Baolu wrote:
> 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.

Yes, I don't think up a scenario except dirty tracking.

Indeed, we'd better not make them as a generic interface.

Do you have any suggestion that underlying iommu drivers can share these code but
not make it as a generic iommu interface?

I have a not so good idea. Make the "split" interfaces as a static function, and
transfer the function pointer to start_dirty_log. But it looks weird and inflexible.

On the other hand, if a driver calls map/unmap with split/merge at the same time,
it's a bug of driver, it should follow the rule.

> 
> This also applies to iommu_merge_page().
>

Thanks,
Keqian