linux-kernel - Re: [PATCH v2 08/16] iommu: define and export iommu_access_remote

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Date:   Tue, 7 Mar 2023 08:33:07 -0800
From:   Fenghua Yu <fenghua.yu@...el.com>
To:     Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>
CC:     Vinod Koul <vkoul@...nel.org>, Dave Jiang <dave.jiang@...el.com>,
        <dmaengine@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Alistair Popple <apopple@...dia.com>,
        Joerg Roedel <joro@...tes.org>,
        "Will Deacon" <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        "Lorenzo Stoakes" <lstoakes@...il.com>,
        Christoph Hellwig <hch@...radead.org>, <iommu@...ts.linux.dev>
Subject: Re: [PATCH v2 08/16] iommu: define and export
 iommu_access_remote_vm()

Hi, Jean,

On 3/7/23 00:40, Jean-Philippe Brucker wrote:
> Hi Fenghua,
> 
> On Mon, Mar 06, 2023 at 08:31:30AM -0800, Fenghua Yu wrote:
>> Define and export iommu_access_remote_vm() to allow IOMMU related
>> drivers to access user address space by PASID.
>>
>> The IDXD driver would like to use it to write the user's completion
>> record that the hardware device is not able to write to due to user
>> page fault.
>>
>> Without the API, it's complex for IDXD driver to copy completion record
>> to a process' fault address for two reasons:
>> 1. access_remote_vm() is not exported and shouldn't be exported for
>>     drivers because drivers may easily cause mm reference issue.
>> 2. user frees fault address pages to trigger fault by IDXD device.
>>
>> The driver has to call iommu_sva_find(), kthread_use_mm(), re-implement
>> majority of access_remote_vm() etc to access remote vm.
>>
>> This IOMMU specific API hides these details and provides a clean interface
>> for idxd driver and potentially other IOMMU related drivers.
>>
>> Suggested-by: Alistair Popple <apopple@...dia.com>
>> Signed-off-by: Fenghua Yu <fenghua.yu@...el.com>
>> Cc: Joerg Roedel <joro@...tes.org>
>> Cc: Will Deacon <will@...nel.org>
>> Cc: Robin Murphy <robin.murphy@....com>
>> Cc: Alistair Popple <apopple@...dia.com>
>> Cc: Lorenzo Stoakes <lstoakes@...il.com>
>> Cc: Christoph Hellwig <hch@...radead.org>
>> Cc: iommu@...ts.linux.dev
>> ---
>> v2:
>> - Define and export iommu_access_remote_vm() for IDXD driver to write
>>    completion record to user address space. This change removes
>>    patch 8 and 9 in v1 (Alistair Popple)
>>
>>   drivers/iommu/iommu-sva.c | 35 +++++++++++++++++++++++++++++++++++
>>   include/linux/iommu.h     |  9 +++++++++
>>   2 files changed, 44 insertions(+)
>>
>> diff --git a/drivers/iommu/iommu-sva.c b/drivers/iommu/iommu-sva.c
>> index 24bf9b2b58aa..1d7a0aee58f7 100644
>> --- a/drivers/iommu/iommu-sva.c
>> +++ b/drivers/iommu/iommu-sva.c
>> @@ -71,6 +71,41 @@ struct mm_struct *iommu_sva_find(ioasid_t pasid)
>>   }
>>   EXPORT_SYMBOL_GPL(iommu_sva_find);
>>   
>> +/**
>> + * iommu_access_remote_vm - access another process' address space by PASID
>> + * @pasid:	Process Address Space ID assigned to the mm
>> + * @addr:	start address to access
>> + * @buf:	source or destination buffer
>> + * @len:	number of bytes to transfer
>> + * @gup_flags:	flags modifying lookup behaviour
>> + *
>> + * Another process' address space is found by PASID. A reference on @mm
>> + * is taken and released inside the function.
>> + *
>> + * Return: number of bytes copied from source to destination.
>> + */
>> +int iommu_access_remote_vm(ioasid_t pasid, unsigned long addr, void *buf,
>> +			   int len, unsigned int gup_flags)
>> +{
>> +	struct mm_struct *mm;
>> +	int copied;
>> +
>> +	mm = iommu_sva_find(pasid);
> 
> The ability to find a mm by PASID is being removed, see
> https://lore.kernel.org/linux-iommu/20230301235646.2692846-4-jacob.jun.pan@linux.intel.com/
> 

Thank you very much for pointing out this.

I talked to Jacob just now. He will keep iommu_sva_find() function
in his next version because this patch is still using the function. He 
agrees that I can still call iommu_sva_find() in this patch.

-Fenghua