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Subject: Re: [PATCH v1 7/8] vfio/type1: Add VFIO_IOMMU_CACHE_INVALIDATE
To:     "Tian, Kevin" <kevin.tian@intel.com>,
        "Liu, Yi L" <yi.l.liu@intel.com>,
        Alex Williamson <alex.williamson@redhat.com>
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        "joro@8bytes.org" <joro@8bytes.org>,
        "Raj, Ashok" <ashok.raj@intel.com>,
        "Tian, Jun J" <jun.j.tian@intel.com>,
        "Sun, Yi Y" <yi.y.sun@intel.com>,
        "jean-philippe@linaro.org" <jean-philippe@linaro.org>,
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        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Wu, Hao" <hao.wu@intel.com>
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From:   Auger Eric <eric.auger@redhat.com>
Message-ID: <7d13bdbb-e972-c301-0970-90f63ecf69fc@redhat.com>
Date:   Thu, 16 Apr 2020 14:42:38 +0200
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Hi Kevin,
On 4/16/20 2:09 PM, Tian, Kevin wrote:
>> From: Liu, Yi L <yi.l.liu@intel.com>
>> Sent: Thursday, April 16, 2020 6:40 PM
>>
>> Hi Alex,
>> Still have a direction question with you. Better get agreement with you
>> before heading forward.
>>
>>> From: Alex Williamson <alex.williamson@redhat.com>
>>> Sent: Friday, April 3, 2020 11:35 PM
>> [...]
>>>>>> + *
>>>>>> + * returns: 0 on success, -errno on failure.
>>>>>> + */
>>>>>> +struct vfio_iommu_type1_cache_invalidate {
>>>>>> +	__u32   argsz;
>>>>>> +	__u32   flags;
>>>>>> +	struct	iommu_cache_invalidate_info cache_info;
>>>>>> +};
>>>>>> +#define VFIO_IOMMU_CACHE_INVALIDATE      _IO(VFIO_TYPE,
>>> VFIO_BASE
>>>>> + 24)
>>>>>
>>>>> The future extension capabilities of this ioctl worry me, I wonder if
>>>>> we should do another data[] with flag defining that data as
>> CACHE_INFO.
>>>>
>>>> Can you elaborate? Does it mean with this way we don't rely on iommu
>>>> driver to provide version_to_size conversion and instead we just pass
>>>> data[] to iommu driver for further audit?
>>>
>>> No, my concern is that this ioctl has a single function, strictly tied
>>> to the iommu uapi.  If we replace cache_info with data[] then we can
>>> define a flag to specify that data[] is struct
>>> iommu_cache_invalidate_info, and if we need to, a different flag to
>>> identify data[] as something else.  For example if we get stuck
>>> expanding cache_info to meet new demands and develop a new uapi to
>>> solve that, how would we expand this ioctl to support it rather than
>>> also create a new ioctl?  There's also a trade-off in making the ioctl
>>> usage more difficult for the user.  I'd still expect the vfio layer to
>>> check the flag and interpret data[] as indicated by the flag rather
>>> than just passing a blob of opaque data to the iommu layer though.
>>> Thanks,
>>
>> Based on your comments about defining a single ioctl and a unified
>> vfio structure (with a @data[] field) for pasid_alloc/free, bind/
>> unbind_gpasid, cache_inv. After some offline trying, I think it would
>> be good for bind/unbind_gpasid and cache_inv as both of them use the
>> iommu uapi definition. While the pasid alloc/free operation doesn't.
>> It would be weird to put all of them together. So pasid alloc/free
>> may have a separate ioctl. It would look as below. Does this direction
>> look good per your opinion?
>>
>> ioctl #22: VFIO_IOMMU_PASID_REQUEST
>> /**
>>   * @pasid: used to return the pasid alloc result when flags == ALLOC_PASID
>>   *         specify a pasid to be freed when flags == FREE_PASID
>>   * @range: specify the allocation range when flags == ALLOC_PASID
>>   */
>> struct vfio_iommu_pasid_request {
>> 	__u32	argsz;
>> #define VFIO_IOMMU_ALLOC_PASID	(1 << 0)
>> #define VFIO_IOMMU_FREE_PASID	(1 << 1)
>> 	__u32	flags;
>> 	__u32	pasid;
>> 	struct {
>> 		__u32	min;
>> 		__u32	max;
>> 	} range;
>> };
>>
>> ioctl #23: VFIO_IOMMU_NESTING_OP
>> struct vfio_iommu_type1_nesting_op {
>> 	__u32	argsz;
>> 	__u32	flags;
>> 	__u32	op;
>> 	__u8	data[];
>> };
>>
>> /* Nesting Ops */
>> #define VFIO_IOMMU_NESTING_OP_BIND_PGTBL        0
>> #define VFIO_IOMMU_NESTING_OP_UNBIND_PGTBL      1
>> #define VFIO_IOMMU_NESTING_OP_CACHE_INVLD       2
>>
> 
> Then why cannot we just put PASID into the header since the
> majority of nested usage is associated with a pasid? 
> 
> ioctl #23: VFIO_IOMMU_NESTING_OP
> struct vfio_iommu_type1_nesting_op {
> 	__u32	argsz;
> 	__u32	flags;
> 	__u32	op;
> 	__u32   pasid;
> 	__u8	data[];
> };
> 
> In case of SMMUv2 which supports nested w/o PASID, this field can
> be ignored for that specific case.
On my side I would prefer keeping the pasid in the data[]. This is not
always used.

For instance, in iommu_cache_invalidate_info/iommu_inv_pasid_info we
devised flags to tell whether the PASID is used.

Thanks

Eric
> 
> Thanks
> Kevin
>