Re: [PATCH v9 Kernel 3/5] vfio iommu: Add ioctl defination to unmap IOVA and return dirty bitmap

[Kirti Wankhede <kwankhede@nvidia.com>]

On 11/14/2019 1:52 AM, Alex Williamson wrote:
> On Thu, 14 Nov 2019 01:22:39 +0530
> Kirti Wankhede <kwankhede@nvidia.com> wrote:
> 
>> On 11/13/2019 4:00 AM, Alex Williamson wrote:
>>> On Tue, 12 Nov 2019 22:33:38 +0530
>>> Kirti Wankhede <kwankhede@nvidia.com> wrote:
>>>    
>>>> With vIOMMU, during pre-copy phase of migration, while CPUs are still
>>>> running, IO virtual address unmap can happen while device still keeping
>>>> reference of guest pfns. Those pages should be reported as dirty before
>>>> unmap, so that VFIO user space application can copy content of those pages
>>>> from source to destination.
>>>>
>>>> IOCTL defination added here add bitmap pointer, size and flag. If flag
>>>
>>> definition, adds
>>>    
>>>> VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP is set and bitmap memory is allocated
>>>> and bitmap_size of set, then ioctl will create bitmap of pinned pages and
>>>
>>> s/of/is/
>>>    
>>>> then unmap those.
>>>>
>>>> Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
>>>> Reviewed-by: Neo Jia <cjia@nvidia.com>
>>>> ---
>>>>    include/uapi/linux/vfio.h | 33 +++++++++++++++++++++++++++++++++
>>>>    1 file changed, 33 insertions(+)
>>>>
>>>> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
>>>> index 6fd3822aa610..72fd297baf52 100644
>>>> --- a/include/uapi/linux/vfio.h
>>>> +++ b/include/uapi/linux/vfio.h
>>>> @@ -925,6 +925,39 @@ struct vfio_iommu_type1_dirty_bitmap {
>>>>    
>>>>    #define VFIO_IOMMU_GET_DIRTY_BITMAP             _IO(VFIO_TYPE, VFIO_BASE + 17)
>>>>    
>>>> +/**
>>>> + * VFIO_IOMMU_UNMAP_DMA_GET_BITMAP - _IOWR(VFIO_TYPE, VFIO_BASE + 18,
>>>> + *				      struct vfio_iommu_type1_dma_unmap_bitmap)
>>>> + *
>>>> + * Unmap IO virtual addresses using the provided struct
>>>> + * vfio_iommu_type1_dma_unmap_bitmap.  Caller sets argsz.
>>>> + * VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP should be set to get dirty bitmap
>>>> + * before unmapping IO virtual addresses. If this flag is not set, only IO
>>>> + * virtual address are unmapped without creating pinned pages bitmap, that
>>>> + * is, behave same as VFIO_IOMMU_UNMAP_DMA ioctl.
>>>> + * User should allocate memory to get bitmap and should set size of allocated
>>>> + * memory in bitmap_size field. One bit in bitmap is used to represent per page
>>>> + * consecutively starting from iova offset. Bit set indicates page at that
>>>> + * offset from iova is dirty.
>>>> + * The actual unmapped size is returned in the size field and bitmap of pages
>>>> + * in the range of unmapped size is returned in bitmap if flag
>>>> + * VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP is set.
>>>> + *
>>>> + * No guarantee is made to the user that arbitrary unmaps of iova or size
>>>> + * different from those used in the original mapping call will succeed.
>>>> + */
>>>> +struct vfio_iommu_type1_dma_unmap_bitmap {
>>>> +	__u32        argsz;
>>>> +	__u32        flags;
>>>> +#define VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP (1 << 0)
>>>> +	__u64        iova;                        /* IO virtual address */
>>>> +	__u64        size;                        /* Size of mapping (bytes) */
>>>> +	__u64        bitmap_size;                 /* in bytes */
>>>> +	void __user *bitmap;                      /* one bit per page */
>>>> +};
>>>> +
>>>> +#define VFIO_IOMMU_UNMAP_DMA_GET_BITMAP _IO(VFIO_TYPE, VFIO_BASE + 18)
>>>> +
>>>
>>> Why not extend VFIO_IOMMU_UNMAP_DMA to support this rather than add an
>>> ioctl that duplicates the functionality and extends it??
>>
>> We do want old userspace applications to work with new kernel and
>> vice-versa, right?
>>
>> If I try to change existing VFIO_IOMMU_UNMAP_DMA ioctl structure, say if
>> add 'bitmap_size' and 'bitmap' after 'size', with below code in old
>> kernel, old kernel & new userspace will work.
>>
>>           minsz = offsetofend(struct vfio_iommu_type1_dma_unmap, size);
>>
>>           if (copy_from_user(&unmap, (void __user *)arg, minsz))
>>                   return -EFAULT;
>>
>>           if (unmap.argsz < minsz || unmap.flags)
>>                   return -EINVAL;
>>
>>
>> With new kernel it would change to:
>>           minsz = offsetofend(struct vfio_iommu_type1_dma_unmap, bitmap);
> 
> No, the minimum structure size still ends at size, we interpret flags
> and argsz to learn if the user understands those fields and optionally
> include them.  Therefore old userspace on new kernel continues to work.
> 
>>           if (copy_from_user(&unmap, (void __user *)arg, minsz))
>>                   return -EFAULT;
>>
>>           if (unmap.argsz < minsz || unmap.flags)
>>                   return -EINVAL;
>>
>> Then old userspace app will fail because unmap.argsz < minsz and might
>> be copy_from_user would cause seg fault because userspace sdk doesn't
>> contain new member variables.
>> We can't change the sequence to keep 'size' as last member, because then
>> new userspace app on old kernel will interpret it wrong.
> 
> If we have new userspace on old kernel, that userspace needs to be able
> to learn that this feature exists (new flag in the
> vfio_iommu_type1_info struct as suggested below) and only make use of it
> when available.  This is why the old kernel checks argsz against minsz.
> So long as the user passes something at least minsz in size, we have
> compatibility.  The old kernel doesn't understand the GET_DIRTY_BITMAP
> flag and will return an error if the user attempts to use it.  Thanks,
> 

Ok. So then VFIO_IOMMU_UNMAP_DMA_GET_BITMAP ioctl is not needed. I'll do 
the change. Again bitmap will be created considering smallest page size 
of iova_pgsizes

But VFIO_IOMMU_GET_DIRTY_BITMAP ioctl will still required, right?

Thanks,
Kirti

> Alex
>   
>>> Otherwise
>>> same comments as previous, in fact it's too bad we can't use this ioctl
>>> for both, but a DONT_UNMAP flag on the UNMAP_DMA ioctl seems a bit
>>> absurd.
>>>
>>> I suspect we also want a flags bit in VFIO_IOMMU_GET_INFO to indicate
>>> these capabilities are supported.
>>>    
>>
>> Ok. I'll add that.
>>
>>> Maybe for both ioctls we also want to define it as the user's
>>> responsibility to zero the bitmap, requiring the kernel to only set
>>> bits as necessary.
>>
>> Ok. Updating comment.
>>
>> Thanks,
>> Kirti
>>
>>> Thanks,
>>>
>>> Alex
>>>    
>>>>    /* -------- Additional API for SPAPR TCE (Server POWERPC) IOMMU -------- */
>>>>    
>>>>    /*
>>>    
>>
>