Re: [PATCH v2 1/2] vfio: introduce vfio_dma_rw to read/write a range of IOVAs

["Mika Penttilä" <mika.penttila@nextfour.com>]

On 16.1.2020 4.59, Alex Williamson wrote:
> On Thu, 16 Jan 2020 02:30:52 +0000
> Mika Penttilä <mika.penttila@nextfour.com> wrote:
>
>> On 15.1.2020 22.06, Alex Williamson wrote:
>>> On Tue, 14 Jan 2020 22:53:03 -0500
>>> Yan Zhao <yan.y.zhao@intel.com> wrote:
>>>   
>>>> vfio_dma_rw will read/write a range of user space memory pointed to by
>>>> IOVA into/from a kernel buffer without pinning the user space memory.
>>>>
>>>> TODO: mark the IOVAs to user space memory dirty if they are written in
>>>> vfio_dma_rw().
>>>>
>>>> Cc: Kevin Tian <kevin.tian@intel.com>
>>>> Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
>>>> ---
>>>>    drivers/vfio/vfio.c             | 45 +++++++++++++++++++
>>>>    drivers/vfio/vfio_iommu_type1.c | 76 +++++++++++++++++++++++++++++++++
>>>>    include/linux/vfio.h            |  5 +++
>>>>    3 files changed, 126 insertions(+)
>>>>
>>>> diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
>>>> index c8482624ca34..8bd52bc841cf 100644
>>>> --- a/drivers/vfio/vfio.c
>>>> +++ b/drivers/vfio/vfio.c
>>>> @@ -1961,6 +1961,51 @@ int vfio_unpin_pages(struct device *dev, unsigned long *user_pfn, int npage)
>>>>    }
>>>>    EXPORT_SYMBOL(vfio_unpin_pages);
>>>>    
>>>> +/*
>>>> + * Read/Write a range of IOVAs pointing to user space memory into/from a kernel
>>>> + * buffer without pinning the user space memory
>>>> + * @dev [in]  : device
>>>> + * @iova [in] : base IOVA of a user space buffer
>>>> + * @data [in] : pointer to kernel buffer
>>>> + * @len [in]  : kernel buffer length
>>>> + * @write     : indicate read or write
>>>> + * Return error code on failure or 0 on success.
>>>> + */
>>>> +int vfio_dma_rw(struct device *dev, dma_addr_t iova, void *data,
>>>> +		   size_t len, bool write)
>>>> +{
>>>> +	struct vfio_container *container;
>>>> +	struct vfio_group *group;
>>>> +	struct vfio_iommu_driver *driver;
>>>> +	int ret = 0;
>> Do you know the iova given to vfio_dma_rw() is indeed a gpa and not iova
>> from a iommu mapping? So isn't it you actually assume all the guest is
>> pinned,
>> like from device assignment?
>>
>> Or who and how is the vfio mapping added before the vfio_dma_rw() ?
> vfio only knows about IOVAs, not GPAs.  It's possible that IOVAs are
> identity mapped to the GPA space, but a VM with a vIOMMU would quickly
> break any such assumption.  Pinning is also not required.  This access
> is via the CPU, not the I/O device, so we don't require the memory to
> be pinning and it potentially won't be for a non-IOMMU backed mediated
> device.  The intention here is that via the mediation of an mdev
> device, a vendor driver would already know IOVA ranges for the device
> to access via the guest driver programming of the device.  Thanks,
>
> Alex

Thanks Alex... you mean IOVA is in the case of iommu already a 
iommu-translated address to a user space VA in VM host space?
How does it get to hold on that? What piece of meditation is responsible 
for this?

--Mika