Re: [PATCH v7] i2c: virtio: add a virtio i2c frontend driver

[Jie Deng]

On 2021/3/12 16:58, Arnd Bergmann wrote:
> On Fri, Mar 12, 2021 at 2:33 PM Jie Deng <jie.deng@xxxxxxxxx> wrote:
>
> > +
> > +/**
> > + * struct virtio_i2c_req - the virtio I2C request structure
> > + * @out_hdr: the OUT header of the virtio I2C message
> > + * @buf: the buffer into which data is read, or from which it's written
> > + * @in_hdr: the IN header of the virtio I2C message
> > + */
> > +struct virtio_i2c_req {
> > +       struct virtio_i2c_out_hdr out_hdr;
> > +       uint8_t *buf;
> > +       struct virtio_i2c_in_hdr in_hdr;
> > +};
> The simpler request structure clearly looks better than the previous version,
> but I think I found another problem here, at least a theoretical one:
>
> When you map the headers into the DMA address space, they should
> be in separate cache lines, to allow the DMA mapping interfaces to
> perform cache management on each one without accidentally clobbering
> another member.
>
> So far I think there is an assumption that virtio buffers are always
> on cache-coherent devices, but if you ever have a virtio-i2c device
> backend on a physical interconnect that is not cache coherent (e.g. a
> microcontroller that shares the memory bus), this breaks down.
>
> You could avoid this by either allocating arrays of each type separately,
> or by marking each member that you pass to the device as
> ____cacheline_aligned.
>
>        Arnd
The virtio devices are software emulated. The backend software may need to
consider this since it may exchange data with physical devices. But I 
don't think
we need it for this interface, because no DMA operation is involved 
between the
frontend driver and backend driver.

Regards,

Jie