Re: [PATCH RFC v2 4/5] virtio-mmio: read shm region page size

[Dmitry Osipenko <dmitry.osipenko@collabora.com>]

On 2/20/25 13:24, Sergio Lopez Pascual wrote:
> Dmitry Osipenko <dmitry.osipenko@collabora.com> writes:
> 
>> On 2/14/25 18:16, Sergio Lopez wrote:
>>> Use the newly introduced SHM_PAGE_SHIFT register to read the page shift
>>> for the shared memory region, derive the page size from it and store the
>>> resulting value into virtio_shm_region.
>>>
>>> Signed-off-by: Sergio Lopez <slp@redhat.com>
>>> ---
>>>  drivers/virtio/virtio_mmio.c     | 11 ++++++++++-
>>>  include/uapi/linux/virtio_mmio.h |  3 +++
>>>  2 files changed, 13 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/virtio/virtio_mmio.c b/drivers/virtio/virtio_mmio.c
>>> index 1f594b626d7a7734e8ec58766737a118c26bad94..0f892770739ea84b3e7be5615332773049b10ab1 100644
>>> --- a/drivers/virtio/virtio_mmio.c
>>> +++ b/drivers/virtio/virtio_mmio.c
>>> @@ -537,6 +537,7 @@ static bool vm_get_shm_region(struct virtio_device *vdev,
>>>  			      struct virtio_shm_region *region, u8 id)
>>>  {
>>>  	struct virtio_mmio_device *vm_dev = to_virtio_mmio_device(vdev);
>>> +	u8 page_shift = 0;
>>>  	u64 len, addr;
>>>
>>>  	/* Select the region we're interested in */
>>> @@ -560,7 +561,15 @@ static bool vm_get_shm_region(struct virtio_device *vdev,
>>>
>>>  	region->addr = addr;
>>>
>>> -	region->page_size = 4096;
>>> +	/* If supported by the device transport, read the region page size.
>>> +	 * The page_shift variable is initialized to zero above, so if this
>>> +	 * feature isn't supported it will result in a page_size of 4096, a
>>> +	 * default safe value.
>>> +	 */
>>> +	if (__virtio_test_bit(vdev, VIRTIO_F_SHM_PAGE_SIZE))
>>> +		page_shift = (u8) readl(vm_dev->base + VIRTIO_MMIO_SHM_PAGE_SHIFT);
>>> +
>>> +	region->page_size = 1 << (page_shift + 12);
>>>
>>>  	return true;
>>>  }
>>> diff --git a/include/uapi/linux/virtio_mmio.h b/include/uapi/linux/virtio_mmio.h
>>> index 0650f91bea6c70f935764070d825d181a2379afb..43348be30eff90ee228b6490b9d3c35ba4c50aa5 100644
>>> --- a/include/uapi/linux/virtio_mmio.h
>>> +++ b/include/uapi/linux/virtio_mmio.h
>>> @@ -133,6 +133,9 @@
>>>  #define VIRTIO_MMIO_SHM_BASE_LOW        0x0b8
>>>  #define VIRTIO_MMIO_SHM_BASE_HIGH       0x0bc
>>>
>>> +/* Shared memory region page shift */
>>> +#define VIRTIO_MMIO_SHM_PAGE_SHIFT      0x0c4
>>
>> What's the logic behind choosing 0x0c4, why not 0x0c0?
> 
> It seems like Linux doesn't support it yet, but in the specs 0x0c0 is
> already reserved for QueueReset.

See QueueReset in the spec now, thanks.

Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>

-- 
Best regards,
Dmitry