[PATCH v2 1/1] s390/cio: fix virtio-ccw DMA without PV

[Halil Pasic]

Commit 37db8985b211 ("s390/cio: add basic protected virtualization
support") breaks virtio-ccw devices with VIRTIO_F_IOMMU_PLATFORM for non
Protected Virtualization (PV) guests. The problem is that the dma_mask
of the ccw device, which is used by virtio core, gets changed from 64 to
31 bit, because some of the DMA allocations do require 31 bit
addressable memory. For PV the only drawback is that some of the virtio
structures must end up in ZONE_DMA because we have the bounce the
buffers mapped via DMA API anyway.

But for non PV guests we have a problem: because of the 31 bit mask
guests bigger than 2G are likely to try bouncing buffers. The swiotlb
however is only initialized for PV guests, because we don't want to
bounce anything for non PV guests. The first such map kills the guest.

Since the DMA API won't allow us to specify for each allocation whether
we need memory from ZONE_DMA (31 bit addressable) or any DMA capable
memory will do, let us use coherent_dma_mask (which is used for
allocations) to force allocating form ZONE_DMA while changing dma_mask
to DMA_BIT_MASK(64) so that at least the streaming API will regard
the whole memory DMA capable.

Signed-off-by: Halil Pasic <pasic@xxxxxxxxxxxxx>
Reported-by: Marc Hartmayer <mhartmay@xxxxxxxxxxxxx>
Suggested-by: Robin Murphy <robin.murphy@xxxxxxx>
Fixes: 37db8985b211 ("s390/cio: add basic protected virtualization support")
---

v1 --> v2:
* Fixed comment: dropped the sentence with workaround.

The idea of enabling the client code to specify on s390 whether a chunk
of allocated DMA memory is to be allocated form ZONE_DMA for each
allocation was not well received [1]. 

Making the streaming API threat all addresses as DMA capable, while
restricting the DMA API allocations to  ZONE_DMA (regardless of needed
or not) is the next best thing we can do (from s390 perspective).

[1] https://lkml.org/lkml/2019/9/23/531 
---
---
 drivers/s390/cio/cio.h    | 1 +
 drivers/s390/cio/css.c    | 7 ++++++-
 drivers/s390/cio/device.c | 2 +-
 3 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/s390/cio/cio.h b/drivers/s390/cio/cio.h
index ba7d2480613b..dcdaba689b20 100644
--- a/drivers/s390/cio/cio.h
+++ b/drivers/s390/cio/cio.h
@@ -113,6 +113,7 @@ struct subchannel {
 	enum sch_todo todo;
 	struct work_struct todo_work;
 	struct schib_config config;
+	u64 dma_mask;
 	char *driver_override; /* Driver name to force a match */
 } __attribute__ ((aligned(8)));
 
diff --git a/drivers/s390/cio/css.c b/drivers/s390/cio/css.c
index 22c55816100b..a05dbf2e97db 100644
--- a/drivers/s390/cio/css.c
+++ b/drivers/s390/cio/css.c
@@ -232,7 +232,12 @@ struct subchannel *css_alloc_subchannel(struct subchannel_id schid,
 	 * belong to a subchannel need to fit 31 bit width (e.g. ccw).
 	 */
 	sch->dev.coherent_dma_mask = DMA_BIT_MASK(31);
-	sch->dev.dma_mask = &sch->dev.coherent_dma_mask;
+	/*
+	 * But we don't have such restrictions imposed on the stuff that
+	 * is handled by the streaming API.
+	 */
+	sch->dma_mask = DMA_BIT_MASK(64);
+	sch->dev.dma_mask = &sch->dma_mask;
 	return sch;
 
 err:
diff --git a/drivers/s390/cio/device.c b/drivers/s390/cio/device.c
index 131430bd48d9..0c6245fc7706 100644
--- a/drivers/s390/cio/device.c
+++ b/drivers/s390/cio/device.c
@@ -710,7 +710,7 @@ static struct ccw_device * io_subchannel_allocate_dev(struct subchannel *sch)
 	if (!cdev->private)
 		goto err_priv;
 	cdev->dev.coherent_dma_mask = sch->dev.coherent_dma_mask;
-	cdev->dev.dma_mask = &cdev->dev.coherent_dma_mask;
+	cdev->dev.dma_mask = sch->dev.dma_mask;
 	dma_pool = cio_gp_dma_create(&cdev->dev, 1);
 	if (!dma_pool)
 		goto err_dma_pool;
-- 
2.17.1