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

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Message-Id: <20250213-virtio-shm-page-size-v1-4-5ee1f9984350@redhat.com>
Date: Thu, 13 Feb 2025 16:49:18 +0100
From: Sergio Lopez <slp@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>, Jason Wang <jasowang@...hat.com>, 
 Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, 
 Eugenio Pérez <eperezma@...hat.com>, 
 David Airlie <airlied@...hat.com>, Gerd Hoffmann <kraxel@...hat.com>, 
 Gurchetan Singh <gurchetansingh@...omium.org>, 
 Chia-I Wu <olvaffe@...il.com>, 
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, 
 Simona Vetter <simona@...ll.ch>, Rob Clark <robdclark@...il.com>, 
 Dmitry Osipenko <dmitry.osipenko@...labora.com>, fnkl.kernel@...il.com
Cc: virtualization@...ts.linux.dev, linux-kernel@...r.kernel.org, 
 dri-devel@...ts.freedesktop.org, Sergio Lopez <slp@...hat.com>
Subject: [PATCH RFC 4/5] virtio-mmio: read shm region page size

Use the newly introduced SHM_PAGE_SIZE register to read the page size
and store it into virtio_shm_region.

Signed-off-by: Sergio Lopez <slp@...hat.com>
---
 drivers/virtio/virtio_mmio.c     | 13 ++++++++++++-
 include/uapi/linux/virtio_mmio.h |  3 +++
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/drivers/virtio/virtio_mmio.c b/drivers/virtio/virtio_mmio.c
index cd0a0407659517e6a318b117ba67258c59f1f983..92590c77901bee453d8c1a1ac60ad9cca90d1b59 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);
+	u16 page_size = 0;
 	u64 len, addr;

 	/* Select the region we're interested in */
@@ -560,7 +561,17 @@ static bool vm_get_shm_region(struct virtio_device *vdev,

 	region->addr = addr;

-	region->page_size = 4096 >> 12;
+	/* If supported by the device transport, read the region page size */
+	if (__virtio_test_bit(vdev, VIRTIO_F_SHM_PAGE_SIZE))
+		page_size = (u16) readl(vm_dev->base + VIRTIO_MMIO_SHM_PAGE_SIZE);
+
+	/* For backwards compatibility, if the device didn't specify a
+	 * page size, assume it to be 4096.
+	 */
+	if (page_size == 0)
+		page_size = 4096 >> 12;
+
+	region->page_size = page_size;

 	return true;
 }
diff --git a/include/uapi/linux/virtio_mmio.h b/include/uapi/linux/virtio_mmio.h
index 0650f91bea6c70f935764070d825d181a2379afb..c5a1cdad5a1c9bc954d7facfd8c05e2acbb28e96 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 size, in format PAGE_SIZE >> 12 */
+#define VIRTIO_MMIO_SHM_PAGE_SIZE       0x0c4
+
 /* Configuration atomicity value */
 #define VIRTIO_MMIO_CONFIG_GENERATION	0x0fc

-- 
2.48.1