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

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Message-Id: <20250214-virtio-shm-page-size-v2-4-aa1619e6908b@redhat.com>
Date: Fri, 14 Feb 2025 16:16:58 +0100
From: Sergio Lopez <slp@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>, 
 Daniel Verkamp <dverkamp@...omium.org>, 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 v2 4/5] virtio-mmio: read shm region page size

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@...hat.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
+
 /* Configuration atomicity value */
 #define VIRTIO_MMIO_CONFIG_GENERATION	0x0fc

-- 
2.48.1