linux-kernel - [PATCH RFC 3/5] virtio-pci: extend virtio_pci_cap to hold page

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Message-Id: <20250213-virtio-shm-page-size-v1-3-5ee1f9984350@redhat.com>
Date: Thu, 13 Feb 2025 16:49:17 +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 3/5] virtio-pci: extend virtio_pci_cap to hold
 page_size

Turn the 16 bit padding into a page_size field to allow the device to
pass its required page size with format PAGE_SIZE >> 12.

Signed-off-by: Sergio Lopez <slp@...hat.com>
---
 drivers/virtio/virtio_pci_modern.c | 29 +++++++++++++++++++++++++----
 include/uapi/linux/virtio_pci.h    |  2 +-
 2 files changed, 26 insertions(+), 5 deletions(-)

diff --git a/drivers/virtio/virtio_pci_modern.c b/drivers/virtio/virtio_pci_modern.c
index 79616ce5057bf3b2b88cae7e8fb7729efa9dd632..26e9cd5148c0f10209c34d12e65d64490a855d75 100644
--- a/drivers/virtio/virtio_pci_modern.c
+++ b/drivers/virtio/virtio_pci_modern.c
@@ -770,14 +770,16 @@ static void del_vq(struct virtio_pci_vq_info *info)
 	vring_del_virtqueue(vq);
 }
 
-static int virtio_pci_find_shm_cap(struct pci_dev *dev, u8 required_id,
-				   u8 *bar, u64 *offset, u64 *len)
+static int virtio_pci_find_shm_cap(struct virtio_device *vdev, struct pci_dev *dev,
+				   u8 required_id, u8 *bar, u64 *offset, u64 *len,
+				   u16 *page_size)
 {
 	int pos;
 
 	for (pos = pci_find_capability(dev, PCI_CAP_ID_VNDR); pos > 0;
 	     pos = pci_find_next_capability(dev, pos, PCI_CAP_ID_VNDR)) {
 		u8 type, cap_len, id, res_bar;
+		u16 res_psize = 0;
 		u32 tmp32;
 		u64 res_offset, res_length;
 
@@ -808,6 +810,23 @@ static int virtio_pci_find_shm_cap(struct pci_dev *dev, u8 required_id,
 		 * Looks good.
 		 */
 
+		if (__virtio_test_bit(vdev, VIRTIO_F_SHM_PAGE_SIZE)) {
+			pci_read_config_word(dev, pos + offsetof(struct virtio_pci_cap,
+								 page_size), &res_psize);
+			if (!res_psize) {
+				dev_err(&dev->dev, "%s: shm cap with invalid page size on "
+					"a device with VIRTIO_F_SHM_PAGE_SIZE feature\n",
+					__func__);
+				res_psize = 4096 >> 12;
+			}
+		}
+
+		/* For backwards compatibility, if the device didn't specify a
+		 * page size, assume it to be 4096.
+		 */
+		if (!res_psize)
+			res_psize = 4096 >> 12;
+
 		/* Read the lower 32bit of length and offset */
 		pci_read_config_dword(dev, pos + offsetof(struct virtio_pci_cap,
 							  offset), &tmp32);
@@ -829,6 +848,7 @@ static int virtio_pci_find_shm_cap(struct pci_dev *dev, u8 required_id,
 		*bar = res_bar;
 		*offset = res_offset;
 		*len = res_length;
+		*page_size = res_psize;
 
 		return pos;
 	}
@@ -841,11 +861,12 @@ static bool vp_get_shm_region(struct virtio_device *vdev,
 	struct virtio_pci_device *vp_dev = to_vp_device(vdev);
 	struct pci_dev *pci_dev = vp_dev->pci_dev;
 	u8 bar;
+	u16 page_size;
 	u64 offset, len;
 	phys_addr_t phys_addr;
 	size_t bar_len;
 
-	if (!virtio_pci_find_shm_cap(pci_dev, id, &bar, &offset, &len))
+	if (!virtio_pci_find_shm_cap(vdev, pci_dev, id, &bar, &offset, &len, &page_size))
 		return false;
 
 	phys_addr = pci_resource_start(pci_dev, bar);
@@ -865,7 +886,7 @@ static bool vp_get_shm_region(struct virtio_device *vdev,
 
 	region->len = len;
 	region->addr = (u64) phys_addr + offset;
-	region->page_size = 4096 >> 12;
+	region->page_size = page_size;
 
 	return true;
 }
diff --git a/include/uapi/linux/virtio_pci.h b/include/uapi/linux/virtio_pci.h
index 8549d4571257142ac6c9dad5c01369923791a85a..fb0ccb7a125d8178c1f78333c4d2f43540e1764b 100644
--- a/include/uapi/linux/virtio_pci.h
+++ b/include/uapi/linux/virtio_pci.h
@@ -127,7 +127,7 @@ struct virtio_pci_cap {
 	__u8 cfg_type;		/* Identifies the structure. */
 	__u8 bar;		/* Where to find it. */
 	__u8 id;		/* Multiple capabilities of the same type */
-	__u8 padding[2];	/* Pad to full dword. */
+	__u16 page_size;	/* Device page size (PAGE_SIZE >> 12). */
 	__le32 offset;		/* Offset within bar. */
 	__le32 length;		/* Length of the structure, in bytes. */
 };

-- 
2.48.1