[PATCH] virtio: limit ring size to a maximum of 128 descriptor entries

[Mark Cave-Ayland]

QEMU commit c9b7d9ec21 "virtio: increase virtqueue size for virtio-scsi and
virtio-blk" increased the number of queue descriptors from 128 to 256 which
caused OpenBIOS to fail when booting from a virtio-blk device under
qemu-system-ppc due to a memory overflow.

Update the virtio-blk driver so that it uses a maximum of 128 descriptor
entries (the previous value) to fix the issue, and also ensure that any
further increases in virtqueue size will not cause the same failure.

Note that this commit also fixes the vring_area size calculation which was
incorrectly based upon the number of virtqueues, and not the number of
descriptor entries.

Signed-off-by: Mark Cave-Ayland 
---
 drivers/virtio.c | 7 ++++++-
 drivers/virtio.h | 5 +++--
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/drivers/virtio.c b/drivers/virtio.c
index ff7ed6f..7808d94 100644
--- a/drivers/virtio.c
+++ b/drivers/virtio.c
@@ -303,7 +303,12 @@ ob_virtio_configure_device(VDev *vdev)
         };

virtio_cfg_write16(vdev->common_cfg, VIRTIO_PCI_COMMON_Q_SELECT, i);
+
         info.num = virtio_cfg_read16(vdev->common_cfg, VIRTIO_PCI_COMMON_Q_SIZE);
+        if (info.num > VIRTIO_MAX_RING_ENTRIES) {
+            info.num = VIRTIO_MAX_RING_ENTRIES;
+            virtio_cfg_write16(vdev->common_cfg, VIRTIO_PCI_COMMON_Q_SIZE, info.num);
+        }

vring_init(&vdev->vrings[i], &info);

@@ -503,7 +508,7 @@ void ob_virtio_init(const char *path, const char *dev_name, uint64_t common_cfg,
     addr = POP();
     vdev->vrings = cell2pointer(addr);

-    PUSH(VIRTIO_RING_SIZE * VIRTIO_MAX_VQS);
+    PUSH((VIRTIO_RING_SIZE * 2 + VIRTIO_PCI_VRING_ALIGN) * VIRTIO_MAX_VQS);
     feval("dma-alloc");
     addr = POP();
     vdev->ring_area = cell2pointer(addr);
diff --git a/drivers/virtio.h b/drivers/virtio.h
index 64b49e7..36cb205 100644
--- a/drivers/virtio.h
+++ b/drivers/virtio.h
@@ -147,8 +147,9 @@ struct VirtioDev {
 };
 typedef struct VirtioDev VirtioDev;

-#define VIRTIO_RING_SIZE            (PAGE_SIZE * 8)
-#define VIRTIO_MAX_VQS              3
+#define VIRTIO_MAX_RING_ENTRIES     128
+#define VIRTIO_RING_SIZE            (sizeof(VRingDesc) * VIRTIO_MAX_RING_ENTRIES)
+#define VIRTIO_MAX_VQS              1
 #define KVM_S390_VIRTIO_RING_ALIGN  4096

#define VRING_USED_F_NO_NOTIFY  1

[Mark Cave-Ayland]

On 16/05/2020 14:46, Mark Cave-Ayland wrote:
> QEMU commit c9b7d9ec21 "virtio: increase virtqueue size for virtio-scsi and
> virtio-blk" increased the number of queue descriptors from 128 to 256 which
> caused OpenBIOS to fail when booting from a virtio-blk device under
> qemu-system-ppc due to a memory overflow.
>
> Update the virtio-blk driver so that it uses a maximum of 128 descriptor
> entries (the previous value) to fix the issue, and also ensure that any
> further increases in virtqueue size will not cause the same failure.
>
> Note that this commit also fixes the vring_area size calculation which was
> incorrectly based upon the number of virtqueues, and not the number of
> descriptor entries.
> Signed-off-by: Mark Cave-Ayland
>
> drivers/virtio.c | 7 ++++++-
>  drivers/virtio.h | 5 +++--
>  2 files changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/virtio.c b/drivers/virtio.c
> index ff7ed6f..7808d94 100644
> --- a/drivers/virtio.c
> +++ b/drivers/virtio.c
> @@ -303,7 +303,12 @@ ob_virtio_configure_device(VDev *vdev)
>          };
>      virtio_cfg_write16(vdev->common_cfg, VIRTIO_PCI_COMMON_Q_SELECT, i);
>    info.num = virtio_cfg_read16(vdev->common_cfg, VIRTIO_PCI_COMMON_Q_SIZE);
>    if (info.num > VIRTIO_MAX_RING_ENTRIES) {
>        info.num = VIRTIO_MAX_RING_ENTRIES;
>        virtio_cfg_write16(vdev->common_cfg, VIRTIO_PCI_COMMON_Q_SIZE, info.num);
>    }
>
>    vring_init(&vdev->vrings[i], &info);
>
> @@ -503,7 +508,7 @@ void ob_virtio_init(const char *path, const char *dev_name, uint64_t common_cfg,
>      addr = POP();
>      vdev->vrings = cell2pointer(addr);
> PUSH(VIRTIO_RING_SIZE * VIRTIO_MAX_VQS);
> PUSH((VIRTIO_RING_SIZE * 2 + VIRTIO_PCI_VRING_ALIGN) * VIRTIO_MAX_VQS);
> feval("dma-alloc");
> addr = POP();
> vdev->ring_area = cell2pointer(addr);
>
> diff --git a/drivers/virtio.h b/drivers/virtio.h
> index 64b49e7..36cb205 100644
> --- a/drivers/virtio.h
> +++ b/drivers/virtio.h
> @@ -147,8 +147,9 @@ struct VirtioDev {
>  };
>  typedef struct VirtioDev VirtioDev;
>
> -#define VIRTIO_RING_SIZE            (PAGE_SIZE * 8)
> -#define VIRTIO_MAX_VQS              3
> +#define VIRTIO_MAX_RING_ENTRIES     128
> +#define VIRTIO_RING_SIZE            (sizeof(VRingDesc) * VIRTIO_MAX_RING_ENTRIES)
> +#define VIRTIO_MAX_VQS              1
>  #define KVM_S390_VIRTIO_RING_ALIGN  4096
>
> #define VRING_USED_F_NO_NOTIFY  1

Applied to master.

ATB,

Mark.