Re: [PATCH v2 8/8] vhost-user-blk: dynamically resize config space based on features

[Raphael Norwitz]

On Fri, Aug 26, 2022 at 05:32:48PM +0300, Daniil Tatianin wrote:
> Make vhost-user-blk backwards compatible when migrating from older VMs
> running with modern features turned off, the same way it was done for
> virtio-blk in 20764be0421c ("virtio-blk: set config size depending on the 
> features enabled")
> 
> It's currently impossible to migrate from an older VM with
> vhost-user-blk (with disable-legacy=off) because of errors like this:
> 
> qemu-system-x86_64: get_pci_config_device: Bad config data: i=0x10 read: 41 
> device: 1 cmask: ff wmask: 80 w1cmask:0
> qemu-system-x86_64: Failed to load PCIDevice:config
> qemu-system-x86_64: Failed to load virtio-blk:virtio
> qemu-system-x86_64: error while loading state for instance 0x0 of device 
> '0000:00:05.0:00.0:02.0/virtio-blk'
> qemu-system-x86_64: load of migration failed: Invalid argument
> 
> This is caused by the newer (destination) VM requiring a bigger BAR0
> alignment because it has to cover a bigger configuration space, which
> isn't actually needed since those additional config fields are not
> active (write-zeroes/discard).
>

With the relevant meson.build and MAINTAINERS bits rebased here:

Reviewed-by: Raphael Norwitz <raphael.norwitz@nutanix.com>


> Signed-off-by: Daniil Tatianin <d-tatianin@yandex-team.ru>
> ---
>  hw/block/vhost-user-blk.c | 17 ++++++++++-------
>  1 file changed, 10 insertions(+), 7 deletions(-)
> 
> diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
> index 0d916edefa..8dd063eb96 100644
> --- a/hw/block/vhost-user-blk.c
> +++ b/hw/block/vhost-user-blk.c
> @@ -23,6 +23,7 @@
>  #include "hw/qdev-core.h"
>  #include "hw/qdev-properties.h"
>  #include "hw/qdev-properties-system.h"
> +#include "hw/virtio/virtio-blk-common.h"
>  #include "hw/virtio/vhost.h"
>  #include "hw/virtio/vhost-user-blk.h"
>  #include "hw/virtio/virtio.h"
> @@ -63,7 +64,7 @@ static void vhost_user_blk_update_config(VirtIODevice 
> *vdev, uint8_t *config)
>      /* Our num_queues overrides the device backend */
>      virtio_stw_p(vdev, &s->blkcfg.num_queues, s->num_queues);
>  
> -    memcpy(config, &s->blkcfg, sizeof(struct virtio_blk_config));
> +    memcpy(config, &s->blkcfg, vdev->config_len);
>  }
>  
>  static void vhost_user_blk_set_config(VirtIODevice *vdev, const uint8_t 
> *config)
> @@ -92,12 +93,12 @@ static int vhost_user_blk_handle_config_change(struct 
> vhost_dev *dev)
>  {
>      int ret;
>      struct virtio_blk_config blkcfg;
> +    VirtIODevice *vdev = dev->vdev;
>      VHostUserBlk *s = VHOST_USER_BLK(dev->vdev);
>      Error *local_err = NULL;
>  
>      ret = vhost_dev_get_config(dev, (uint8_t *)&blkcfg,
> -                               sizeof(struct virtio_blk_config),
> -                               &local_err);
> +                               vdev->config_len, &local_err);
>      if (ret < 0) {
>          error_report_err(local_err);
>          return ret;
> @@ -106,7 +107,7 @@ static int vhost_user_blk_handle_config_change(struct 
> vhost_dev *dev)
>      /* valid for resize only */
>      if (blkcfg.capacity != s->blkcfg.capacity) {
>          s->blkcfg.capacity = blkcfg.capacity;
> -        memcpy(dev->vdev->config, &s->blkcfg, sizeof(struct 
> virtio_blk_config));
> +        memcpy(dev->vdev->config, &s->blkcfg, vdev->config_len);
>          virtio_notify_config(dev->vdev);
>      }
>  
> @@ -442,7 +443,7 @@ static int vhost_user_blk_realize_connect(VHostUserBlk 
> *s, Error **errp)
>      assert(s->connected);
>  
>      ret = vhost_dev_get_config(&s->dev, (uint8_t *)&s->blkcfg,
> -                               sizeof(struct virtio_blk_config), errp);
> +                               s->parent_obj.config_len, errp);
>      if (ret < 0) {
>          qemu_chr_fe_disconnect(&s->chardev);
>          vhost_dev_cleanup(&s->dev);
> @@ -457,6 +458,7 @@ static void vhost_user_blk_device_realize(DeviceState 
> *dev, Error **errp)
>      ERRP_GUARD();
>      VirtIODevice *vdev = VIRTIO_DEVICE(dev);
>      VHostUserBlk *s = VHOST_USER_BLK(vdev);
> +    size_t config_size;
>      int retries;
>      int i, ret;
>  
> @@ -487,8 +489,9 @@ static void vhost_user_blk_device_realize(DeviceState 
> *dev, Error **errp)
>          return;
>      }
>  
> -    virtio_init(vdev, VIRTIO_ID_BLOCK,
> -                sizeof(struct virtio_blk_config));
> +    config_size = virtio_get_config_size(&virtio_blk_cfg_size_params,
> +                                         vdev->host_features);
> +    virtio_init(vdev, VIRTIO_ID_BLOCK, config_size);
>  
>      s->virtqs = g_new(VirtQueue *, s->num_queues);
>      for (i = 0; i < s->num_queues; i++) {
> -- 
> 2.25.1
>