Re: [PATCH v3 08/26] DAX: virtio-fs: Add cache BAR

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Wed, Apr 28, 2021 at 12:00:42PM +0100, Dr. David Alan Gilbert (git) wrote:
> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> 
> Add a cache BAR into which files will be directly mapped.
> The size can be set with the cache-size= property, e.g.
>    -device vhost-user-fs-pci,chardev=char0,tag=myfs,cache-size=16G
> 
> The default is no cache.
> 
> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> with PPC fixes by:
> Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
> ---
>  hw/virtio/vhost-user-fs-pci.c     | 32 +++++++++++++++++++++++++++++++
>  hw/virtio/vhost-user-fs.c         | 32 +++++++++++++++++++++++++++++++
>  include/hw/virtio/vhost-user-fs.h |  2 ++
>  3 files changed, 66 insertions(+)
> 
> diff --git a/hw/virtio/vhost-user-fs-pci.c b/hw/virtio/vhost-user-fs-pci.c
> index 2ed8492b3f..20e447631f 100644
> --- a/hw/virtio/vhost-user-fs-pci.c
> +++ b/hw/virtio/vhost-user-fs-pci.c
> @@ -12,14 +12,19 @@
>   */
>  
>  #include "qemu/osdep.h"
> +#include "qapi/error.h"
>  #include "hw/qdev-properties.h"
>  #include "hw/virtio/vhost-user-fs.h"
>  #include "virtio-pci.h"
>  #include "qom/object.h"
> +#include "standard-headers/linux/virtio_fs.h"
> +
> +#define VIRTIO_FS_PCI_CACHE_BAR 2
>  
>  struct VHostUserFSPCI {
>      VirtIOPCIProxy parent_obj;
>      VHostUserFS vdev;
> +    MemoryRegion cachebar;
>  };
>  
>  typedef struct VHostUserFSPCI VHostUserFSPCI;
> @@ -38,7 +43,9 @@ static Property vhost_user_fs_pci_properties[] = {
>  static void vhost_user_fs_pci_realize(VirtIOPCIProxy *vpci_dev, Error **errp)
>  {
>      VHostUserFSPCI *dev = VHOST_USER_FS_PCI(vpci_dev);
> +    bool modern_pio = vpci_dev->flags & VIRTIO_PCI_FLAG_MODERN_PIO_NOTIFY;
>      DeviceState *vdev = DEVICE(&dev->vdev);
> +    uint64_t cachesize;
>  
>      if (vpci_dev->nvectors == DEV_NVECTORS_UNSPECIFIED) {
>          /* Also reserve config change and hiprio queue vectors */
> @@ -46,6 +53,31 @@ static void vhost_user_fs_pci_realize(VirtIOPCIProxy *vpci_dev, Error **errp)
>      }
>  
>      qdev_realize(vdev, BUS(&vpci_dev->bus), errp);
> +    cachesize = dev->vdev.conf.cache_size;
> +
> +    if (cachesize && modern_pio) {
> +        error_setg(errp, "DAX Cache can not be used together with modern_pio");

It's not necessary to respin but it would help to capture the reason for
this limitation either in the error message or at least in a comment.

The problem is that PCI BARs are limited resources and enabling modern
PIO notify conflicts with the DAX Window BAR usage.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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