[libvirt] [PATCH] libxl: add support for cache=directsync to to libxl_device_disk

[Jim Fehlig]

Olaf Hering wrote:
> Up to now xend and libxl did not have any way and need to specify a cache=
> mode for block devices. At least the libxl driver did just ignore the
> cachemode value.
>   

Even though cache mode was ignored before, should we now error on
unsupported values?

> With xen-4.5 a new knob was added to libxl which instructs qemu to use
> threads=native cache=directsync in the qdisk driver. This mode was disabled
> early due to a pvops kernel bug. As a workaround cache=writeback was set as
> default. The kernel bug is long fixed. I assume the forward ported xenlinux
> kernel never had this bug.
>
> This change exposes the knob to libvirt. If cache=directsync is set qemu is
> instructed to use native AIO.
>
> This change also lays the ground work to set cache=unsafe at some point.
> It is currently under investigation whether libxl should expose such knob.
>
> Signed-off-by: Olaf Hering <olaf at aepfle.de>
> ---
>  src/libxl/libxl_conf.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
>
> diff --git a/src/libxl/libxl_conf.c b/src/libxl/libxl_conf.c
> index 4cb062e..79f4a82 100644
> --- a/src/libxl/libxl_conf.c
> +++ b/src/libxl/libxl_conf.c
> @@ -739,6 +739,20 @@ libxlDiskSetDiscard(libxl_device_disk *x_disk, int discard)
>  #endif
>  }
>  
> +static void
> +libxlDiskSetCacheMode(libxl_device_disk *x_disk, int cachemode)
> +{
> +    switch (cachemode) {
> +#if defined(LIBXL_HAVE_DEVICE_DISK_DIRECT_IO_SAFE)
> +    case VIR_DOMAIN_DISK_CACHE_DIRECTSYNC:
> +        x_disk->direct_io_safe = 1;
> +        break;
> +#endif
> +    default:
> +        break;
> +    }
>   

If the answer to the above is yes, this should be changed to something like

    switch (cachemode) {
    case VIR_DOMAIN_DISK_CACHE_DEFAULT:
        break;

    case VIR_DOMAIN_DISK_CACHE_DISABLE:
    case VIR_DOMAIN_DISK_CACHE_WRITETHRU:
    case VIR_DOMAIN_DISK_CACHE_WRITEBACK:
    case VIR_DOMAIN_DISK_CACHE_UNSAFE:
    case VIR_DOMAIN_DISK_CACHE_LAST:
        error...
        break;

    case VIR_DOMAIN_DISK_CACHE_DIRECTSYNC:
#if defined(LIBXL_HAVE_DEVICE_DISK_DIRECT_IO_SAFE)
        x_disk->direct_io_safe = 1;
#else
        error...
#endif
        break;
    }

Regards,
Jim

> +}
> +
>  
>  int
>  libxlMakeDisk(virDomainDiskDefPtr l_disk, libxl_device_disk *x_disk)
> @@ -855,6 +869,7 @@ libxlMakeDisk(virDomainDiskDefPtr l_disk, libxl_device_disk *x_disk)
>      x_disk->readwrite = !l_disk->readonly;
>      x_disk->is_cdrom = l_disk->device == VIR_DOMAIN_DISK_DEVICE_CDROM ? 1 : 0;
>      libxlDiskSetDiscard(x_disk, l_disk->discard);
> +    libxlDiskSetCacheMode(x_disk, l_disk->cachemode);
>      /* An empty CDROM must have the empty format, otherwise libxl fails. */
>      if (x_disk->is_cdrom && !x_disk->pdev_path)
>          x_disk->format = LIBXL_DISK_FORMAT_EMPTY;
>
>