On Thu, Jul 02, 2015 at 04:20:22PM +0200, Paolo Bonzini wrote:
> VIRTIO_BLK_F_CONFIG_WCE is important in order to achieve good performance
> (up to 2x, though more realistically +30-40%) in latency-bound workloads.
> However, it was removed by mistake together with VIRTIO_BLK_F_FLUSH.
>
> In addition, even removing VIRTIO_BLK_F_FLUSH was probably not a great
> idea, because it simplifies simple drivers (e.g. firmware) that are okay
> with a writethrough cache but still need data to persist after power loss.
> What really should have been removed is just the possibility that devices
> not propose VIRTIO_BLK_F_FLUSH, but even that only deserves a "SHOULD" in
> the new world of conformance statements.
>
> Restore these, with the following changes:
>
> * clarify and use conformance statements in order to define writeback
> and writethrough caching according to what is commonly done by high-end
> storage.
>
> * clarify (with conformance statements) the influence of the
> VIRTIO_BLK_F_FLUSH feature on caching and how to proceed if only one of
> VIRTIO_BLK_F_FLUSH and VIRTIO_BLK_F_CONFIG_WCE is negotiated.
>
> * strengthen the requirement for persisting writes to MUST after
> a VIRTIO_BLK_T_FLUSH request (and in other cases too involving the
> new features).
>
> The suggested behavior upon feature negotiation is okay for the Linux
> implementation of virtio1, even after the implementation is modified to
> support the two new features.
>
> This fixes VIRTIO-144.
>
> Signed-off-by: Paolo Bonzini <>
> ---
> conformance.tex | 2 +
> content.tex | 130 +++++++++++++++++++++++++++++++++++++++++++++++---------
> 2 files changed, 111 insertions(+), 21 deletions(-)
Reviewed-by: Stefan Hajnoczi <>
