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Re: [Qemu-devel] [PATCH V2 0/5] Set correct blk feature for virtio 1.0

From: Cornelia Huck
Subject: Re: [Qemu-devel] [PATCH V2 0/5] Set correct blk feature for virtio 1.0
Date: Wed, 15 Jul 2015 11:11:18 +0200

On Wed, 15 Jul 2015 13:29:56 +0800
Jason Wang <address@hidden> wrote:

> Hi all:
> This series tries to set feature correctly for virtio-blk when virtio
> 1.0 is supported. Two isssues were addressed according to the spec:
> - scsi passthrough was not support in 1.0. This is done by, 1) disable
>   scsi by defautl for 2.4 machine type and fail the initialization
>   when both scsi and 1.0 were set.
> - any layout must be set for transitional device. This is done by set
>   any layout when 1.0 is supported.
> Please review
> Changes from V1:
> - Split virtio-net changes out of the series
> - Enable VIRTIO_BLK_F_SCSI only when scsi is set
> - Disable scsi by default and compat it for legacy machine types
> - Let get_features() can fail and fail the initialization of
>   virito-blk when both 1.0 and scsi were supported.

Hm, this seems confusing to me mainly due to the different way
transitional devices are handled by pci and ccw.

For virtio-pci: (please correct me if I misunderstood)
- devices (except input) are transitional by default
- user can disable legacy or modern support
- drivers can use method they prefer, depending on VERSION_1

For virtio-ccw:
- transitional means "in limbo" regarding legacy or modern
- devices become legacy if features are read without negotiation of a
  revision, or if revision 0 is negotiated
- they become modern if revision 1 is negotiated

That implies that for ccw, a transitional device does not offer any
features: It either transitions to the legacy or modern state if a
revision was negotiated, or it becomes a legacy device by reading
features (which are the legacy features, then). While pci has "real"
transitional devices needing to offer a certain feature set.

I'm not sure what the solution is here: basically, ccw needs a dynamic
feature set, while pci does not.

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