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Re: [Xen-devel] [PATCH 0 of 3] blktap3/libxl: add support for blktap3 in libxl

On Fri, Apr 19, 2013 at 04:40:48PM +0100, Thanos Makatos wrote:
> This patch series implements support for blktap3 in libxl.
> Supporting blktap3 requires rather few changes:
> 1. We introduce a new disk back-end type (TAP3) and a new device kind (VBD3) 
> to
>    allow blktap3 to co-exist with blktap3.

"allow blktap3 to co-exist with blktap3" ? 

I guess the the other one should be blktap2 .. 

-- Pasi

>  blktap2 remains the default back-end
>    for tap devices. Switching in the future to blktap3 as the default handler
>    of tap devices should be trivial.
> 2. libxl doesn't spawn the tapdisk process any more, as is the case for
>    blktap2, since the tapback daemon is responsible for that. Thus, libxl only
>    needs to write to XenStore the file/partition/whatever backing the virtual
>    disk so that the tapback daemon can pass it to tapdisk.
> 3. Since there is no block device in dom0 any more, pygrub won't be able to
>    boot from VHD files. To get around this problem, pygrub can use the NBD
>    functionality (existing in blktap2.5): it can explicitly ask tapback to
>    create a NDB in order to access the virtual disk. This functionality will 
> be
>    implemented in a future patch series. However, this will only work for 
> Linux
>    dom0's. As a generic solution that would work on any dom0, we could
>    implement a simple protocol for data exchange between pygrub and tapdisk.
> Signed-off-by: Thanos Makatos <thanos.makatos@xxxxxxxxxx>
> ---
> Changed since v1:
>   * Check whether tapback's control socket exists instead of checking whether
>     a process named "tapback" exists.
>   * Don't duplicate code for writing the type:/path/to/file to XenStore.
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