Re: [Xen-devel] [PATCH v6 1/4] x86/ioreq server: Add HVMOP to map guest ram with p2m_ioreq_server to an ioreq server.

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 02/09/16 11:47, Yu Zhang wrote:
> A new HVMOP - HVMOP_map_mem_type_to_ioreq_server, is added to
> let one ioreq server claim/disclaim its responsibility for the
> handling of guest pages with p2m type p2m_ioreq_server. Users
> of this HVMOP can specify which kind of operation is supposed
> to be emulated in a parameter named flags. Currently, this HVMOP
> only support the emulation of write operations. And it can be
> further extended to support the emulation of read ones if an
> ioreq server has such requirement in the future.
> 
> For now, we only support one ioreq server for this p2m type, so
> once an ioreq server has claimed its ownership, subsequent calls
> of the HVMOP_map_mem_type_to_ioreq_server will fail. Users can also
> disclaim the ownership of guest ram pages with p2m_ioreq_server, by
> triggering this new HVMOP, with ioreq server id set to the current
> owner's and flags parameter set to 0.
> 
> Note both HVMOP_map_mem_type_to_ioreq_server and p2m_ioreq_server
> are only supported for HVMs with HAP enabled.
> 
> Also note that only after one ioreq server claims its ownership
> of p2m_ioreq_server, will the p2m type change to p2m_ioreq_server
> be allowed.
> 
> Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
> Signed-off-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
> Acked-by: Tim Deegan <tim@xxxxxxx>
> ---
> Cc: Paul Durrant <paul.durrant@xxxxxxxxxx>
> Cc: Jan Beulich <jbeulich@xxxxxxxx>
> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Cc: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
> Cc: Jun Nakajima <jun.nakajima@xxxxxxxxx>
> Cc: Kevin Tian <kevin.tian@xxxxxxxxx>
> Cc: Tim Deegan <tim@xxxxxxx>
> 
> changes in v6: 
>   - Clarify logic in hvmemul_do_io().
>   - Use recursive lock for ioreq server lock.
>   - Remove debug print when mapping ioreq server.
>   - Clarify code in ept_p2m_type_to_flags() for consistency.
>   - Remove definition of P2M_IOREQ_HANDLE_WRITE_ACCESS.
>   - Add comments for HVMMEM_ioreq_server to note only changes
>     to/from HVMMEM_ram_rw are permitted.
>   - Add domain_pause/unpause() in hvm_map_mem_type_to_ioreq_server()
>     to avoid the race condition when a vm exit happens on a write-
>     protected page, just to find the ioreq server has been unmapped
>     already.

Why do we need to do this?  Won't the default case just DTRT if it finds
that the ioreq server has been unmapped?

 -George

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