Re: [PATCH v2 2/3] x86/ioreq server: Rename p2m_mmio_write_dm to p2m_ioreq_server

[Paul Durrant <Paul.Durrant@citrix.com>]

> -----Original Message-----
> From: Xen-devel [mailto:xen-devel-bounces@lists.xen.org] On Behalf Of Jan
> Beulich
> Sent: 20 April 2016 17:53
> To: George Dunlap; Paul Durrant; Wei Liu; yu.c.zhang@linux.intel.com
> Cc: Kevin Tian; Keir (Xen.org); Andrew Cooper; Tim (Xen.org); xen-
> devel@lists.xen.org; zhiyuan.lv@intel.com; jun.nakajima@intel.com
> Subject: Re: [Xen-devel] [PATCH v2 2/3] x86/ioreq server: Rename
> p2m_mmio_write_dm to p2m_ioreq_server
> 
> >>> George Dunlap <george.dunlap@citrix.com> 04/20/16 6:30 PM >>>
> >On Wed, Apr 20, 2016 at 4:02 PM, George Dunlap
> <george.dunlap@citrix.com> wrote:
> >> On 19/04/16 12:02, Yu, Zhang wrote:
> >>> So I suppose the only place we need change for this patch is
> >>> for hvmmem_type_t, which should be defined like this?
> >>>
> >>> typedef enum {
> >>>     HVMMEM_ram_rw,             /* Normal read/write guest RAM */
> >>>     HVMMEM_ram_ro,             /* Read-only; writes are discarded */
> >>>     HVMMEM_mmio_dm,            /* Reads and write go to the device
> model */
> >>> #if __XEN_INTERFACE_VERSION__ >= 0x00040700
> >>>     HVMMEM_ioreq_server
> >>> #else
> >>>     HVMMEM_mmio_write_dm
> >>> #endif
> >>> } hvmmem_type_t;
> >>>
> >>> Besides, does 4.7 still accept freeze exception? It would be great
> >>> if we can get an approval for this.
> >>
> >> Wait, do we *actually* need this?  Is anyone actually using this?
> >>
> >> I'd say remove it, and if anyone complains, *then* do the #ifdef'ery as
> >> a bug-fix.  I'm pretty sure that's Linux's policy -- You Must Keep
> >> Userspace Working, but you can break it to see if anyone complains first.
> 
> We don't normally do it like that - we aim at keeping things compatible
> right away. I don't know of a case where we would have knowingly broken
> compatibility for users of the public headers (leaving aside tool stack only
> stuff of course).
> 
> >Going further than this:
> >
> >The proposed patch series not only changes the name, it changes the
> >functionality.  We do not want code to *compile* against 4.7 and then
> >not *work* against 4.7; and the worst of all is to compile and sort of
> >work but do it incorrectly.
> 
> I had the impression that the renaming patch was what it is - a renaming
> patch, without altering behavior.
> 
> >Does the ioreq server have a way of asking Xen what version of the ABI
> >it's providing?  I'm assuming the answer is "no"; in which case code
> >that is compiled against the 4.6 interface but run on a 4.8 interface
> >that looks like this will fail in a somewhat unpredictable way.
> 
> The only thing it can do is ask for the Xen version. The ABI version is not
> being returned by anything (but perhaps should be).
> 
> >Given that:
> >
> >1. When we do check the ioreq server functionality in, what's the
> >correct way to deal with code that wants to use the old interface, and
> >what do we do with code compiled against the old interface but running
> >on the new one?
> 
> For the full series I'm not sure I can really tell.But as said, for the rename
> patch alone I thought it is just a rename. And that's what we want to get
> in (see Paul's earlier reply - he wants to see the old name gone, so it won't
> be used any further).
> 
> >2. What's the best thing to do for this release?
> 
> If the entire series (no matter whether to go in now or later) is changing
> behavior, then the only choice is to consider the currently used enum
> value burnt, and use a fresh one for the new semantics.

It sounds like that would be best way. If we don't so that then we have to maintain the write-dm semantics for pages of that type unless the type is claimed (by using the new hypercall) and that's bit icky. I much prefer that pages of the new type are treated as RAM until claimed.

  Paul

> 
> >If it's the case that the only code that uses this is in XenServer,
> >then I'd say the answer to #1 can be simply, "Don't compile" and
> >"Don't do that" respectively; and the answer to #2 can be either
> >"Leave it be" or "Remove the enum from the public interface".
> >
> >If there are other projects that have started to use this interface,
> >then we need a better answer to #1 than "Compile but fail in
> >unpredicatble ways".
> 
> How would we know whether there are other users?
> 
> Jan
> 
> 
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