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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: George Dunlap [mailto:george.dunlap@xxxxxxxxxx]
> Sent: 20 April 2016 18:07
> To: Paul Durrant; Jan Beulich; Wei Liu; yu.c.zhang@xxxxxxxxxxxxxxx
> Cc: Kevin Tian; Keir (Xen.org); Andrew Cooper; Tim (Xen.org); xen-
> devel@xxxxxxxxxxxxx; zhiyuan.lv@xxxxxxxxx; jun.nakajima@xxxxxxxxx
> Subject: Re: [Xen-devel] [PATCH v2 2/3] x86/ioreq server: Rename
> p2m_mmio_write_dm to p2m_ioreq_server
> 
> On 20/04/16 17:58, Paul Durrant wrote:
> >> -----Original Message-----
> >> From: Xen-devel [mailto:xen-devel-bounces@xxxxxxxxxxxxx] On Behalf Of
> Jan
> >> Beulich
> >> Sent: 20 April 2016 17:53
> >> To: George Dunlap; Paul Durrant; Wei Liu; yu.c.zhang@xxxxxxxxxxxxxxx
> >> Cc: Kevin Tian; Keir (Xen.org); Andrew Cooper; Tim (Xen.org); xen-
> >> devel@xxxxxxxxxxxxx; zhiyuan.lv@xxxxxxxxx; jun.nakajima@xxxxxxxxx
> >> Subject: Re: [Xen-devel] [PATCH v2 2/3] x86/ioreq server: Rename
> >> p2m_mmio_write_dm to p2m_ioreq_server
> >>
> >>>>> George Dunlap <george.dunlap@xxxxxxxxxx> 04/20/16 6:30 PM >>>
> >>> On Wed, Apr 20, 2016 at 4:02 PM, George Dunlap
> >> <george.dunlap@xxxxxxxxxx> 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.
> 
> I think the only sensible way to keep the enum is to also keep the
> functionality, which would mean using *another* p2m type for ioreq_server.
> 
> Given that the functionality isn't going away for 4.7, I don't see an
> urgent need to remove the enum; but if Paul does, then a patch renaming
> it to HVMMEM_unused would be the way forward then I guess.  Once the
> underlying p2m type goes away, you'll want to return -EINVAL for this
> enum value.
> 

Since the old semantics made it into the wild in 4.6.1 (which I was unaware of 
until a couple of days ago) then I guess we are going to need some form of 
deprecation like this.

  Paul

>  -George
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