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: Yu, Zhang [mailto:yu.c.zhang@xxxxxxxxxxxxxxx]
> Sent: 21 April 2016 14:49
> To: Paul Durrant; George Dunlap; Jan Beulich; Wei Liu
> 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 4/21/2016 9:31 PM, Paul Durrant wrote:
> >> -----Original Message-----
> >> From: Yu, Zhang [mailto:yu.c.zhang@xxxxxxxxxxxxxxx]
> >> Sent: 21 April 2016 13:25
> >> To: George Dunlap; Paul Durrant; Jan Beulich; Wei Liu
> >> 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 4/21/2016 1:06 AM, George Dunlap wrote:
> >>> 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.
> >>>
> >>
> >> So the enum would be sth. 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_mmio_write_dm, /* Read-only; writes go to the device
> model
> >> */
> >> #else
> >>      HVMMEM_unused,
> >> #endif
> >>      HVMMEM_ioreq_server
> >> } hvmmem_type_t;
> >>
> >
> > I believe that's correct, but presumably there's need to be a change to the
> hypervisor since any reference there to HVMMEM_mmio_write_dm (which I
> think is limited to the get and set mem type code in hvm.c) will now need to
> map HVMMEM_unused to the old p2m_mmio_write_dm type.
> >
> Thank you, Paul.
> 
> But p2m_mmio_write_dm will not exist any more...
> E.g. if in hvmop_get_mem_type(), if type 0xf(p2m_ioreq_server) is
> returned, we could just return HVMMEM_ioreq_server. No need to
> worry about the HVMMEM_mmio_write_dm.
> 
> Maybe we only need to change the beginning of hvmop_set_mem_type()
> to sth. like this:
> 
> /* Interface types to internal p2m types */
> static const p2m_type_t memtype[] = {
>      [HVMMEM_ram_rw]  = p2m_ram_rw,
>      [HVMMEM_ram_ro]  = p2m_ram_ro,
>      [HVMMEM_mmio_dm] = p2m_mmio_dm,
>      [HVMMEM_unused] = p2m_invalid,  /* this will be rejected later */
>      [HVMMEM_ioreq_server] = p2m_ioreq_server
> };
> and later in the same routine, just reject the HVMMEM_unused type, in
> an if(with unlikely) statement.
> 

As long as everyone is in agreement then we can break the functionality that 
exists in 4.6.1 (and presumably 4.7 now) then that’s ok.

  Paul

> >   Paul
> 
> B.R.
> Yu
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