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 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.

  Paul

> Thanks
> Yu
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