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

["Yu, Zhang" <yu.c.zhang@linux.intel.com>]

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@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.
>
> 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;

Thanks
Yu

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