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

[George Dunlap <george.dunlap@citrix.com>]

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.

 -George

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