Re: [Xen-devel] [PATCH v4 3/3] x86/ioreq server: Add HVMOP to map guest ram with p2m_ioreq_server to an ioreq server.

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 16.06.16 at 11:30, <yu.c.zhang@xxxxxxxxxxxxxxx> wrote:
> On 6/15/2016 6:21 PM, Jan Beulich wrote:
>>>>> On 15.06.16 at 11:50, <george.dunlap@xxxxxxxxxx> wrote:
>>> On 14/06/16 14:31, Jan Beulich wrote:
>>>>>>> On 14.06.16 at 15:13, <george.dunlap@xxxxxxxxxx> wrote:
>>>>> On 14/06/16 11:45, Jan Beulich wrote:
>>>>>> Locking is somewhat strange here: You protect against the "set"
>>>>>> counterpart altering state while you retrieve it, but you don't
>>>>>> protect against the returned data becoming stale by the time
>>>>>> the caller can consume it. Is that not a problem? (The most
>>>>>> concerning case would seem to be a race of hvmop_set_mem_type()
>>>>>> with de-registration of the type.)
>>>>> How is that different than calling set_mem_type() first, and then
>>>>> de-registering without first unmapping all the types?
>>>> Didn't we all agree this is something that should be disallowed
>>>> anyway (not that I've seen this implemented, i.e. just being
>>>> reminded of it by your reply)?
>>> I think I suggested it as a good idea, but Paul and Yang both thought it
>>> wasn't necessary.  Do you think it should be a requirement?
>> I think things shouldn't be left in a half-adjusted state.
>>
>>> We could have the de-registering operation fail in those circumstances;
>>> but probably a more robust thing to do would be to have Xen go change
>>> all the ioreq_server entires back to ram_rw (since if the caller just
>>> ignores the failure, things are in an even worse state).
>> If that's reasonable to do without undue delay (e.g. by using
>> the usual "recalculate everything" forced to trickle down through
>> the page table levels, then that's as good.
> 
> Thanks for your advices, Jan & George.
> 
> Previously in the 2nd version, I used p2m_change_entry_type_global() to 
> reset the
> outstanding p2m_ioreq_server entries back to p2m_ram_rw asynchronously after
> the de-registration. But we realized later that this approach means we 
> can not support
> live migration. And to recalculate the whole p2m table forcefully when 
> de-registration
> happens means too much cost.
> 
> And further discussion with Paul was that we can leave the 
> responsibility to reset p2m type
> to the device model side, and even a device model fails to do so, the 
> affected one will only
> be the current VM, neither other VM nor hypervisor will get hurt.
> 
> I thought we have reached agreement in the review process of version 2, 
> so I removed
> this part from version 3.

In which case I would appreciate the commit message to explain
this (in particular I admit I don't recall why live migration would
be affected by the p2m_change_entry_type_global() approach,
but the request is also so that later readers have at least some
source of information other than searching the mailing list).

> Hah, I guess these 2 #defines are just cloned from similar ones, and I 
> did not expected
> they would receive so much comments. Anyway, I admire your preciseness 
> and thanks
> for pointing this out. :)
> 
> Since the bit number #defines have no special meaning, I'd like to just 
> define the flags
> directly:
> 
> #define HVMOP_IOREQ_MEM_ACCESS_READ (1u << 0)
> #define HVMOP_IOREQ_MEM_ACCESS_WRITE (1u << 1)

XEN_HVMOP_* then please.

Jan


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