Re: [PATCH V3] X86/vMCE: handle broken page with regard to migration

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

On 21/11/12 12:11, Ian Campbell wrote:
> On Wed, 2012-11-21 at 11:34 +0000, George Dunlap wrote:
>> On 20/11/12 18:42, Ian Jackson wrote:
>>> Liu, Jinsong writes ("RE: [Xen-devel] [PATCH V3] X86/vMCE: handle broken page with regard to migration"):
>>>> Ian Jackson wrote:
>>>>> Liu, Jinsong writes ("RE: [Xen-devel] [PATCH V3] X86/vMCE: handle
>>>>> broken page with regard to migration"):
>>>>>> No, at last lter, there are 4 points:
>>>>>> 1. start last iter
>>>>>> 2. get and transfer pfn_type to target
>>>>>> 3. copy page to target
>>>>>> 4. end last iter
>>> ...
>>>> It indeed checks mce after point 3 for each page, but what's the
>>>> advantage of keeping a separate list?
>>> It avoids yet another loop over all the pages.  Unless I have
>>> misunderstood.  Which I may have, because: if it checks for mce after
>>> point 3 then surely that is sufficient ?  We don't need to worry about
>>> mces after that check.
>> It's sufficient, but wouldn't each check require a separate hypercall?
>> That would surely be slower than just a single hypercall and a loop
>> (which is what Jinsong's patch does).
>>
>> We don't actually need a list -- I think we just need to know, "Have any
>> pages broken between reading the p2m table ( xc_get_pfn_type_batch() );
>> if so, we do another full iteration.
> If a page fails between 2. and 3. above then what happens at point 3? I
> presume we can't map and send the page (since it is broken), do we get
> some sort of failure to map?

At that point the pages are already mapped.  The order is:
1. From dirty bitmap / whatever, get a batch of pfns to map
2. Map them with foreign_batch
3. Read the p2m table
4. Send a list of PFNs (this is where it will list the broken ones)
5. Send all the data from those pfns

So JS is adding:

6. Read the p2m table again to see if anything has broken between 3 and 
the end of 5

> What happens if the failure occurs during stage 3, i.e. while the page
> is mapped and we are reading from it?

I was rather wondering the same thing. :-)

  -George