[Xen-devel] Re: some questions of IO ring in xenpaging

[Olaf Hering <olaf@xxxxxxxxx>]

On Thu, Sep 01, zhen shi wrote:

> Hi Olaf --
> 
> I have two questions to ask you about xenpaging.
> 1) When guest os causes page_fault for the accessed page is paging_out
> or paged, it will execute p2m_mem_paging_populate(). and in
> p2m_mem_paging_populate() it will first check if the ring is full.
> when I ran with domU suse11 4G memory and 8vcpus, I found there will
> be a corruption in checking the ring.
> For example, if 4vcpus are met with page faults when they access
> different pages, and there is only four free-requests for the ring.
> and then they call p2m_mem_paging_populate(),and execute
> mem_event_check_ring(d) at the same time.All will find ring is not
> full,and will fill the requests. It will cause the latter request to
> cover the front request.
> and I think there should a lock before the mem_event_check_ring(d),
> and normally it unlock after mem_event_put_request(d, &req).
> You can review the attached doc of xenpaging_IO_ring.txt to see if my
> opnion is right.

Yes, you are right.
I think mem_event_check_ring() should reserve a reference, and
mem_event_put_request() should use that reference.
mem_sharing_alloc_page() even has a comment that this should be done.


> 2) mem_sharing and xenpaging are shared with one IO ring for domU. In
> the function of mem_sharing_alloc_page(), if alloc_domheap_page(d, 0)
> returns NULL, then it will pause VCPU, check if the ring is full, and
> fill the request at last.
> I think there is also a corruption of mem_event_check_ring(d) with it
> in p2m_mem_paging_populate(). We should assure exclusively in reading
> the free_request and puting requests.  What's more, although it hardly
> fails in alloc_domheap_page(d, 0) from mem_sharing_alloc_page(), it
> will fill the requests in IO ring.  But in xenpaging when handling the
> page_in requests, we have not distinguished the requests with flag
> "MEM_EVENT_FLAG_VCPU_PAUSED" from paging or sharing. It will cause if
> the request is from mem_sharing_alloc_page(), it will go to
> p2m_mem_paging_resume() at last, and the page's p2mt is p2m_ram_rw. I
> think this is wrong. Maybe we should add the req.type when page in.

Yes, get_request() in xenpaging should check the type before popping the
request from the ring. Perhaps memsharing and xenpaging should use its
own rings.

Olaf

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