Re: [Xen-devel] [PATCH v3 0/3] map grant refs at pfn = mfn

[Thomas Leonard <talex5@xxxxxxxxx>]

On 6 August 2014 14:46, Stefano Stabellini
<stefano.stabellini@xxxxxxxxxxxxx> wrote:
> On Wed, 6 Aug 2014, Thomas Leonard wrote:
>> On 6 August 2014 14:35, Stefano Stabellini
>> <stefano.stabellini@xxxxxxxxxxxxx> wrote:
>> > On Wed, 6 Aug 2014, Thomas Leonard wrote:
>> >> On 1 August 2014 19:12, Thomas Leonard <talex5@xxxxxxxxx> wrote:
>> >> > On 1 August 2014 18:16, Stefano Stabellini
>> >> > <stefano.stabellini@xxxxxxxxxxxxx> wrote:
>> >> >> On Fri, 1 Aug 2014, Thomas Leonard wrote:
>> >> >>> On 1 August 2014 18:01, Stefano Stabellini
>> >> >>> <stefano.stabellini@xxxxxxxxxxxxx> wrote:
>> >> >>> > On Fri, 1 Aug 2014, Thomas Leonard wrote:
>> >> >>> >> On 1 August 2014 17:25, Stefano Stabellini
>> >> >>> >> <stefano.stabellini@xxxxxxxxxxxxx> wrote:
>> >> >>> >> > On Fri, 1 Aug 2014, Julien Grall wrote:
>> >> >>> >> >> On 01/08/14 17:16, Thomas Leonard wrote:
>> >> >>> >> >> > On 1 August 2014 16:13, Stefano Stabellini
>> >> >>> >> >> > <stefano.stabellini@xxxxxxxxxxxxx> wrote:
>> >> >>> >> >> > > On Fri, 1 Aug 2014, Thomas Leonard wrote:
>> >> >>> >> >> > > > On 24/07/14 14:30, Stefano Stabellini wrote:
>> >> >>> >> >> > > > > Hi all,
>> >> >>> >> >> > > > > this patch series introduces a second p2m mapping of 
>> >> >>> >> >> > > > > grant reference
>> >> >>> >> >> > > > > on
>> >> >>> >> >> > > > > ARM at guest physical address == machine address of the 
>> >> >>> >> >> > > > > grant ref.  It
>> >> >>> >> >> > > > > is safe because dom0 is already mapped 1:1. We export
>> >> >>> >> >> > > > > XENFEAT_grant_map_identity to signal the guest that 
>> >> >>> >> >> > > > > this second p2m
>> >> >>> >> >> > > > > mapping is
>> >> >>> >> >> > > > > available.
>> >> >>> >> >> > > > >
>> >> >>> >> >> > > > > One reason for wanting the second p2m mapping is to 
>> >> >>> >> >> > > > > avoid tracking mfn
>> >> >>> >> >> > > > > to pfn mappings in the guest kernel. Since the same mfn 
>> >> >>> >> >> > > > > can be granted
>> >> >>> >> >> > > > > multiple times to the backend, finding the right pfn 
>> >> >>> >> >> > > > > corresponding to
>> >> >>> >> >> > > > > a
>> >> >>> >> >> > > > > given mfn can be difficult and expensive. Providing a 
>> >> >>> >> >> > > > > second mapping
>> >> >>> >> >> > > > > at
>> >> >>> >> >> > > > > a known address allow the kernel to access the page 
>> >> >>> >> >> > > > > without knowing
>> >> >>> >> >> > > > > the
>> >> >>> >> >> > > > > pfn.
>> >> >>> >> >> > > >
>> >> >>> >> >> > > > Is there a version of these patches for Xen 4.4 that I 
>> >> >>> >> >> > > > can test? The
>> >> >>> >> >> > > > restriction on duplicate pages is causing trouble for 
>> >> >>> >> >> > > > networking on
>> >> >>> >> >> > > > Mirage too
>> >> >>> >> >> > > > (http://roscidus.com/blog/blog/2014/07/28/my-first-unikernel/#tcp-retransmissions).
>> >> [...]
>> >> >> Checkout the branch grant_map_identity_4.4_2
>> >> >
>> >> > Thanks - it's working now!
>> >>
>> >> Update: if my VM uses the network and then exits quickly, dom0 gets
>> >> stuck.
>> >
>> > What do you mean by exit quickly? xl destroy domain?
>>
>> The guest starts, sends a few packets and then exits (possibly before
>> the packets have been transmitted). This happens before "xl create"
>> manages to attach to the console.
>
> I haven't seen anything like this and I cannot reproduce the issue on
> unstable.

Here's a test case (tested on stable-4.4 with your patches:
https://github.com/talex5/xen/commits/stable-4.4):

$ wget http://test.roscidus.com/static/breaks.xen

$ cat > breaks.xl << EOF
name = 'breaks'
kernel = 'breaks.xen'
builder = 'linux'
memory = 256
on_crash = 'preserve'
vif = [ 'mac=c0:ff:ee:c0:ff:ee,bridge=br0' ]
EOF

$ sudo xl create -c breaks.xl
Parsing config from breaks.xl
xenconsole: Could not read tty from store: No such file or directory
libxl: error: libxl_exec.c:118:libxl_report_child_exitstatus: console
child [0] exited with error status 2

$ sudo xl list
Name                                        ID   Mem VCPUs      State   Time(s)
Domain-0                                     0   512     2     r-----     335.1
(null)                                       8     0     1     --ps-d       0.1


> What if you delay the network initialization within the guest to a bit
> later?

Yes, it's fine with a delay before using the network.


-- 
Dr Thomas Leonard        http://0install.net/
GPG: 9242 9807 C985 3C07 44A6  8B9A AE07 8280 59A5 3CC1
GPG: DA98 25AE CAD0 8975 7CDA  BD8E 0713 3F96 CA74 D8BA

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