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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Wed, 6 Aug 2014, Thomas Leonard wrote:
> 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

I cannot repro the issue even using "breaks.xen" as guest kernel.
What dom0 kernel are you using?


> $ 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.

Very interesting.

> 
> -- 
> 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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