Re: [Xen-devel] [PATCH v3] xen-blkfront: set pages are FOREIGN_FRAME when sharing them

[Stefano Stabellini <stefano.stabellini@eu.citrix.com>]

On Mon, 7 May 2012, Konrad Rzeszutek Wilk wrote:
> On Tue, Apr 17, 2012 at 11:58:58AM +0100, Stefano Stabellini wrote:
> > On Mon, 16 Apr 2012, Konrad Rzeszutek Wilk wrote:
> > > On Tue, Apr 10, 2012 at 05:25:19PM +0100, Stefano Stabellini wrote:
> > > > Set pages as FOREIGN_FRAME whenever blkfront shares them with another
> > > > domain. Then when blkfront un-share them, also removes the
> > > > FOREIGN_FRAME_BIT from the p2m.
> > > >
> > > > We do it so that when the source and the destination domain are the same
> > > > (blkfront connected to blkback in the same domain) we can more easily
> 
> So I've been testing it with my mini config and it worked great. But
> when I started using a distro .config it blew up. I am not really
> sure why it does that, but here is the dmesg and .config.
> 
> Nothing fancy with the guest config:
> 
> cat /crash.xm  | grep -v \#
> memory = 4096
> name = "OL6_X86_64_PVHVM"
> vcpus=12
> vif = [ 'mac=00:0f:4b:00:00:72,bridge=switch' ]
> disk= ['phy:/dev/guests/OL6_X86_64_PVHVM,hda,w']
> vfb = [ 'vnc=1, vnclisten=0.0.0.0,vncunused=1']
> vnc=1
> vnclisten="0.0.0.0"
> hvm_loader="/usr/bin/pygrub"
> 
> 
> This is the #testing branch, but just using my #linux-next along
> with stable/for-jens-3.5 should reproduce this.
> 
> I added a bit of debug statement and found that 0xffffffffffffffff
> was passed in as a PFN in the set_phys_to_machine.

I have been unable to reproduce this problem (I haven't given up yet)
but I bet that the following patch fixes it:


diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index e3a9945..88e9304 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -333,7 +333,7 @@ static int blkif_queue_request(struct request *req)
 					buffer_mfn,
 					rq_data_dir(req));
 
-			info->shadow[id].frame[i] = mfn_to_pfn(buffer_mfn);
+			info->shadow[id].frame[i] = buffer_pfn;
 			ring_req->u.rw.seg[i] =
 					(struct blkif_request_segment) {
 						.gref       = ref,


The idea is that the request contains a page for which

pfn->mfn->pfn == 0xffffffffffffffff

I am not sure exactly how it could be possible to get into this state in
blkfront, I hope that some more tracing and code reading will be able to
shed some lights on the issue.