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Re: [Xen-devel] Re: [PATCH 12/13] Nested Virtualization: vram

On Monday 13 September 2010 15:16:41 Tim Deegan wrote:
> At 16:47 +0100 on 08 Sep (1283964447), Christoph Egger wrote:
> > On Wednesday 08 September 2010 17:07:35 Tim Deegan wrote:
> > > Hi,
> > >
> > > Is this needed as part of the nested-HVM series or just an independent
> > > interface change?
> >
> > This is an independent interface change.
> > It is not needed to make nested-HVM work in general but it is part of
> > a larger change to make log dirty code nested virtualization aware.
> Ah; does the current nested-SVM patchset break logdirty?

I would say hap-on-hap in general does.
I mean that also counts for ept-on-ept.

See below.

> I don't think making the vram structures per-P2M is the best approach.
> We're never going to have more than one vram area to track per guest so
> it can just operate on the host-p2m, like it does already.
> In general, the log-dirty code operates on N1 pfns, and we won't want a
> per-p2m log-dirty bitmap either; we'd only have to fold them together to
> use them in the tools.

Look at this trace:

(XEN)    [<ffff82c4801f953e>] hap_write_p2m_entry+0x3e/0x1cb
(XEN)    [<ffff82c4801cf285>] p2m_set_entry+0x4a7/0x782
(XEN)    [<ffff82c4801c88e1>] set_p2m_entry+0xb3/0x101
(XEN)    [<ffff82c4801cba46>] p2m_change_type+0x120/0x17a
(XEN)    [<ffff82c4801f94ce>] hap_clean_vram_tracking+0x44/0x76
(XEN)    [<ffff82c4801c7a6e>] paging_log_dirty_range+0x33/0x8b4
(XEN)    [<ffff82c4801f9420>] hap_track_dirty_vram+0x109/0x173
(XEN)    [<ffff82c4801a7afe>] do_hvm_op+0xc1a/0x12a5
(XEN)    [<ffff82c4802000d2>] syscall_enter+0xf2/0x14c

The problem is in paging_write_p2m_entry():

     struct vcpu *v = current;
     if ( v->domain != d )
         v = d->vcpu ? d->vcpu[0] ? NULL;

The chosen vcpu can be in guest mode and fill the vram / logdirty
host p2m with l2 guest related data.


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