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

[Christoph Egger <Christoph.Egger@xxxxxxx>]

On Monday 13 September 2010 15:50:40 Tim Deegan wrote:
> At 14:32 +0100 on 13 Sep (1284388352), Christoph Egger wrote:
> > > 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.
>
> OK.  That's certainly confusing.  I think the fix is to have all the
> outward-facing interfaces to the p2m code always operate on the host
> (L1->L0) p2m.  None of their callers would know what to do with an L2
> pfn anyway.  Only code that explicitly asks for it (e.g. the NPF
> handler) should see the L2->L0 p2m.

The instruction emulator also must see the L2 -> L0 p2m
- to be more precise it is __hvm_copy() that fetches the
instruction - in order to be able to emulate instructions
for the L2 guest the L1 guest does not intercept.

Christoph



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