Re: [Xen-devel] Questions regarding implementing mem_event for PV

["Aravindh Puthiyaparambil (aravindp)" <aravindp@xxxxxxxxx>]

> > I am looking in to implementing mem_event for PV guests that use PV
> mmu_ops. Initially I am thinking of supporting 64-bit PV guests. I think only
> RW, RX and R permissions would be possible. I am looking at the hypercall
> interfaces that do the pagetable manipulations. In addition to the hypercalls 
> I
> am also looking at the pagetable write emulation code.
> I'm assuming mem_access here, rather than generic mem_event...

Ah yes, I should have said mem_access rather than mem_event :-)

 > >
> > My plan is to set the page permissions in:
> > do_mmu_update()
> > do_mmuext_op(MMUEXT_PIN_L1_TABLE)
> > __do_update_va_mapping()
> > ptwr_emulated_update()
> >
> > I would then hook in to do_page_fault() to perform the access check and
> send the request to the mem_event handler. Please let me know if I am on
> the right track and if this is a sane approach?
> >
> > Questions...
> > I am not sure where to store the p2m_access value in the PTE. EPT uses bits
> 61:58. The spec says they are ignored for regular pagetables, so is it safe to
> use these bits? If not, do you have any suggestions about where I can stash
> these bits?
> Depends on the PV guest. If using auto-translated physmap (i.e. PVH but not
> for dom0), then you will have a proper EPT. You can probably just rely on the
> same men-event implementation that exists for HVMs (hand-waving
> though).
> 
> On a traditional PV, you will need to turn on shadow. I don't think you will 
> get
> to where you want by changing the PT entries the guest sees. Or you will, but
> more painfully (hand waving, redux)
> 
> Once you turn on shadows, that uses the pt implementation of the p2m
> (arch/x86/mm/p2m-pt.c), which AMD also uses. There is room in there for
> mem access bits. From a previous thread on xen-devel the IOMMU bits can
> be sacrificed to host mem access bits, and the hypervisor already implements
> XOR for IOMMU and "fancy memory stuff"

That was helpful. Looks like I would have to go down the PV + shadow path if I 
want this working for PV guests.

Thanks,
Aravindh

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