[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.

 

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?

The default functions for p2m->set_entry() and p2m->get_entry() default to the shadow PT variants which I am assuming cannot be used in the PV cases and I would need to do something along the lines of mod_l1_entry(). Is this correct?

 

Thanks,

Aravindh

 

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