[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] Questions regarding implementing mem_event for PV
> -----Original Message----- > From: Aravindh Puthiyaparambil (aravindp) > Sent: Thursday, October 17, 2013 12:00 PM > To: 'Tim Deegan' > Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx > Subject: RE: Questions regarding implementing mem_event for PV > > > > 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'm afraid you can't do that, and still support the PV API as it > > stands. The guest had read access to its pagetables, and it does not > > expect Xen to change them underfoot. E.g. if you temporarily mark a > > page as inaccessible and the guest walks its own pagetables to > > diagnose a page fault, it could see your change and send SIGSEGV to > > the current process. > > > > So if you want to have permissions in the pagetables that aren't the > > ones the guest asked for, your choices are: > > > > (a) use shadow pagetables. This is what the live migration code > > does in order to intercept writes for tracking memory dirtying. > > Slow, but should work for most things. > > > > (b) Wait for the PVH patch series to go in (and the equivalent > > series in Linux and FreeBSD kernels). PVH guests are enough > > like HVM ones in the Xen code that mem_events will be much easier. > > > > (b) is probably much eaier, if you have the choice. > > I don't have the choice of waiting for (b) so I guess I would have to go down > the harder (a) route. Let me explore that and get back to you with my > thoughts. Is there an xl config file option to bring up a PV guest with shadow? Or do I have to call xc_shadow_control() from the mem_event handler to enable shadow paging for a PV guest? Thanks, Aravindh _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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