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xense-devel
Re: [Xense-devel] Xen memory management
So with full virtualization using HVM's Xen is responsible for
maintaining shadow page tables for each HVM. So does that mean Xen has
to trap every write _and read_ to the HVM page table and do address
translation from P2M(write) and M2P(read)?
Pacifica has an optional feature which allows the processor to walk the
guest page table and do physical-to-machine translation from a host table
before filling the TLB. I don't think the initial wave of CPUs implements
this (?) but I guess it'll appear later - at which point you get the effect
of shadow PTs implemented in hardware.
Reads are an interesting question... The guest will be reading its own
pagetables, which will have physical addresses in (which it will therefore
understand). The only thing you might want to emulate are accesses to the
accessed/dirty bits, since these won't be directly updated by the hardware.
I believe Xen somehow propagates these back from the host pagetables into
the guest's pagetables, but I'm not clear on exactly when this happens...
I assume if that's true the
global tables go away, a HVM wouldn't know they existed anyway.
HVM systems don't see the global tables. They're only needed for
paravirtualised guests, because those guests need to do the M2P mapping
efficiently themselves.
Are
there any other strategies for safe page table access that don't involve
Xen (writable page tables with HVM)? It seems like the full
virtualization strategy could be quite expensive, although it seems
pretty safe.
What exactly do you mean by "safe" here? Do you mean "without the global
M2P table"? You *could* implement a per-domain M2P table that'd otherwise
work the same. It'd just be a bit memory hungry. Or you could get the guest
to maintain its own M2P table, again having a per-guest cost.
Thanks for the clarification.
Did that help some?
P.S. Is it safe to send pictures to the list? I don't know if that's
kosher or not so I'm holding off.
I think they've been sent before without any major explosions occurring...
I'm not sure if it'll need moderator approval.
Cheers,
Mark
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