Messages in this thread | | | Date | Mon, 25 Oct 2021 18:05:05 +0200 | Subject | Re: [PATCH v2 0/4] KVM: x86: APICv cleanups | From | Paolo Bonzini <> |
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On 25/10/21 17:59, Sean Christopherson wrote: >> No, checking for the update is worse and with this example, I can now point >> my finger on why I preferred the VM check even before: because even though >> the page fault path runs in vCPU context and uses a vCPU-specific role, >> overall the page tables are still per-VM. > Arguably the lack of incorporation into the page role is the underlying bug, and > all the shenanigans with synchronizing updates are just workarounds for that bug. > I.e. page tables are never strictly per-VM, they're per-role, but we fudge it in > this case because we don't want to take on the overhead of maintaining two sets > of page tables to handle APICv.
Yes, that makes sense as well:
- you can have simpler code by using the vCPU state, but then correctness requires that the APICv state be part of the vCPU-specific MMU state. That is, of the role.
- if you don't want to do that, because you want to maintain one set of page tables only, the price to pay is the synchronization shenanigans, both those involving apicv_update mutex^Wrwsem (which ensure no one uses the old state) and those involving kvm_faultin_pfn/kvm_zap_pfn_range (to ensure the one state used by the MMU is the correct one).
So it's a pick your poison situation.
Paolo
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