Re: [RFC KVM 00/27] KVM Address Space Isolation

From: Liran Alon
Date: Mon May 13 2019 - 17:19:11 EST

> On 13 May 2019, at 22:31, Nakajima, Jun <jun.nakajima@xxxxxxxxx> wrote:
> On 5/13/19, 7:43 AM, "kvm-owner@xxxxxxxxxxxxxxx on behalf of Alexandre Chartre" wrote:
> Proposal
> ========
> To handle both these points, this series introduce the mechanism of KVM
> address space isolation. Note that this mechanism completes (a)+(b) and
> don't contradict. In case this mechanism is also applied, (a)+(b) should
> still be applied to the full virtual address space as a defence-in-depth).
> The idea is that most of KVM #VMExit handlers code will run in a special
> KVM isolated address space which maps only KVM required code and per-VM
> information. Only once KVM needs to architectually access other (sensitive)
> data, it will switch from KVM isolated address space to full standard
> host address space. At this point, KVM will also need to kick all sibling
> hyperthreads to get out of guest (note that kicking all sibling hyperthreads
> is not implemented in this serie).
> Basically, we will have the following flow:
> - qemu issues KVM_RUN ioctl
> - KVM handles the ioctl and calls vcpu_run():
> . KVM switches from the kernel address to the KVM address space
> . KVM transfers control to VM (VMLAUNCH/VMRESUME)
> . VM returns to KVM
> . KVM handles VM-Exit:
> . if handling need full kernel then switch to kernel address space
> . else continues with KVM address space
> . KVM loops in vcpu_run() or return
> - KVM_RUN ioctl returns
> So, the KVM_RUN core function will mainly be executed using the KVM address
> space. The handling of a VM-Exit can require access to the kernel space
> and, in that case, we will switch back to the kernel address space.
> Once all sibling hyperthreads are in the host (either using the full kernel address space or user address space), what happens to the other sibling hyperthreads if one of them tries to do VM entry? That VCPU will switch to the KVM address space prior to VM entry, but others continue to run? Do you think (a) + (b) would be sufficient for that case?

The description here is missing and important part: When a hyperthread needs to switch from KVM isolated address space to kernel full address space, it should first kick all sibling hyperthreads outside of guest and only then safety switch to full kernel address space. Only once all sibling hyperthreads are running with KVM isolated address space, it is safe to enter guest.

The main point of this address space is to avoid kicking all sibling hyperthreads on *every* VMExit from guest but instead only kick them when switching address space. The assumption is that the vast majority of exits can be handled in KVM isolated address space and therefore do not require to kick the sibling hyperthreads outside of guest.


> ---
> Jun
> Intel Open Source Technology Center