Messages in this thread | | | Subject | Re: [PATCH 2/2] KVM: VMX: Extend VMX's #AC handding | From | Xiaoyao Li <> | Date | Sun, 2 Feb 2020 00:58:15 +0800 |
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On 2/1/2020 5:33 AM, Andy Lutomirski wrote: > > >> On Jan 31, 2020, at 1:04 PM, Sean Christopherson <sean.j.christopherson@intel.com> wrote: >> >> On Fri, Jan 31, 2020 at 12:57:51PM -0800, Andy Lutomirski wrote: >>> >>>>> On Jan 31, 2020, at 12:18 PM, Sean Christopherson <sean.j.christopherson@intel.com> wrote: >>>> >>>> This is essentially what I proposed a while back. KVM would allow enabling >>>> split-lock #AC in the guest if and only if SMT is disabled or the enable bit >>>> is per-thread, *or* the host is in "warn" mode (can live with split-lock #AC >>>> being randomly disabled/enabled) and userspace has communicated to KVM that >>>> it is pinning vCPUs. >>> >>> How about covering the actual sensible case: host is set to fatal? In this >>> mode, the guest gets split lock detection whether it wants it or not. How do >>> we communicate this to the guest? >> >> KVM doesn't advertise split-lock #AC to the guest and returns -EFAULT to the >> userspace VMM if the guest triggers a split-lock #AC. >> >> Effectively the same behavior as any other userspace process, just that KVM >> explicitly returns -EFAULT instead of the process getting a SIGBUS. > > > Which helps how if the guest is actually SLD-aware? > > I suppose we could make the argument that, if an SLD-aware guest gets #AC at CPL0, it’s a bug, but it still seems rather nicer to forward the #AC to the guest instead of summarily killing it.
If KVM does advertise split-lock detection to the guest, then kvm/host can know whether a guest is SLD-aware by checking guest's MSR_TEST_CTRL.SPLIT_LOCK_DETECT bit.
- If guest's MSR_TEST_CTRL.SPLIT_LOCK_DETECT is set, it indicates guest is SLD-aware so KVM forwards #AC to guest.
- If not set. It may be a old guest or a malicious guest or a guest without SLD support, and we cannot figure it out. So we have to kill the guest when host is SLD-fatal, and let guest survive when SLD-WARN for old sane buggy guest.
In a word, all the above is on the condition that KVM advertise split-lock detection to guest. But this patch doesn't do this. Maybe I should add that part in v2.
> ISTM, on an SLD-fatal host with an SLD-aware guest, the host should tell the guest “hey, you may not do split locks — SLD is forced on” and the guest should somehow acknowledge it so that it sees the architectural behavior instead of something we made up. Hence my suggestion. >
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