Messages in this thread | | | Subject | Re: [PATCH 2/2] KVM: VMX: Extend VMX's #AC handding | From | Xiaoyao Li <> | Date | Sun, 2 Feb 2020 12:33:22 +0800 |
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On 2/2/2020 1:56 AM, Andy Lutomirski wrote: > > >> On Feb 1, 2020, at 8:58 AM, Xiaoyao Li <xiaoyao.li@intel.com> wrote: >> >> 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. >> > > I disagree. If you advertise split-lock detection with the current core capability bit, it should *work*. And it won’t. The choices you’re actually giving the guest are: > > a) Guest understands SLD and wants it on. The guest gets the same behavior as in bare metal. > > b) Guest does not understand. Guest gets killed if it screws up as described below. > >> - 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. > > All true, but the result of running a Linux guest in SLD-warn mode will be broken. > >> >> 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. > > I think you should think the details all the way through, and I think you’re likely to determine that the Intel architecture team needs to do *something* to clean up this mess. > > There are two independent problems here. First, SLD *can’t* be virtualized sanely because it’s per-core not per-thread.
Sadly, it's the fact we cannot change. So it's better virtualized only when SMT is disabled to make thing simple.
> Second, most users *won’t want* to virtualize it correctly even if they could: if a guest is allowed to do split locks, it can DoS the system.
To avoid DoS attack, it must use sld_fatal mode. In this case, guest are forbidden to do split locks.
> So I think there should be an architectural way to tell a guest that SLD is on whether it likes it or not. And the guest, if booted with sld=warn, can print a message saying “haha, actually SLD is fatal” and carry on.
OK. Let me sort it out.
If SMT is disabled/unsupported, so KVM advertises SLD feature to guest. below are all the case:
----------------------------------------------------------------------- Host Guest Guest behavior ----------------------------------------------------------------------- 1. off same as in bare metal ----------------------------------------------------------------------- 2. warn off allow guest do split lock (for old guest): hardware bit set initially, once split lock happens, clear hardware bit when vcpu is running So, it's the same as in bare metal 3. warn 1. user space: get #AC, then clear MSR bit, but hardware bit is not cleared, #AC again, finally clear hardware bit when vcpu is running. So it's somehow the same as in bare-metal
2. kernel: same as in bare metal. 4. fatal same as in bare metal ---------------------------------------------------------------------- 5.fatal off guest is killed when split lock, or forward #AC to guest, this way guest gets an unexpected #AC 6. warn 1. user space: get #AC, then clear MSR bit, but hardware bit is not cleared, #AC again, finally guest is killed, or KVM forwards #AC to guest then guest gets an unexpected #AC. 2. kernel: same as in bare metal, call die(); 7. fatal same as in bare metal ----------------------------------------------------------------------
Based on the table above, if we want guest has same behavior as in bare metal, we can set host to sld_warn mode. If we want prevent DoS from guest, we should set host to sld_fatal mode.
Now, let's analysis what if there is an architectural way to tell a guest that SLD is forced on. Assume it's a SLD_forced_on cpuid bit.
- Host is sld_off, SLD_forced_on cpuid bit is not set, no change for case #1
- Host is sld_fatal, SLD_forced_on cpuid bit must be set: - if guest is SLD-aware, guest is supposed to only use fatal mode that goes to case #7. And guest is not recommended using warn mode. if guest persists, it goes to case #6
- if guest is not SLD-aware, maybe it's an old guest or it's a malicious guest that pretends not SLD-aware, it goes to case #5.
- Host is sld_warn, we have two choice - set SLD_forced_on cpuid bit, it's the same as host is fatal. - not set SLD_force_on_cpuid bit, it's the same as case #2,#3,#4
So I think introducing an architectural way to tell a guest that SLD is forced on can make the only difference is that, there is a way to tell guest not to use warn mode, thus eliminating case #6.
If you think it really matters, I can forward this requirement to our Intel architecture people. >> >>> 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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