Messages in this thread | | | Date | Thu, 28 Mar 2024 21:21:37 +0800 | Subject | Re: [PATCH v19 059/130] KVM: x86/tdp_mmu: Don't zap private pages for unsupported cases | From | Xiaoyao Li <> |
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On 3/28/2024 6:17 PM, Chao Gao wrote: > On Thu, Mar 28, 2024 at 11:40:27AM +0800, Xiaoyao Li wrote: >> On 3/28/2024 11:04 AM, Edgecombe, Rick P wrote: >>> On Thu, 2024-03-28 at 09:30 +0800, Xiaoyao Li wrote: >>>>> The current ABI of KVM_EXIT_X86_RDMSR when TDs are created is nothing. So I don't see how this >>>>> is >>>>> any kind of ABI break. If you agree we shouldn't try to support MTRRs, do you have a different >>>>> exit >>>>> reason or behavior in mind? >>>> >>>> Just return error on TDVMCALL of RDMSR/WRMSR on TD's access of MTRR MSRs. >>> >>> MTRR appears to be configured to be type "Fixed" in the TDX module. So the guest could expect to be >>> able to use it and be surprised by a #GP. >>> >>> { >>> "MSB": "12", >>> "LSB": "12", >>> "Field Size": "1", >>> "Field Name": "MTRR", >>> "Configuration Details": null, >>> "Bit or Field Virtualization Type": "Fixed", >>> "Virtualization Details": "0x1" >>> }, >>> >>> If KVM does not support MTRRs in TDX, then it has to return the error somewhere or pretend to >>> support it (do nothing but not return an error). Returning an error to the guest would be making up >>> arch behavior, and to a lesser degree so would ignoring the WRMSR. >> >> The root cause is that it's a bad design of TDX to make MTRR fixed1. When >> guest reads MTRR CPUID as 1 while getting #VE on MTRR MSRs, it already breaks >> the architectural behavior. (MAC faces the similar issue , MCA is fixed1 as > > I won't say #VE on MTRR MSRs breaks anything. Writes to other MSRs (e.g. > TSC_DEADLINE MSR) also lead to #VE. If KVM can emulate the MSR accesses, #VE > should be fine. > > The problem is: MTRR CPUID feature is fixed 1 while KVM/QEMU doesn't know how > to virtualize MTRR especially given that KVM cannot control the memory type in > secure-EPT entries.
yes, I partly agree on that "#VE on MTRR MSRs breaks anything". #VE is not a problem, the problem is if the #VE is opt-in or unconditional.
For the TSC_DEADLINE_MSR, #VE is opt-in actually. CPUID(1).EXC[24].TSC_DEADLINE is configurable by VMM. Only when VMM configures the bit to 1, will the TD guest get #VE. If VMM configures it to 0, TD guest just gets #GP. This is the reasonable design.
>> well while accessing MCA related MSRs gets #VE. This is why TDX is going to >> fix them by introducing new feature and make them configurable) >> >>> So that is why I lean towards >>> returning to userspace and giving the VMM the option to ignore it, return an error to the guest or >>> show an error to the user. >> >> "show an error to the user" doesn't help at all. Because user cannot fix it, >> nor does QEMU. > > The key point isn't who can fix/emulate MTRR MSRs. It is just KVM doesn't know > how to handle this situation and ask userspace for help. > > Whether or how userspace can handle the MSR writes isn't KVM's problem. It may be > better if KVM can tell userspace exactly in which cases KVM will exit to > userspace. But there is no such an infrastructure. > > An example is: in KVM CET series, we find it is complex for KVM instruction > emulator to emulate control flow instructions when CET is enabled. The > suggestion is also to punt to userspace (w/o any indication to userspace that > KVM would do this).
Please point me to decision of CET? I'm interested in how userspace can help on that.
>> >>> If KVM can't support the behavior, better to get an actual error in >>> userspace than a mysterious guest hang, right? >> What behavior do you mean? >> >>> Outside of what kind of exit it is, do you object to the general plan to punt to userspace? >>> >>> Since this is a TDX specific limitation, I guess there is KVM_EXIT_TDX_VMCALL as a general category >>> of TDVMCALLs that cannot be handled by KVM. > > Using KVM_EXIT_TDX_VMCALL looks fine. > > We need to explain why MTRR MSRs are handled in this way unlike other MSRs. > > It is better if KVM can tell userspace that MTRR virtualization isn't supported > by KVM for TDs. Then userspace should resolve the conflict between KVM and TDX > module on MTRR. But to report MTRR as unsupported, we need to make > GET_SUPPORTED_CPUID a vm-scope ioctl. I am not sure if it is worth the effort.
My memory is that Sean dislike the vm-scope GET_SUPPORTED_CPUID for TDX when he was at Intel.
Anyway, we can provide TDX specific interface to report SUPPORTED_CPUID in KVM_TDX_CAPABILITIES, if we really need it.
> >> >> I just don't see any difference between handling it in KVM and handling it in >> userspace: either a) return error to guest or b) ignore the WRMSR.
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