Re: [PATCH v19 059/130] KVM: x86/tdp_mmu: Don't zap private pages for unsupported cases

Xiaoyao Li
2024-03-28T03:40:34Z

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 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.

> 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.

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.