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

Xiaoyao Li
2024-03-28T00:07:00Z

On 3/28/2024 1:36 AM, Edgecombe, Rick P wrote:
> On Wed, 2024-03-27 at 10:54 +0800, Xiaoyao Li wrote:
>>>> If QEMU doesn't configure the msr filter list correctly, KVM has to handle
>>>> guest's MTRR MSR accesses.
>>>> In my understanding, the suggestion is KVM zap private memory mappings.

TDX spec states that

18.2.1.4.1 Memory Type for Private and Opaque Access

The memory type for private and opaque access semantics, which use a
private HKID, is WB.

18.2.1.4.2 Memory Type for Shared Accesses

Intel SDM, Vol. 3, 28.2.7.2 Memory Type Used for Translated Guest-
Physical Addresses

The memory type for shared access semantics, which use a shared HKID,
is determined as described below. Note that this is different from the
way memory type is determined by the hardware during non-root mode
operation. Rather, it is a best-effort approximation that is designed
to still allow the host VMM some control over memory type.
• For shared access during host-side (SEAMCALL) flows, the memory
type is determined by MTRRs.
• For shared access during guest-side flows (VM exit from the guest
TD), the memory type is determined by a combination of the Shared
EPT and MTRRs.
o If the memory type determined during Shared EPT walk is WB, then
the effective memory type for the access is determined by MTRRs.
o Else, the effective memory type for the access is UC.

My understanding is that guest MTRR doesn't affect the memory type for
private memory. So we don't need to zap private memory mappings.

>>>> But guests won't accept memory again because no one
>>>> currently requests guests to do this after writes to MTRR MSRs. In this case,
>>>> guests may access unaccepted memory, causing infinite EPT violation loop
>>>> (assume SEPT_VE_DISABLE is set). This won't impact other guests/workloads on
>>>> the host. But I think it would be better if we can avoid wasting CPU resource
>>>> on the useless EPT violation loop.
>>>
>>> Qemu is expected to do it correctly.  There are manyways for userspace to go
>>> wrong.  This isn't specific to MTRR MSR.
>>
>> This seems incorrect. KVM shouldn't force userspace to filter some
>> specific MSRs. The semantic of MSR filter is userspace configures it on
>> its own will, not KVM requires to do so.
>
> I'm ok just always doing the exit to userspace on attempt to use MTRRs in a TD, and not rely on the
> MSR list. At least I don't see the problem.

What is the exit reason in vcpu->run->exit_reason?
KVM_EXIT_X86_RDMSR/WRMSR? If so, it breaks the ABI on
KVM_EXIT_X86_RDMSR/WRMSR.