Re: [PATCH 7/7] KVM: TDX: Add TSX_CTRL msr into uret_msrs list

[Adrian Hunter]

On 4/12/24 17:33, Xiaoyao Li wrote:
> On 12/4/2024 7:55 PM, Adrian Hunter wrote:
>> On 4/12/24 13:13, Chao Gao wrote:
>>> On Wed, Dec 04, 2024 at 08:57:23AM +0200, Adrian Hunter wrote:
>>>> On 4/12/24 08:37, Chao Gao wrote:
>>>>> On Wed, Dec 04, 2024 at 08:18:32AM +0200, Adrian Hunter wrote:
>>>>>> On 4/12/24 03:25, Chao Gao wrote:
>>>>>>>> +#define TDX_FEATURE_TSX (__feature_bit(X86_FEATURE_HLE) | __feature_bit(X86_FEATURE_RTM))
>>>>>>>> +
>>>>>>>> +static bool has_tsx(const struct kvm_cpuid_entry2 *entry)
>>>>>>>> +{
>>>>>>>> +    return entry->function == 7 && entry->index == 0 &&
>>>>>>>> +           (entry->ebx & TDX_FEATURE_TSX);
>>>>>>>> +}
>>>>>>>> +
>>>>>>>> +static void clear_tsx(struct kvm_cpuid_entry2 *entry)
>>>>>>>> +{
>>>>>>>> +    entry->ebx &= ~TDX_FEATURE_TSX;
>>>>>>>> +}
>>>>>>>> +
>>>>>>>> +static bool has_waitpkg(const struct kvm_cpuid_entry2 *entry)
>>>>>>>> +{
>>>>>>>> +    return entry->function == 7 && entry->index == 0 &&
>>>>>>>> +           (entry->ecx & __feature_bit(X86_FEATURE_WAITPKG));
>>>>>>>> +}
>>>>>>>> +
>>>>>>>> +static void clear_waitpkg(struct kvm_cpuid_entry2 *entry)
>>>>>>>> +{
>>>>>>>> +    entry->ecx &= ~__feature_bit(X86_FEATURE_WAITPKG);
>>>>>>>> +}
>>>>>>>> +
>>>>>>>> +static void tdx_clear_unsupported_cpuid(struct kvm_cpuid_entry2 *entry)
>>>>>>>> +{
>>>>>>>> +    if (has_tsx(entry))
>>>>>>>> +        clear_tsx(entry);
>>>>>>>> +
>>>>>>>> +    if (has_waitpkg(entry))
>>>>>>>> +        clear_waitpkg(entry);
>>>>>>>> +}
>>>>>>>> +
>>>>>>>> +static bool tdx_unsupported_cpuid(const struct kvm_cpuid_entry2 *entry)
>>>>>>>> +{
>>>>>>>> +    return has_tsx(entry) || has_waitpkg(entry);
>>>>>>>> +}
>>>>>>>
>>>>>>> No need to check TSX/WAITPKG explicitly because setup_tdparams_cpuids() already
>>>>>>> ensures that unconfigurable bits are not set by userspace.
>>>>>>
>>>>>> Aren't they configurable?
>>>>>
>>>>> They are cleared from the configurable bitmap by tdx_clear_unsupported_cpuid(),
>>>>> so they are not configurable from a userspace perspective. Did I miss anything?
>>>>> KVM should check user inputs against its adjusted configurable bitmap, right?
>>>>
>>>> Maybe I misunderstand but we rely on the TDX module to reject
>>>> invalid configuration.  We don't check exactly what is configurable
>>>> for the TDX Module.
>>>
>>> Ok, this is what I missed. I thought KVM validated user input and masked
>>> out all unsupported features. sorry for this.
>>>
>>>>
>>>> TSX and WAITPKG are not invalid for the TDX Module, but KVM
>>>> must either support them by restoring their MSRs, or disallow
>>>> them.  This patch disallows them for now.
>>>
>>> Yes. I agree. what if a new feature (supported by a future TDX module) also
>>> needs KVM to restore some MSRs? current KVM will allow it to be exposed (since
>>> only TSX/WAITPKG are checked); then some MSRs may get corrupted. I may think
>>> this is not a good design. Current KVM should work with future TDX modules.
>>
>> With respect to CPUID, I gather this kind of thing has been
>> discussed, such as here:
>>
>>     https://lore.kernel.org/all/ZhVsHVqaff7AKagu@xxxxxxxxxx/
>>
>> and Rick and Xiaoyao are working on something.
>>
>> In general, I would expect a new TDX Module would advertise support for
>> new features, but KVM would have to opt in to use them.
>>
> 
> There were discussion[1] on whether KVM to gatekeep the configurable/supported CPUIDs for TDX. I stand by Sean that KVM needs to do so.
> 
> Regarding KVM opt in the new feature, KVM gatekeeps the CPUID bit that can be set by userspace is exactly the behavior of opt-in. i.e., for a given KVM, it only allows a CPUID set {S} to be configured by userspace, if new TDX module supports new feature X, it needs KVM to opt-in X by adding X to {S} so that X is allowed to be configured by userspace.
> 
> Besides, I find current interface between KVM and userspace lacks the ability to tell userspace what bits are not supported by KVM. KVM_TDX_CAPABILITIES.cpuid doesn't work because it represents the configurable CPUIDs, not supported CPUIDs (I think we might rename it to configurable_cpuid to better reflect its meaning). So userspace has to hardcode that TSX and WAITPKG is not support itself.

I don't follow why hardcoding would be necessary.

If the leaf is represented in KVM_TDX_CAPABILITIES.cpuid, and
the bits are 0 there, why would userspace try to set them to 1?

> 
> [1] https://lore.kernel.org/all/ZuM12EFbOXmpHHVQ@xxxxxxxxxx/
>