On 6/12/24 05:37, Xiaoyao Li wrote:
> On 12/6/2024 1:31 AM, Adrian Hunter wrote:
>> 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?
>
> Userspace doesn't set the bit to 1 in kvm_tdx_init_vm.cpuid, doesn't mean userspace wants the bit to be 0.
>
> Note, KVM_TDX_CAPABILITIES.cpuid reports the configurable bits. The value 0 of a bit in KVM_TDX_CAPABILITIES.cpuid means the bit is not configurable, not means the bit is unsupported.
For features configurable by CPUID like TSX and WAITPKG,
a value of 0 does mean unsupported, because the value
has to be 1 to enable the feature.
