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

[Chao Gao]

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.