Re: [PATCH v6 56/60] i386/tdx: Don't treat SYSCALL as unavailable

Xiaoyao Li
2025-01-16T08:54:00Z

On 11/5/2024 5:59 PM, Paolo Bonzini wrote:
> On 11/5/24 07:24, Xiaoyao Li wrote:
>> Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
>> ---
>>   target/i386/kvm/tdx.c | 7 +++++++
>>   1 file changed, 7 insertions(+)
>>
>> diff --git a/target/i386/kvm/tdx.c b/target/i386/kvm/tdx.c
>> index 9cb099e160e4..05475edf72bd 100644
>> --- a/target/i386/kvm/tdx.c
>> +++ b/target/i386/kvm/tdx.c
>> @@ -734,6 +734,13 @@ static int
>> tdx_check_features(X86ConfidentialGuest *cg, CPUState *cs)
>>           requested = env->features[w];
>>           unavailable = requested & ~actual;
>> +        /*
>> +         * Intel enumerates SYSCALL bit as 1 only when processor in
>> 64-bit
>> +         * mode and before vcpu running it's not in 64-bit mode.
>> +         */
>> +        if (w == FEAT_8000_0001_EDX && unavailable &
>> CPUID_EXT2_SYSCALL) {
>> +            unavailable &= ~CPUID_EXT2_SYSCALL;
>> +        }
>>           mark_unavailable_features(cpu, w, unavailable, unav_prefix);
>>           if (unavailable) {
>>               mismatch = true;
>
> This seems like a TDX module bug?

I don't think so. The value of CPUID_EXT2_SYSCALL depends on the mode of
the vcpu. Per SDM, it's 0 outside 64-bit mode.

The initial state of TDX vcpu is 32-bit protected mode. At the time of
calling KVM_TDX_GET_CPUID, vcpu hasn't started running. So the value
should be 0.

There indeed is a TDX module. After vcpu starts running and TD guest
switches to 64-bit mode. The value of this bit returned by TDX module
via global metadata CPUID value still remains 0.

Off the topic, for me, it's really a bad API to return TDX's CPUID value
via TD-scope metadata. It fits better with TD VCPU scope metadata.

> It's the kind of thing that I guess
> could be worked around in KVM.
>
> If we do it in QEMU, I'd rather see it as
>
>             actual = cpuid_entry_get_reg(entry, wi->cpuid.reg);
>             switch (w) {
>             case FEAT_8000_0001_EDX:
>                 actual |= CPUID_EXT2_SYSCALL;
>                 break;
>             }
>             break;

I'll change to this way.

> Paolo
>