Re: [V2 PATCH] x86/tdx: add VIRT_CPUID2 virtualization if REDUCE_VE was not successful

[Zhiquan Li <zhiquan1.li@intel.com>]

On 2025/4/30 21:44, Dave Hansen wrote:
>> Currently the most customer's complaints come from the CPUID leaf 0x2
>> not virtualization, and most of access come from user space.  Is it
>> appropriate for such behavior directly cause a guest kernel panic?
> If a VMM doesn't properly configure topology properly and allows
> REDUCE_VE, then panic. They obviously got something wrong because that
> configuration doesn't make any sense.

OK, I agree with you.  Is it better to simplify the logic like this:

    static void reduce_unnecessary_ve(void)
    {
        u64 configured;

        /* Has the VMM provided a valid topology configuration? */
        tdg_vm_rd(TDCS_TOPOLOGY_ENUM_CONFIGURED, &configured);

        if (!configured)
            panic("VMM did not configure X2APIC_IDs properly\n");

        tdg_vm_wr(TDCS_TD_CTLS, TD_CTLS_REDUCE_VE, TD_CTLS_REDUCE_VE);
    }

Since merely fall back to enable ENUM_TOPOLOGY isn't enough, the guest
still suffering the CPUID leaf 0x2 not virtualization regression, like
the glibc bug.  Full REDUCE_VE is really expected.

Best Regards,
Zhiquan