Re: [PATCH 1/2] KVM: TDX: Allow TDs to read MSR_IA32_PLATFORM_ID

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 4/28/2026 10:47 AM, Binbin Wu wrote:
> Add MSR_IA32_PLATFORM_ID to tdx_has_emulated_msr() so that TDs can read
> it.
> 
> Linux kernel reads MSR_IA32_PLATFORM_ID during init since commit
> d8630b67ca1e ("x86/cpu: Add platform ID to CPU info structure").  KVM
> already supports this MSR on read for normal VMs by returning 0.
> Without support for this MSR, TDs get unchecked MSR access errors.
> 
>      unchecked MSR access error: RDMSR from 0x17 at rIP: 0xffffffffba38d6fc (intel_get_platform_id+0x7c/0xb0)
>      Call Trace:
>       <TASK>
>       ? early_init_intel+0x28/0x2c0
>       ? early_cpu_init+0x9b/0x930
>       ? setup_arch+0xbf/0xbb0
>       ? _printk+0x6b/0x90
>       ? start_kernel+0x7f/0xaa0
>       ? x86_64_start_reservations+0x24/0x30
>       ? x86_64_start_kernel+0xda/0xe0
>       ? common_startup_64+0x13e/0x141
>       </TASK>
> 
> Add MSR_IA32_PLATFORM_ID in tdx_has_emulated_msr() to allow TDs to read
> the MSR.  MSR_IA32_PLATFORM_ID is read-only by hardware definition, i.e.
> KVM should never add it as writable, no need to add it in
> tdx_is_read_only_msr().
> 
> Fixes: dd50294f3e3c ("KVM: TDX: Implement callbacks for MSR operations")
> Reported-by: Vishal Verma <vishal.l.verma@intel.com>
> Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>

Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>

Nit: It would be helpful to add the info that read on this MSR from TDs 
always triggers #VE so that Linux guest will request emulation from KVM.

> ---
>   arch/x86/kvm/vmx/tdx.c | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index 04ce321ebdf3..812ad99b11e5 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -2094,6 +2094,7 @@ void tdx_get_exit_info(struct kvm_vcpu *vcpu, u32 *reason,
>   bool tdx_has_emulated_msr(u32 index)
>   {
>   	switch (index) {
> +	case MSR_IA32_PLATFORM_ID:
>   	case MSR_IA32_UCODE_REV:
>   	case MSR_IA32_ARCH_CAPABILITIES:
>   	case MSR_IA32_POWER_CTL: