[PATCH v2] x86/cpu: Skip reading MSR_IA32_PLATFORM_ID in virtualized environment

[PATCH v2] x86/cpu: Skip reading MSR_IA32_PLATFORM_ID in virtualized environment

[Binbin Wu]

The kernel now reads MSR_IA32_PLATFORM_ID during CPU init.  When
running as a guest, if the underlying hypervisor does not emulate the
MSR, the RDMSR from MSR_IA32_PLATFORM_ID can trigger an unchecked MSR
access during early boot.  This MSR is not emulated in the case for KVM
TDX, where the following is observed in the TD guest:

    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>

The platform ID is used for one thing and one thing only: microcode
updates.  Those updates are solely the domain of the bare-metal OS.  The
guest kernel code should not even try to touch the MSR.  Skip reading
the MSR when the kernel is running in a virtualized environment.  0 is a
valid platform ID, however, microcode related logic is skipped in a
virtualized environment.

Since intel_get_platform_id() could be called early before cpuinfo_x86
is fully initialized in the case of CONFIG_MICROCODE_DBG, check whether
the kernel is running in a virtualized environment from CPUID.  Use
cpuid_ecx() instead of native_cpuid_ecx() so that Xen PV guest will see
the virtualized bit.

Fixes: d8630b67ca1ed ("x86/cpu: Add platform ID to CPU info structure")
Reported-by: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
---
v2:
- Drop the patch on KVM side. (Sean, Dave)
- Use X86_FEATURE_HYPERVISOR for better readability. (Dave)
- Use cpuid_ecx() instead of native_cpuid_ecx() to check the hypervisor bit.
- Add RB from Rick.
---
 arch/x86/kernel/cpu/microcode/intel.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/kernel/cpu/microcode/intel.c b/arch/x86/kernel/cpu/microcode/intel.c
index 37ac4afe0972..1bc0c350726c 100644
--- a/arch/x86/kernel/cpu/microcode/intel.c
+++ b/arch/x86/kernel/cpu/microcode/intel.c
@@ -147,6 +147,10 @@ u32 intel_get_platform_id(void)
 	if (intel_cpuid_vfm() <= INTEL_PENTIUM_II_KLAMATH)
 		return 0;
 
+	/* Don't try to read microcode bits when virtualized. */
+	if (cpuid_ecx(1) & BIT(X86_FEATURE_HYPERVISOR & 0x1f))
+		return 0;
+
 	/* get processor flags from MSR 0x17 */
 	native_rdmsr(MSR_IA32_PLATFORM_ID, val[0], val[1]);
 

base-commit: 9974969c14031a097d6b45bcb7a06bb4aa525c40
-- 
2.46.0

Re: [PATCH v2] x86/cpu: Skip reading MSR_IA32_PLATFORM_ID in virtualized environment

[Kiryl Shutsemau]

On Thu, Apr 30, 2026 at 10:09:53AM +0800, Binbin Wu wrote:
> The kernel now reads MSR_IA32_PLATFORM_ID during CPU init.  When
> running as a guest, if the underlying hypervisor does not emulate the
> MSR, the RDMSR from MSR_IA32_PLATFORM_ID can trigger an unchecked MSR
> access during early boot.  This MSR is not emulated in the case for KVM
> TDX, where the following is observed in the TD guest:
> 
>     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>
> 
> The platform ID is used for one thing and one thing only: microcode
> updates.  Those updates are solely the domain of the bare-metal OS.  The
> guest kernel code should not even try to touch the MSR.  Skip reading
> the MSR when the kernel is running in a virtualized environment.  0 is a
> valid platform ID, however, microcode related logic is skipped in a
> virtualized environment.
> 
> Since intel_get_platform_id() could be called early before cpuinfo_x86
> is fully initialized in the case of CONFIG_MICROCODE_DBG, check whether
> the kernel is running in a virtualized environment from CPUID.  Use
> cpuid_ecx() instead of native_cpuid_ecx() so that Xen PV guest will see
> the virtualized bit.
> 
> Fixes: d8630b67ca1ed ("x86/cpu: Add platform ID to CPU info structure")
> Reported-by: Vishal Verma <vishal.l.verma@intel.com>
> Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
> Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>

Acked-by: Kiryl Shutsemau (Meta) <kas@kernel.org>

-- 
  Kiryl Shutsemau / Kirill A. Shutemov

Re: [PATCH v2] x86/cpu: Skip reading MSR_IA32_PLATFORM_ID in virtualized environment

[Borislav Petkov]

On Thu, Apr 30, 2026 at 10:09:53AM +0800, Binbin Wu wrote:
> diff --git a/arch/x86/kernel/cpu/microcode/intel.c b/arch/x86/kernel/cpu/microcode/intel.c
> index 37ac4afe0972..1bc0c350726c 100644
> --- a/arch/x86/kernel/cpu/microcode/intel.c
> +++ b/arch/x86/kernel/cpu/microcode/intel.c
> @@ -147,6 +147,10 @@ u32 intel_get_platform_id(void)
>  	if (intel_cpuid_vfm() <= INTEL_PENTIUM_II_KLAMATH)
>  		return 0;
>  
> +	/* Don't try to read microcode bits when virtualized. */
> +	if (cpuid_ecx(1) & BIT(X86_FEATURE_HYPERVISOR & 0x1f))
> +		return 0;
> +
>  	/* get processor flags from MSR 0x17 */
>  	native_rdmsr(MSR_IA32_PLATFORM_ID, val[0], val[1]);
>  
> 
> base-commit: 9974969c14031a097d6b45bcb7a06bb4aa525c40
> -- 

Does that work too?

diff --git a/arch/x86/kernel/cpu/microcode/intel.c b/arch/x86/kernel/cpu/microcode/intel.c
index 18d2eff7a4b7..1b24de94bdd5 100644
--- a/arch/x86/kernel/cpu/microcode/intel.c
+++ b/arch/x86/kernel/cpu/microcode/intel.c
@@ -139,6 +139,9 @@ u32 intel_get_platform_id(void)
 {
 	unsigned int val[2];
 
+	if (hypervisor_present)
+		return 0;
+
 	/*
 	 * This can be called early. Use CPUID directly instead of
 	 * relying on cpuinfo_x86 which may not be fully initialized.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette

Re: [PATCH v2] x86/cpu: Skip reading MSR_IA32_PLATFORM_ID in virtualized environment

[Binbin Wu]

On 5/11/2026 6:04 PM, Borislav Petkov wrote:
> On Thu, Apr 30, 2026 at 10:09:53AM +0800, Binbin Wu wrote:
>> diff --git a/arch/x86/kernel/cpu/microcode/intel.c b/arch/x86/kernel/cpu/microcode/intel.c
>> index 37ac4afe0972..1bc0c350726c 100644
>> --- a/arch/x86/kernel/cpu/microcode/intel.c
>> +++ b/arch/x86/kernel/cpu/microcode/intel.c
>> @@ -147,6 +147,10 @@ u32 intel_get_platform_id(void)
>>  	if (intel_cpuid_vfm() <= INTEL_PENTIUM_II_KLAMATH)
>>  		return 0;
>>  
>> +	/* Don't try to read microcode bits when virtualized. */
>> +	if (cpuid_ecx(1) & BIT(X86_FEATURE_HYPERVISOR & 0x1f))
>> +		return 0;
>> +
>>  	/* get processor flags from MSR 0x17 */
>>  	native_rdmsr(MSR_IA32_PLATFORM_ID, val[0], val[1]);
>>  
>>
>> base-commit: 9974969c14031a097d6b45bcb7a06bb4aa525c40
>> -- 
> 
> Does that work too?
> 
> diff --git a/arch/x86/kernel/cpu/microcode/intel.c b/arch/x86/kernel/cpu/microcode/intel.c
> index 18d2eff7a4b7..1b24de94bdd5 100644
> --- a/arch/x86/kernel/cpu/microcode/intel.c
> +++ b/arch/x86/kernel/cpu/microcode/intel.c
> @@ -139,6 +139,9 @@ u32 intel_get_platform_id(void)
>  {
>  	unsigned int val[2];
>  
> +	if (hypervisor_present)

hypervisor_present could be uninitialized if dis_ucode_ldr is true.
intel_get_platform_id() is also called during the normal cpu initialization.


> +		return 0;
> +
>  	/*
>  	 * This can be called early. Use CPUID directly instead of
>  	 * relying on cpuinfo_x86 which may not be fully initialized.
>