[PATCH 0/2] Fix MSR_IA32_PLATFORM_ID access for TDX guests

[PATCH 0/2] Fix MSR_IA32_PLATFORM_ID access for TDX guests

[Binbin Wu]

Since commit d8630b67ca1e ("x86/cpu: Add platform ID to CPU info
structure"), the Linux kernel reads MSR_IA32_PLATFORM_ID during CPU
init. This causes unchecked MSR access errors when running as a TDX
guest, because KVM doesn't include MSR_IA32_PLATFORM_ID in
tdx_has_emulated_msr().

On KVM side, this patch set adds MSR_IA32_PLATFORM_ID to
tdx_has_emulated_msr() so that KVM returns 0 for TDs reading this MSR,
consistent with how KVM already handles it for normal VMs.

Optionally, in order to provide back-compatibility for newer Linux guests
running on older KVM hosts without the fix, the second patch skips the
MSR_IA32_PLATFORM_ID read entirely when Linux is running in a
virtualized environment. This could be dropped if we think the fix on
KVM side is sufficient.

Binbin Wu (2):
  KVM: TDX: Allow TDs to read MSR_IA32_PLATFORM_ID
  x86/cpu: Skip reading MSR_IA32_PLATFORM_ID in virtualized environment

 arch/x86/kernel/cpu/microcode/core.c     | 2 +-
 arch/x86/kernel/cpu/microcode/intel.c    | 4 ++++
 arch/x86/kernel/cpu/microcode/internal.h | 5 +++++
 arch/x86/kvm/vmx/tdx.c                   | 1 +
 4 files changed, 11 insertions(+), 1 deletion(-)


base-commit: 39704f00f747aba3144289870b5fd8ac230a9aaf
-- 
2.46.0

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

[Binbin Wu]

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>
---
 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:
-- 
2.46.0

[PATCH 2/2] x86/cpu: Skip reading MSR_IA32_PLATFORM_ID in virtualized environment

[Binbin Wu]

The Linux 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 0x17 in intel_get_platform_id() can trigger an
unchecked MSR access during early boot.

    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>

Currently, KVM does not support guest reads of MSR_IA32_PLATFORM_ID for
TDX.  This should be fixed in the hypervisor, but for compatibility with
newer guests running on older KVM hosts, skip reading MSR_IA32_PLATFORM_ID
and return 0. It's meaningless to read MSR_IA32_PLATFORM_ID since guests
are not allowed to do microcode update.  cpu_has_old_microcode(), which
uses platform ID for CPU match, also skips checking whether the microcode
is old or not in a virtualized environment.

Since intel_get_platform_id() can be called early before cpuinfo_x86
is fully initialized, check whether it's running in a virtualized
environment from CPUID and opportunistically add a helper.

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>
---
 arch/x86/kernel/cpu/microcode/core.c     | 2 +-
 arch/x86/kernel/cpu/microcode/intel.c    | 4 ++++
 arch/x86/kernel/cpu/microcode/internal.h | 5 +++++
 3 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
index 651202e6fefb..ee204c8a90bf 100644
--- a/arch/x86/kernel/cpu/microcode/core.c
+++ b/arch/x86/kernel/cpu/microcode/core.c
@@ -135,7 +135,7 @@ bool __init microcode_loader_disabled(void)
 	 * 3) Certain AMD patch levels are not allowed to be
 	 *    overwritten.
 	 */
-	hypervisor_present = native_cpuid_ecx(1) & BIT(31);
+	hypervisor_present = x86_cpuid_has_hypervisor();
 
 	if ((hypervisor_present && !IS_ENABLED(CONFIG_MICROCODE_DBG)) ||
 	    amd_check_current_patch_level())
diff --git a/arch/x86/kernel/cpu/microcode/intel.c b/arch/x86/kernel/cpu/microcode/intel.c
index 37ac4afe0972..cb93e4ea410e 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 (x86_cpuid_has_hypervisor())
+		return 0;
+
 	/* get processor flags from MSR 0x17 */
 	native_rdmsr(MSR_IA32_PLATFORM_ID, val[0], val[1]);
 
diff --git a/arch/x86/kernel/cpu/microcode/internal.h b/arch/x86/kernel/cpu/microcode/internal.h
index 3b93c0676b4f..0233e074d76b 100644
--- a/arch/x86/kernel/cpu/microcode/internal.h
+++ b/arch/x86/kernel/cpu/microcode/internal.h
@@ -100,6 +100,11 @@ static inline unsigned int x86_cpuid_family(void)
 	return x86_family(eax);
 }
 
+static inline bool x86_cpuid_has_hypervisor(void)
+{
+	return native_cpuid_ecx(1) & BIT(31);
+}
+
 extern bool force_minrev;
 
 #ifdef CONFIG_CPU_SUP_AMD
-- 
2.46.0

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

[Xiaoyao Li]

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:

Re: [PATCH 2/2] x86/cpu: Skip reading MSR_IA32_PLATFORM_ID in virtualized environment

[Xiaoyao Li]

On 4/28/2026 10:47 AM, Binbin Wu wrote:
> The Linux 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 0x17 in intel_get_platform_id() can trigger an
> unchecked MSR access during early boot.
> 
>      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>
> 
> Currently, KVM does not support guest reads of MSR_IA32_PLATFORM_ID for
> TDX.  This should be fixed in the hypervisor, but for compatibility with
> newer guests running on older KVM hosts, skip reading MSR_IA32_PLATFORM_ID
> and return 0. It's meaningless to read MSR_IA32_PLATFORM_ID since guests
> are not allowed to do microcode update.  cpu_has_old_microcode(), which
> uses platform ID for CPU match, also skips checking whether the microcode
> is old or not in a virtualized environment.
> 
> Since intel_get_platform_id() can be called early before cpuinfo_x86
> is fully initialized, check whether it's running in a virtualized
> environment from CPUID and opportunistically add a helper.
> 
> 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>
> ---
>   arch/x86/kernel/cpu/microcode/core.c     | 2 +-
>   arch/x86/kernel/cpu/microcode/intel.c    | 4 ++++
>   arch/x86/kernel/cpu/microcode/internal.h | 5 +++++
>   3 files changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
> index 651202e6fefb..ee204c8a90bf 100644
> --- a/arch/x86/kernel/cpu/microcode/core.c
> +++ b/arch/x86/kernel/cpu/microcode/core.c
> @@ -135,7 +135,7 @@ bool __init microcode_loader_disabled(void)
>   	 * 3) Certain AMD patch levels are not allowed to be
>   	 *    overwritten.
>   	 */
> -	hypervisor_present = native_cpuid_ecx(1) & BIT(31);
> +	hypervisor_present = x86_cpuid_has_hypervisor();
>   
>   	if ((hypervisor_present && !IS_ENABLED(CONFIG_MICROCODE_DBG)) ||
>   	    amd_check_current_patch_level())
> diff --git a/arch/x86/kernel/cpu/microcode/intel.c b/arch/x86/kernel/cpu/microcode/intel.c
> index 37ac4afe0972..cb93e4ea410e 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. */

The platform ID is not only used by microcode update. I don't think 
calling them microcode bits is proper. It's also stashed in struct 
cpuinfo_x86::intel_platform_id and used in x86_match_cpu() as a factor 
as the generic FMS (Family, Model, Stepping) bits.

We skip things due to hypervisor is set usually when the thing is known 
unable to be virtualized. From the perspective of microcode update, it's 
OK to skip reading it due to hypervisor bit. However, from the 
perspective of generic platform ID, it seems not that reasonable to skip 
reading it due to hypervisor bit. Especially KVM has supported this MSR 
for normal VMs.

So how about using the safe version of RDMSR, if we want the enhancement?

> +	if (x86_cpuid_has_hypervisor())
> +		return 0;
> +
>   	/* get processor flags from MSR 0x17 */
>   	native_rdmsr(MSR_IA32_PLATFORM_ID, val[0], val[1]);
>   
> diff --git a/arch/x86/kernel/cpu/microcode/internal.h b/arch/x86/kernel/cpu/microcode/internal.h
> index 3b93c0676b4f..0233e074d76b 100644
> --- a/arch/x86/kernel/cpu/microcode/internal.h
> +++ b/arch/x86/kernel/cpu/microcode/internal.h
> @@ -100,6 +100,11 @@ static inline unsigned int x86_cpuid_family(void)
>   	return x86_family(eax);
>   }
>   
> +static inline bool x86_cpuid_has_hypervisor(void)
> +{
> +	return native_cpuid_ecx(1) & BIT(31);
> +}
> +
>   extern bool force_minrev;
>   
>   #ifdef CONFIG_CPU_SUP_AMD

Re: [PATCH 2/2] x86/cpu: Skip reading MSR_IA32_PLATFORM_ID in virtualized environment

[Binbin Wu]

On 4/28/2026 2:01 PM, Xiaoyao Li wrote:
[...]

>> diff --git a/arch/x86/kernel/cpu/microcode/intel.c b/arch/x86/kernel/cpu/microcode/intel.c
>> index 37ac4afe0972..cb93e4ea410e 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. */
> 
> The platform ID is not only used by microcode update. I don't think calling them microcode bits is proper. It's also stashed in struct cpuinfo_x86::intel_platform_id and used in x86_match_cpu() as a factor as the generic FMS (Family, Model, Stepping) bits.

It's still related to microcode though.
The platform id is used to detect whether the microcode is old or not in this
case.

> 
> We skip things due to hypervisor is set usually when the thing is known unable to be virtualized. From the perspective of microcode update, it's OK to skip reading it due to hypervisor bit. However, from the perspective of generic platform ID, it seems not that reasonable to skip reading it due to hypervisor bit. Especially KVM has supported this MSR for normal VMs.

Considering the microcode related code, Linux kernel does special handling when
running as a guest, microcode_loader_disabled() sets dis_ucode_ldr and
cpu_has_old_microcode() also skips to check whether the microcode is old
if it's running in a virtualized environment. I.e. the special handling for
virtualization make it meaningless to read the MSR.

> 
> So how about using the safe version of RDMSR, if we want the enhancement?

I am open to this.
Let's wait to see if the change on guest side is wanted or not.

> 
>> +    if (x86_cpuid_has_hypervisor())
>> +        return 0;
>> +
>>       /* get processor flags from MSR 0x17 */
>>       native_rdmsr(MSR_IA32_PLATFORM_ID, val[0], val[1]);
>>   diff --git a/arch/x86/kernel/cpu/microcode/internal.h b/arch/x86/kernel/cpu/microcode/internal.h
>> index 3b93c0676b4f..0233e074d76b 100644
>> --- a/arch/x86/kernel/cpu/microcode/internal.h
>> +++ b/arch/x86/kernel/cpu/microcode/internal.h
>> @@ -100,6 +100,11 @@ static inline unsigned int x86_cpuid_family(void)
>>       return x86_family(eax);
>>   }
>>   +static inline bool x86_cpuid_has_hypervisor(void)
>> +{
>> +    return native_cpuid_ecx(1) & BIT(31);
>> +}
>> +
>>   extern bool force_minrev;
>>     #ifdef CONFIG_CPU_SUP_AMD
> 
> 

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

[Chao Gao]

On Tue, Apr 28, 2026 at 10:47:45AM +0800, 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>
>---
> 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:

This patch looks good to me. But the rule for which MSRs should be emulated
by KVM for TDX is not very clear to me.

Maybe we can document the rule here, if there is one. That would make it
much easier to tell whether future issues like this are guest regressions
or missing KVM handling.