Re: [PATCH 1/2] x86/microcode: Do not access MSR_IA32_PLATFORM_ID when running as a guest

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

On 5/14/2026 4:06 AM, Borislav Petkov wrote:
> Patch in Fixes: causes the usual:
> 
>    unchecked MSR access error: RDMSR from 0x17 at ... (intel_get_platform_id)
>    Call Trace:
>     early_init_intel
>     early_cpu_init
>     setup_arch
>     _printk
>     start_kernel
>     x86_64_start_reservations
>     x86_64_start_kernel
>     common_startup_64
> 
> because the kernel is booted in a guest.
> 
> In order to avoid it, this MSR access needs to be prevented when running
> virtualized. That is usually done by checking X86_FEATURE_HYPERVISOR but
> for this particular case it is too early yet.
> 
> The platform ID needs to be read as early as when microcode is loaded on
> the BSP:
> 
>    load_ucode_bsp ... -> get_microcode_blob ... -> intel_find_matching_signature
> 
> and by that time, CPUID leafs haven't been parsed yet.
> 
> The microcode loader already has logic to check early whether the kernel
> is running virtualized so make that globally available to arch/x86/. The
> query whether running virtualized is getting more and more prominent in
> recent times so might as well make it an arch-global var which the rest
> of the code can use.
> 
> Fixes: d8630b67ca1ed ("x86/cpu: Add platform ID to CPU info structure")
> Reported-by: Vishal Verma <vishal.l.verma@intel.com>
> Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
> Tested-by: Binbin Wu <binbin.wu@linux.intel.com>
> Link: https://lore.kernel.org/r/20260430020953.1405535-1-binbin.wu@linux.intel.com

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

> @@ -179,6 +172,11 @@ void __init load_ucode_bsp(void)
>   
>   	early_parse_cmdline();
>   
> +	if (!cpuid_feature())
> +		dis_ucode_ldr = true;
> +	else
> +		x86_hypervisor_present = native_cpuid_ecx(1) & BIT(31);
> +

IIUC, the reason why move the initialization of x86_hypervisor_present 
from microcode_loader_disabled to here is to make sure 
x86_hypervisor_present is always initialized, right?

But initializing x86_hypervisor_present by load_ucode_bsp() depends on 
CONFIG_MICROCODE, which depends on CPU_SUP_AMD || CPU_SUP_INTEL. So when 
both CPU_SUP_AMD and CPU_SUP_INTEL are not enabled, 
x86_hypervisor_present will always be 0. It's not a problem for now, we 
can solve it when x86_hypervisor_present is going to be used by CENTAUR 
or ZHAOXIN.