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

[Binbin Wu <binbin.wu@linux.intel.com>]

On 5/13/2026 6:14 PM, Borislav Petkov wrote:
> On Tue, May 12, 2026 at 09:57:58AM +0800, Binbin Wu wrote:
>> hypervisor_present could be uninitialized if dis_ucode_ldr is true.
>> intel_get_platform_id() is also called during the normal cpu initialization.
> 
> Right, that needs more surgery. See if the below works instead pls.
> 
> We might as well do it - the question whether we run on a HV comes very often
> recenly - might as well make it an "official" variable.

Make sense.
My version treated it as a bug fix, so I tried to minimize the code change. 

[...]

>  
> @@ -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);
> +

There is one case not covered by setting the global x86_hypervisor_present at this point.
Xen PV guest will not go this path, so Xen PV guest will not see the hypervisor bit.

But maybe we can just ignore Xen PV guest case today?


>  	if (microcode_loader_disabled())
>  		return;
>  
> diff --git a/arch/x86/kernel/cpu/microcode/intel.c b/arch/x86/kernel/cpu/microcode/intel.c
> index 37ac4afe0972..a4c0a0cf928b 100644
> --- a/arch/x86/kernel/cpu/microcode/intel.c
> +++ b/arch/x86/kernel/cpu/microcode/intel.c
> @@ -138,6 +138,9 @@ u32 intel_get_platform_id(void)
>  {
>  	unsigned int val[2];
>  
> +	if (x86_hypervisor_present)
> +		return 0;
> +
>  	/*
>  	 * This can be called early. Use CPUID directly instead of
>  	 * relying on cpuinfo_x86 which may not be fully initialized.
> diff --git a/arch/x86/kernel/cpu/microcode/internal.h b/arch/x86/kernel/cpu/microcode/internal.h
> index 3b93c0676b4f..a10b547eda1e 100644
> --- a/arch/x86/kernel/cpu/microcode/internal.h
> +++ b/arch/x86/kernel/cpu/microcode/internal.h
> @@ -48,7 +48,6 @@ extern struct early_load_data early_data;
>  extern struct ucode_cpu_info ucode_cpu_info[];
>  extern u32 microcode_rev[NR_CPUS];
>  extern u32 base_rev;
> -extern bool hypervisor_present;
>  
>  struct cpio_data find_microcode_in_initrd(const char *path);
>