Re: [PATCH] x86/fred: Fix early boot failures on SEV-ES/SNP guests

["Nikunj A. Dadhania" <nikunj@amd.com>]

On 2/5/2026 11:09 PM, Tom Lendacky wrote:
> On 2/5/26 11:20, Dave Hansen wrote:
>> On 2/5/26 08:10, Dave Hansen wrote:
>>> Shouldn't we flip the FRED CR4 bit _last_, once all the MSRs are set up?
>>> Why is it backwards in the first place? Why can't it be fixed?
>>
>> Ahhh, it was done by CR4 pinning. It's the first thing in C code for
>> booting secondaries:
>>
>> static void notrace __noendbr start_secondary(void *unused)
>> {
>>         cr4_init();
>>
>> Since FRED is set in 'cr4_pinned_mask', cr4_init() sets the FRED bit far
>> before the FRED MSRs are ready. Anyone else doing native_write_cr4()
>> will do the same thing. That's obviously not what was intended from the
>> pinning code or the FRED init code.
>>
>> Shouldn't we fix this properly rather than moving printk()'s around?
> 
> I believe that is what this part of the thread decided on:
> 
> https://lore.kernel.org/kvm/02df7890-83c2-4047-8c88-46fbc6e0a892@intel.com/T/#m3e44c2c53aca3bcd872de4ce1e50a14500e62e4e
> 
> Thanks,
> Tom
> 
>>
>> One idea is just to turn off all the CR-pinning logic while bringing
>> CPUs up. That way, nothing before:
>>
>> 	set_cpu_online(smp_processor_id(), true);
>>
>> can get tripped up by CR pinning. I've attached a completely untested
>> patch to do that.

Yes, this works as well. And Xin Li's patch also resolves the issue by
moving the cr4_init() later after initializing FRED MSRs.

Regards,
Nikunj