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

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Message-ID: <5cf9358a-a5c3-4d4b-b82f-16d69fa30f3e@amd.com>
Date: Fri, 6 Feb 2026 18:08:38 +0530
From: "Nikunj A. Dadhania" <nikunj@....com>
To: Tom Lendacky <thomas.lendacky@....com>, Dave Hansen
	<dave.hansen@...el.com>, <linux-kernel@...r.kernel.org>,
	<kvm@...r.kernel.org>, <bp@...en8.de>
CC: <tglx@...nel.org>, <mingo@...hat.com>, <dave.hansen@...ux.intel.com>,
	<hpa@...or.com>, <xin@...or.com>, <seanjc@...gle.com>, <pbonzini@...hat.com>,
	<x86@...nel.org>, <jon.grimm@....com>, <stable@...r.kernel.org>
Subject: Re: [PATCH] x86/fred: Fix early boot failures on SEV-ES/SNP guests



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