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

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Message-ID: <103bc1df-4caf-430a-9c8b-fcee78b3dd1d@amd.com>
Date: Thu, 5 Feb 2026 11:39:09 -0600
From: Tom Lendacky <thomas.lendacky@....com>
To: Dave Hansen <dave.hansen@...el.com>, Nikunj A Dadhania <nikunj@....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 2/5/26 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.
> 
> The other thing would be to make pinning actually per-cpu:
> 'cr4_pinned_bits' could be per-cpu and we'd just keep it empty until the
> CPU is actually booted and everything is fully set up.
> 
> Either way, this is looking like it'll be a bit more than one patch to
> do properly.