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Date: Fri, 3 Apr 2026 11:52:09 -0600
From: Tycho Andersen <tycho@kernel.org>
To: Borislav Petkov <bp@alien8.de>
Cc: Tom Lendacky <thomas.lendacky@amd.com>,
	Thomas Gleixner <tglx@kernel.org>, Ingo Molnar <mingo@redhat.com>,
	Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
	"H. Peter Anvin" <hpa@zytor.com>,
	Ashish Kalra <ashish.kalra@amd.com>,
	John Allen <john.allen@amd.com>,
	Herbert Xu <herbert@gondor.apana.org.au>,
	"David S. Miller" <davem@davemloft.net>,
	Ard Biesheuvel <ardb@kernel.org>,
	Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>,
	Kishon Vijay Abraham I <kvijayab@amd.com>,
	Alexey Kardashevskiy <aik@amd.com>,
	Nikunj A Dadhania <nikunj@amd.com>,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>,
	Kim Phillips <kim.phillips@amd.com>,
	Sean Christopherson <seanjc@google.com>,
	linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org
Subject: Re: [PATCH v1 1/2] x86/sev: Do not initialize SNP if missing CPUs
Message-ID: <ac_-Sfa6qAhdG8rR@tycho.pizza>
References: <20260401143552.3038979-1-tycho@kernel.org>
 <70635612-76e5-488a-bb82-e66752dc9857@amd.com>
 <20260403171833.GXac_2aVdvz9gTb_DL@fat_crate.local>
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In-Reply-To: <20260403171833.GXac_2aVdvz9gTb_DL@fat_crate.local>

On Fri, Apr 03, 2026 at 07:18:33PM +0200, Borislav Petkov wrote:
> On Fri, Apr 03, 2026 at 08:31:24AM -0500, Tom Lendacky wrote:
> > > +	if (!cpumask_equal(cpu_online_mask, cpu_possible_mask))
> > 
> > If CONFIG_INIT_ALL_POSSIBLE is set, won't that set cpu_possible_mask to
> > include all CPUs up to NR_CPUS? That would result in this always failing.

Thanks, I didn't know about this config.

> Yah,
> 
> sashiko gave another possible situation where this can fail:
> 
> https://sashiko.dev/#/patchset/20260401143552.3038979-1-tycho%40kernel.org

> disabled ACPI MADT entries for hotplug

I suppose depends on how the firmware reasons about such things too.
But it seems like the suggestion of cpus_present_mask is right.

> > Not sure if this change is worth it.
> 
> Well, it would save us a lot of effort if we can check at module load time
> whether some CPUs are offlined. A very simple use case:
> 
> echo 0 > /sys/devices/system/cpu.../online
> modprobe ccp

This is how I was testing this, but I'll see about testing the ACPI
thing too.

Tycho