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From: Michael Ellerman <mpe@ellerman.id.au>
To: Stephen Rothwell <sfr@canb.auug.org.au>, Sean Christopherson
 <seanjc@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>, Thomas Gleixner <tglx@linutronix.de>,
 Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>, Dave
 Hansen <dave.hansen@linux.intel.com>, x86@kernel.org, Peter Zijlstra
 <peterz@infradead.org>, Josh Poimboeuf <jpoimboe@kernel.org>,
 linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, Pawan Gupta
 <pawan.kumar.gupta@linux.intel.com>, Daniel Sneddon
 <daniel.sneddon@linux.intel.com>, linuxppc-dev@lists.ozlabs.org,
 linux-arch@vger.kernel.org, Catalin Marinas <catalin.marinas@arm.com>,
 Will Deacon <will@kernel.org>, Heiko Carstens <hca@linux.ibm.com>
Subject: Re: [PATCH 1/3] x86/cpu: Actually turn off mitigations by default
 for SPECULATION_MITIGATIONS=n
In-Reply-To: <20240413115324.53303a68@canb.auug.org.au>
References: <20240409175108.1512861-1-seanjc@google.com>
 <20240409175108.1512861-2-seanjc@google.com>
 <20240413115324.53303a68@canb.auug.org.au>
Date: Sat, 13 Apr 2024 19:27:36 +1000
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Stephen Rothwell <sfr@canb.auug.org.au> writes:
> Hi Sean,
>
> I noticed this commit in linux-next.
>
> On Tue,  9 Apr 2024 10:51:05 -0700 Sean Christopherson <seanjc@google.com=
> wrote:
>>
>> Initialize cpu_mitigations to CPU_MITIGATIONS_OFF if the kernel is built
>> with CONFIG_SPECULATION_MITIGATIONS=3Dn, as the help text quite clearly
>> states that disabling SPECULATION_MITIGATIONS is supposed to turn off all
>> mitigations by default.
>>=20
>>   =E2=94=82 If you say N, all mitigations will be disabled. You really
>>   =E2=94=82 should know what you are doing to say so.
>>=20
>> As is, the kernel still defaults to CPU_MITIGATIONS_AUTO, which results =
in
>> some mitigations being enabled in spite of SPECULATION_MITIGATIONS=3Dn.
>>=20
>> Fixes: f43b9876e857 ("x86/retbleed: Add fine grained Kconfig knobs")
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Sean Christopherson <seanjc@google.com>
>> ---
>>  kernel/cpu.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>=20
>> diff --git a/kernel/cpu.c b/kernel/cpu.c
>> index 8f6affd051f7..07ad53b7f119 100644
>> --- a/kernel/cpu.c
>> +++ b/kernel/cpu.c
>> @@ -3207,7 +3207,8 @@ enum cpu_mitigations {
>>  };
>>=20=20
>>  static enum cpu_mitigations cpu_mitigations __ro_after_init =3D
>> -	CPU_MITIGATIONS_AUTO;
>> +	IS_ENABLED(CONFIG_SPECULATION_MITIGATIONS) ? CPU_MITIGATIONS_AUTO :
>> +						     CPU_MITIGATIONS_OFF;
>>=20=20
>>  static int __init mitigations_parse_cmdline(char *arg)
>>  {
>> --=20
>> 2.44.0.478.gd926399ef9-goog
>>=20
>
> I noticed because it turned off all mitigations for my PowerPC qemu
> boot tests - probably because CONFIG_SPECULATION_MITIGATIONS only
> exists in arch/x86/Kconfig ... thus for other architectures that have
> cpu mitigations, this will always default them to off, right?

Yep.

The patch has the effect of changing the default for non-x86 arches from
auto to off.

I see at least powerpc, arm64 and s390 use cpu_mitigations_off() and
will be affected.

cheers