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
Date: Sat, 13 Apr 2024 19:27:36 +1000 [thread overview]
Message-ID: <87edb9d33r.fsf@mail.lhotse> (raw)
In-Reply-To: <20240413115324.53303a68@canb.auug.org.au>
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=n, as the help text quite clearly
>> states that disabling SPECULATION_MITIGATIONS is supposed to turn off all
>> mitigations by default.
>>
>> │ If you say N, all mitigations will be disabled. You really
>> │ should know what you are doing to say so.
>>
>> As is, the kernel still defaults to CPU_MITIGATIONS_AUTO, which results in
>> some mitigations being enabled in spite of SPECULATION_MITIGATIONS=n.
>>
>> 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(-)
>>
>> 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 {
>> };
>>
>> static enum cpu_mitigations cpu_mitigations __ro_after_init =
>> - CPU_MITIGATIONS_AUTO;
>> + IS_ENABLED(CONFIG_SPECULATION_MITIGATIONS) ? CPU_MITIGATIONS_AUTO :
>> + CPU_MITIGATIONS_OFF;
>>
>> static int __init mitigations_parse_cmdline(char *arg)
>> {
>> --
>> 2.44.0.478.gd926399ef9-goog
>>
>
> 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
next prev parent reply other threads:[~2024-04-13 9:27 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-09 17:51 [PATCH 0/3] x86/cpu: Fix SPECULATIVE_MITIGATION=n kernels Sean Christopherson
2024-04-09 17:51 ` [PATCH 1/3] x86/cpu: Actually turn off mitigations by default for SPECULATION_MITIGATIONS=n Sean Christopherson
2024-04-13 1:53 ` Stephen Rothwell
2024-04-13 9:27 ` Michael Ellerman [this message]
2024-04-13 9:38 ` Michael Ellerman
2024-04-14 22:42 ` Stephen Rothwell
2024-04-15 11:16 ` Geert Uytterhoeven
2024-04-15 14:31 ` Sean Christopherson
2024-04-16 11:06 ` Michael Ellerman
2024-04-19 14:03 ` Will Deacon
2024-04-19 14:06 ` Sean Christopherson
2024-04-19 14:38 ` Will Deacon
2024-04-15 11:13 ` Geert Uytterhoeven
2024-04-09 17:51 ` [PATCH 2/3] x86/cpu: Disable BHI mitigation by default when SPECULATION_MITIGATIONS=n Sean Christopherson
2024-04-09 17:51 ` [PATCH 3/3] x86/cpu: Ignore "mitigations" kernel parameter if SPECULATION_MITIGATIONS=n Sean Christopherson
2024-04-10 14:18 ` Ingo Molnar
2024-04-09 18:55 ` [PATCH 0/3] x86/cpu: Fix SPECULATIVE_MITIGATION=n kernels Daniel Sneddon
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