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    Subject[tip:core/speculation] cpu/speculation: Add 'mitigations=' cmdline option
    Commit-ID:  98af8452945c55652de68536afdde3b520fec429
    Gitweb: https://git.kernel.org/tip/98af8452945c55652de68536afdde3b520fec429
    Author: Josh Poimboeuf <jpoimboe@redhat.com>
    AuthorDate: Fri, 12 Apr 2019 15:39:28 -0500
    Committer: Thomas Gleixner <tglx@linutronix.de>
    CommitDate: Wed, 17 Apr 2019 21:37:28 +0200

    cpu/speculation: Add 'mitigations=' cmdline option

    Keeping track of the number of mitigations for all the CPU speculation
    bugs has become overwhelming for many users. It's getting more and more
    complicated to decide which mitigations are needed for a given
    architecture. Complicating matters is the fact that each arch tends to
    have its own custom way to mitigate the same vulnerability.

    Most users fall into a few basic categories:

    a) they want all mitigations off;

    b) they want all reasonable mitigations on, with SMT enabled even if
    it's vulnerable; or

    c) they want all reasonable mitigations on, with SMT disabled if
    vulnerable.

    Define a set of curated, arch-independent options, each of which is an
    aggregation of existing options:

    - mitigations=off: Disable all mitigations.

    - mitigations=auto: [default] Enable all the default mitigations, but
    leave SMT enabled, even if it's vulnerable.

    - mitigations=auto,nosmt: Enable all the default mitigations, disabling
    SMT if needed by a mitigation.

    Currently, these options are placeholders which don't actually do
    anything. They will be fleshed out in upcoming patches.

    Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Tested-by: Jiri Kosina <jkosina@suse.cz> (on x86)
    Reviewed-by: Jiri Kosina <jkosina@suse.cz>
    Cc: Borislav Petkov <bp@alien8.de>
    Cc: "H . Peter Anvin" <hpa@zytor.com>
    Cc: Andy Lutomirski <luto@kernel.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Jiri Kosina <jikos@kernel.org>
    Cc: Waiman Long <longman@redhat.com>
    Cc: Andrea Arcangeli <aarcange@redhat.com>
    Cc: Jon Masters <jcm@redhat.com>
    Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
    Cc: Paul Mackerras <paulus@samba.org>
    Cc: Michael Ellerman <mpe@ellerman.id.au>
    Cc: linuxppc-dev@lists.ozlabs.org
    Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
    Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
    Cc: linux-s390@vger.kernel.org
    Cc: Catalin Marinas <catalin.marinas@arm.com>
    Cc: Will Deacon <will.deacon@arm.com>
    Cc: linux-arm-kernel@lists.infradead.org
    Cc: linux-arch@vger.kernel.org
    Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    Cc: Tyler Hicks <tyhicks@canonical.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Randy Dunlap <rdunlap@infradead.org>
    Cc: Steven Price <steven.price@arm.com>
    Cc: Phil Auld <pauld@redhat.com>
    Link: https://lkml.kernel.org/r/b07a8ef9b7c5055c3a4637c87d07c296d5016fe0.1555085500.git.jpoimboe@redhat.com

    ---
    Documentation/admin-guide/kernel-parameters.txt | 24 ++++++++++++++++++++++++
    include/linux/cpu.h | 24 ++++++++++++++++++++++++
    kernel/cpu.c | 15 +++++++++++++++
    3 files changed, 63 insertions(+)

    diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
    index 858b6c0b9a15..720ffa9c4e04 100644
    --- a/Documentation/admin-guide/kernel-parameters.txt
    +++ b/Documentation/admin-guide/kernel-parameters.txt
    @@ -2513,6 +2513,30 @@
    in the "bleeding edge" mini2440 support kernel at
    http://repo.or.cz/w/linux-2.6/mini2440.git

    + mitigations=
    + Control optional mitigations for CPU vulnerabilities.
    + This is a set of curated, arch-independent options, each
    + of which is an aggregation of existing arch-specific
    + options.
    +
    + off
    + Disable all optional CPU mitigations. This
    + improves system performance, but it may also
    + expose users to several CPU vulnerabilities.
    +
    + auto (default)
    + Mitigate all CPU vulnerabilities, but leave SMT
    + enabled, even if it's vulnerable. This is for
    + users who don't want to be surprised by SMT
    + getting disabled across kernel upgrades, or who
    + have other ways of avoiding SMT-based attacks.
    + This is the default behavior.
    +
    + auto,nosmt
    + Mitigate all CPU vulnerabilities, disabling SMT
    + if needed. This is for users who always want to
    + be fully mitigated, even if it means losing SMT.
    +
    mminit_loglevel=
    [KNL] When CONFIG_DEBUG_MEMORY_INIT is set, this
    parameter allows control of the logging verbosity for
    diff --git a/include/linux/cpu.h b/include/linux/cpu.h
    index 5041357d0297..2d9c6f4b78f5 100644
    --- a/include/linux/cpu.h
    +++ b/include/linux/cpu.h
    @@ -187,4 +187,28 @@ static inline void cpu_smt_disable(bool force) { }
    static inline void cpu_smt_check_topology(void) { }
    #endif

    +/*
    + * These are used for a global "mitigations=" cmdline option for toggling
    + * optional CPU mitigations.
    + */
    +enum cpu_mitigations {
    + CPU_MITIGATIONS_OFF,
    + CPU_MITIGATIONS_AUTO,
    + CPU_MITIGATIONS_AUTO_NOSMT,
    +};
    +
    +extern enum cpu_mitigations cpu_mitigations;
    +
    +/* mitigations=off */
    +static inline bool cpu_mitigations_off(void)
    +{
    + return cpu_mitigations == CPU_MITIGATIONS_OFF;
    +}
    +
    +/* mitigations=auto,nosmt */
    +static inline bool cpu_mitigations_auto_nosmt(void)
    +{
    + return cpu_mitigations == CPU_MITIGATIONS_AUTO_NOSMT;
    +}
    +
    #endif /* _LINUX_CPU_H_ */
    diff --git a/kernel/cpu.c b/kernel/cpu.c
    index d1c6d152da89..e70a90634b41 100644
    --- a/kernel/cpu.c
    +++ b/kernel/cpu.c
    @@ -2279,3 +2279,18 @@ void __init boot_cpu_hotplug_init(void)
    #endif
    this_cpu_write(cpuhp_state.state, CPUHP_ONLINE);
    }
    +
    +enum cpu_mitigations cpu_mitigations __ro_after_init = CPU_MITIGATIONS_AUTO;
    +
    +static int __init mitigations_parse_cmdline(char *arg)
    +{
    + if (!strcmp(arg, "off"))
    + cpu_mitigations = CPU_MITIGATIONS_OFF;
    + else if (!strcmp(arg, "auto"))
    + cpu_mitigations = CPU_MITIGATIONS_AUTO;
    + else if (!strcmp(arg, "auto,nosmt"))
    + cpu_mitigations = CPU_MITIGATIONS_AUTO_NOSMT;
    +
    + return 0;
    +}
    +early_param("mitigations", mitigations_parse_cmdline);
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