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    Subject[PATCH 5.18 311/879] efi: Allow to enable EFI runtime services by default on RT
    Date
    From: Javier Martinez Canillas <javierm@redhat.com>

    [ Upstream commit a031651ff2144a3d81d4916856c093bc1ea0a413 ]

    Commit d9f283ae71af ("efi: Disable runtime services on RT") disabled EFI
    runtime services by default when the CONFIG_PREEMPT_RT option is enabled.

    The rationale for that commit is that some EFI calls could take too much
    time, leading to large latencies which is an issue for Real-Time kernels.

    But a side effect of that change was that now is not possible anymore to
    enable the EFI runtime services by default when CONFIG_PREEMPT_RT is set,
    without passing an efi=runtime command line parameter to the kernel.

    Instead, let's add a new EFI_DISABLE_RUNTIME boolean Kconfig option, that
    would be set to n by default but to y if CONFIG_PREEMPT_RT is enabled.

    That way, the current behaviour is preserved but gives users a mechanism
    to enable the EFI runtimes services in their kernels if that is required.
    For example, if the firmware could guarantee bounded time for EFI calls.

    Also, having a separate boolean config could allow users to disable the
    EFI runtime services by default even when CONFIG_PREEMPT_RT is not set.

    Reported-by: Alexander Larsson <alexl@redhat.com>
    Fixes: d9f283ae71af ("efi: Disable runtime services on RT")
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
    Link: https://lore.kernel.org/r/20220331151654.184433-1-javierm@redhat.com
    Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    drivers/firmware/efi/Kconfig | 15 +++++++++++++++
    drivers/firmware/efi/efi.c | 2 +-
    2 files changed, 16 insertions(+), 1 deletion(-)

    diff --git a/drivers/firmware/efi/Kconfig b/drivers/firmware/efi/Kconfig
    index 2c3dac5ecb36..243882f5e5f9 100644
    --- a/drivers/firmware/efi/Kconfig
    +++ b/drivers/firmware/efi/Kconfig
    @@ -284,3 +284,18 @@ config EFI_CUSTOM_SSDT_OVERLAYS

    See Documentation/admin-guide/acpi/ssdt-overlays.rst for more
    information.
    +
    +config EFI_DISABLE_RUNTIME
    + bool "Disable EFI runtime services support by default"
    + default y if PREEMPT_RT
    + help
    + Allow to disable the EFI runtime services support by default. This can
    + already be achieved by using the efi=noruntime option, but it could be
    + useful to have this default without any kernel command line parameter.
    +
    + The EFI runtime services are disabled by default when PREEMPT_RT is
    + enabled, because measurements have shown that some EFI functions calls
    + might take too much time to complete, causing large latencies which is
    + an issue for Real-Time kernels.
    +
    + This default can be overridden by using the efi=runtime option.
    diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
    index 5502e176d51b..ff57db8f8d05 100644
    --- a/drivers/firmware/efi/efi.c
    +++ b/drivers/firmware/efi/efi.c
    @@ -66,7 +66,7 @@ struct mm_struct efi_mm = {

    struct workqueue_struct *efi_rts_wq;

    -static bool disable_runtime = IS_ENABLED(CONFIG_PREEMPT_RT);
    +static bool disable_runtime = IS_ENABLED(CONFIG_EFI_DISABLE_RUNTIME);
    static int __init setup_noefi(char *arg)
    {
    disable_runtime = true;
    --
    2.35.1


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