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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Thu, 31 Mar 2022 18:26:37 +0200
Message-ID: <CAMj1kXHgyjB_BVzXx+CK0tBuJpZ3h=8XKus7nWiyovECjVQ0gw@mail.gmail.com>
Subject: Re: [PATCH v2] efi: Allow to enable EFI runtime services by default
 on RT
To:     Javier Martinez Canillas <javierm@redhat.com>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-efi <linux-efi@vger.kernel.org>,
        Brian Masney <bmasney@redhat.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Al Stone <ahs3@redhat.com>,
        Peter Robinson <pbrobinson@gmail.com>,
        Robbie Harwood <rharwood@redhat.com>,
        Peter Jones <pjones@redhat.com>,
        Alexander Larsson <alexl@redhat.com>,
        Andrew Halaney <ahalaney@redhat.com>,
        linux-rt-users@vger.kernel.org,
        Thomas Gleixner <tglx@linutronix.de>
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On Thu, 31 Mar 2022 at 17:17, Javier Martinez Canillas
<javierm@redhat.com> wrote:
>
> 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>
> ---
>
> Changes in v2:
> - Improve commit description to make clear the motivation for the change
>   (Sebastian Andrzej Siewior).
>

This looks ok to me. I'll queue this up once the merge window closes.

>  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
>