linux-kernel - Re: [PATCH v2] efi: Allow to enable EFI runtime services by default on RT

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Date:   Fri, 1 Apr 2022 10:32:56 +0200
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     "Ahmed S. Darwish" <a.darwish@...utronix.de>,
        Ard Biesheuvel <ardb@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-efi <linux-efi@...r.kernel.org>,
        Brian Masney <bmasney@...hat.com>, Al Stone <ahs3@...hat.com>,
        Peter Robinson <pbrobinson@...il.com>,
        Robbie Harwood <rharwood@...hat.com>,
        Peter Jones <pjones@...hat.com>,
        Alexander Larsson <alexl@...hat.com>,
        Andrew Halaney <ahalaney@...hat.com>,
        linux-rt-users@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2] efi: Allow to enable EFI runtime services by default
 on RT

Hello Sebastian,

On 4/1/22 09:42, Sebastian Andrzej Siewior wrote:
> On 2022-04-01 00:19:57 [+0200], Javier Martinez Canillas wrote:
>>> In case of (CONFIG_PREEMPT_RT=y && CONFIG_EFI_DISABLE_RUNTIME=n),
>>> shouldn't we add a small message in the kernel log warning that EFI
>>> runtime services are enabled for the RT kernel?
>>>
>>> In almost all HW, except custom ones with "verified" firmware, such a
>>> warning would be useful... This is especially true since in the embedded
>>
>> I considered that as well but was not sure about what that message should be.
> 
> This makes sense and we had this in the past but dropped it for some
> reason.
> 

Ok, something like the following maybe? If you agree, I'll squash in v3:

diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
index ff57db8f8d05..08d329a5179b 100644
--- a/drivers/firmware/efi/efi.c
+++ b/drivers/firmware/efi/efi.c
@@ -362,6 +362,8 @@ static int __init efisubsys_init(void)
 
        if (!efi_enabled(EFI_RUNTIME_SERVICES))
                efi.runtime_supported_mask = 0;
+       else if (IS_ENABLED(CONFIG_PREEMPT_RT))
+               pr_warn("EFI runtime services can lead to high latencies on Real-Time kernels\n");
 
        if (!efi_enabled(EFI_BOOT))
                return 0;

>> Since it will be printed even on systems whose EFI firmwares do not
>> have such long call times as the ones described in the commit that
>> disabled the runtime services for RT.
>>
>> And in that case the warning may be misleading and make users believe
>> that a problem exists, which might not be accurate.
> 
> Does this matter? The efi-rtc driver is known to cause latencies but it
> does not happen if the driver is not used. The same is probably true for
> efi-vars: It won't cause high latencies on _read_ but then a certain
> number of bit flips during read _may_ lead to write+erase which will
> cause higher latencies.
> Having a warning at boot (similar to trace_printk's warning) with the
> options listed that are known to case high latencies might be a help.
> There are some options that nobody will argue about like LOCKDEP. Then
> there are other like WATCHDOG or this one, where a debate might start ;)
>

Yes, you are correct.
 
>> Best regards,
>> Javier
> 
> Sebastian
> 

-- 
Best regards,

Javier Martinez Canillas
Linux Engineering
Red Hat