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

From: Javier Martinez Canillas <javierm@redhat.com>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: "Ahmed S. Darwish" <a.darwish@linutronix.de>,
	Ard Biesheuvel <ardb@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-efi <linux-efi@vger.kernel.org>,
	Brian Masney <bmasney@redhat.com>, 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>
Subject: Re: [PATCH v2] efi: Allow to enable EFI runtime services by default on RT
Date: Fri, 1 Apr 2022 10:32:56 +0200	[thread overview]
Message-ID: <78a0360d-1a27-5280-10bf-d27d1d306fa5@redhat.com> (raw)
In-Reply-To: <Ykas9iX/D3WURx8T@linutronix.de>

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

