From: Ard Biesheuvel <ard.biesheuvel@linaro.org>
To: Ingo Molnar <mingo@kernel.org>
Cc: Matt Fleming <matt@codeblueprint.co.uk>,
"H . Peter Anvin" <hpa@zytor.com>,
Thomas Gleixner <tglx@linutronix.de>,
"linux-efi@vger.kernel.org" <linux-efi@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Andy Lutomirski <luto@kernel.org>,
Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [PATCH 07/14] efi: runtime-wrappers: Run UEFI Runtime Services with interrupts enabled
Date: Wed, 3 Feb 2016 10:57:24 +0100 [thread overview]
Message-ID: <CAKv+Gu-AWTQNG3ObSZt0cHV1YFb8UbYpkq-uVBqMW0wryfv_ig@mail.gmail.com> (raw)
In-Reply-To: <20160203094340.GA15890@gmail.com>
On 3 February 2016 at 10:43, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Matt Fleming <matt@codeblueprint.co.uk> wrote:
>
>> From: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>>
>> The UEFI spec allows Runtime Services to be invoked with interrupts
>> enabled. The only reason we were disabling interrupts was to prevent
>> recursive calls into the services on the same CPU, which will lead to
>> deadlock. However, the only context where such invocations may occur
>> legally is from efi-pstore via efivars, and that code has been updated
>> to call a non-blocking alternative when invoked from a non-interruptible
>> context.
>>
>> So instead, update the ordinary, blocking UEFI Runtime Services wrappers
>> to execute with interrupts enabled. This aims to prevent excessive interrupt
>> latencies on uniprocessor platforms with slow variable stores.
>
> Well, those excessive latencies would affect SMP platforms as well, just that
> there are (usually) other CPUs free to do execution, right?
>
Correct.
> More fundamentally, this makes me nervous:
>
> > The UEFI spec allows Runtime Services to be invoked with interrupts enabled.
> > [...]
>
> So what really matters is not what the spec says, but how Windows executes UEFI
> firmware code in practice.
>
> If major versions of Windows calls UEFI firmware with interrupts disabled, then
> frankly I don't think we should interrupt them under Linux either, regardless of
> what the spec says ...
>
> Random firmware code getting interrupted by the OS changes timings and might have
> other side effects the firmware code might not expect - so the question is, does
> Windows already de facto allow the IRQ preemption of firmware calls?
>
Good question. I will try to find out.
> Also, this:
>
>> - unsigned long flags;
>> efi_status_t status;
>>
>> - spin_lock_irqsave(&efi_runtime_lock, flags);
>> + BUG_ON(in_irq());
>> +
>> + spin_lock(&efi_runtime_lock);
>
> ... how does crashing the kernel help debuggability?
>
> Please use WARN_ON_ONCE() - or in fact, this assert is probably not needed at all,
> as lockdep will warn about IRQ unsafe lock usage.
>
Actually, reading back the original thread, Matt had already
identified this problem, and v2/v3 of this patch removed all of them
but one, so thanks for spotting that.
> I'd add comments to the efi_runtime_lock definition site explaining that this is
> never taken from IRQ contexts.
>
OK.
next prev parent reply other threads:[~2016-02-03 9:57 UTC|newest]
Thread overview: 54+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-02-01 22:06 [GIT PULL 00/14] EFI changes for v4.6 Matt Fleming
2016-02-01 22:06 ` [PATCH 01/14] efi: Expose non-blocking set_variable() wrapper to efivars Matt Fleming
2016-02-03 11:31 ` [tip:efi/core] " tip-bot for Ard Biesheuvel
2016-02-01 22:06 ` [PATCH 02/14] efi: Remove redundant efi_set_variable_nonblocking prototype Matt Fleming
2016-02-03 11:31 ` [tip:efi/core] efi: Remove redundant efi_set_variable_nonblocking () prototype tip-bot for Ard Biesheuvel
2016-02-01 22:06 ` [PATCH 03/14] efi: runtime-wrappers: Add a nonblocking version of QueryVariableInfo Matt Fleming
2016-02-03 11:31 ` [tip:efi/core] efi/runtime-wrappers: Add a nonblocking version of QueryVariableInfo() tip-bot for Ard Biesheuvel
2016-02-01 22:06 ` [PATCH 04/14] efi: Add nonblocking option to efi_query_variable_store() Matt Fleming
2016-02-03 11:32 ` [tip:efi/core] " tip-bot for Ard Biesheuvel
2016-02-01 22:06 ` [PATCH 05/14] efi: runtime-wrappers: Remove out of date comment regarding in_nmi() Matt Fleming
2016-02-03 11:32 ` [tip:efi/core] efi/runtime-wrappers: " tip-bot for Ard Biesheuvel
2016-02-01 22:07 ` [PATCH 06/14] efi: runtime-wrapper: Get rid of the rtc_lock spinlock Matt Fleming
2016-02-03 11:32 ` [tip:efi/core] efi: Runtime-wrapper: " tip-bot for Ard Biesheuvel
2016-02-01 22:07 ` [PATCH 07/14] efi: runtime-wrappers: Run UEFI Runtime Services with interrupts enabled Matt Fleming
2016-02-03 9:43 ` Ingo Molnar
2016-02-03 9:57 ` Ard Biesheuvel [this message]
2016-02-03 10:58 ` Ingo Molnar
2016-02-03 11:33 ` Ard Biesheuvel
2016-02-03 12:01 ` Matt Fleming
2016-02-04 13:58 ` [PATCH] efi: runtime-wrappers: run " Ard Biesheuvel
2016-02-08 15:16 ` Matt Fleming
2016-02-08 19:37 ` Andy Lutomirski
2016-02-09 16:52 ` Ard Biesheuvel
2016-02-11 16:03 ` Matt Fleming
2016-02-11 16:04 ` Matt Fleming
2016-02-01 22:07 ` [PATCH 08/14] efivars: Use to_efivar_entry Matt Fleming
2016-02-03 11:33 ` [tip:efi/core] " tip-bot for Geliang Tang
2016-02-01 22:07 ` [PATCH 09/14] x86/efi-bgrt: Don't ignore the BGRT if the 'valid' bit is 0 Matt Fleming
2016-02-03 11:33 ` [tip:efi/core] x86/efi/bgrt: " tip-bot for Môshe van der Sterre
2016-02-01 22:07 ` [PATCH 10/14] efi: Make checkpatch complain less about efi.h GUID additions Matt Fleming
2016-02-03 10:33 ` Ingo Molnar
2016-02-03 10:44 ` Matt Fleming
2016-02-03 10:50 ` Ingo Molnar
2016-02-03 11:18 ` Matt Fleming
2016-02-03 11:27 ` Ingo Molnar
2016-02-03 11:09 ` Joe Perches
2016-02-01 22:07 ` [PATCH 11/14] x86/efi: Show actual ending addresses in efi_print_memmap Matt Fleming
2016-02-02 8:49 ` Laszlo Ersek
2016-02-03 11:33 ` [tip:efi/core] " tip-bot for Robert Elliott
2016-02-01 22:07 ` [PATCH 12/14] efi: Add NV memory attribute Matt Fleming
2016-02-02 8:54 ` Laszlo Ersek
2016-02-03 11:34 ` [tip:efi/core] " tip-bot for Robert Elliott
2016-02-01 22:07 ` [PATCH 13/14] efi: Add Persistent Memory type name Matt Fleming
2016-02-02 8:56 ` Laszlo Ersek
2016-02-03 11:34 ` [tip:efi/core] " tip-bot for Robert Elliott
2016-02-01 22:07 ` [PATCH 14/14] x86/efi: Print size in binary units in efi_print_memmap Matt Fleming
2016-02-02 9:22 ` Laszlo Ersek
2016-02-03 10:40 ` Ingo Molnar
2016-02-03 11:28 ` Matt Fleming
2016-02-03 12:36 ` Andy Shevchenko
2016-02-03 15:25 ` Elliott, Robert (Persistent Memory)
2016-02-09 12:20 ` Ingo Molnar
2016-02-09 12:53 ` Laszlo Ersek
2016-02-09 13:14 ` Ingo Molnar
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