linux-kernel - RE: [PATCH V4 0/3] Use efi_rts_wq to invoke EFI Runtime Services

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Date:   Fri, 25 May 2018 23:08:55 +0000
From:   "Prakhya, Sai Praneeth" <sai.praneeth.prakhya@...el.com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
CC:     "linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        Lee Chun-Yi <jlee@...e.com>, Borislav Petkov <bp@...en8.de>,
        "Luck, Tony" <tony.luck@...el.com>,
        "Will Deacon" <will.deacon@....com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Mark Rutland" <mark.rutland@....com>,
        Bhupesh Sharma <bhsharma@...hat.com>,
        "Naresh Bhat" <naresh.bhat@...aro.org>,
        "Neri, Ricardo" <ricardo.neri@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Subject: RE: [PATCH V4 0/3] Use efi_rts_wq to invoke EFI Runtime Services

> > Changes from V3 to V4:
> > ----------------------
> > 1. As suggested by Peter, use completions instead of flush_work() as the
> >   former is cheaper
> > 2. Call efi_delete_dummy_variable() from efisubsys_init(). Sorry! Ard,
> >   wasn't able to find a better alternative to keep this change local to
> >   arch/x86.
> >
> 
> Two questions:
> - Should the non-blocking variants of the query and set_variable_store use the
> work queue? Doesn't that make them blocking?

That's a good question . I think you are right, calling non-blocking variants of efi_rts 
using work queues makes them blocking. But, I have a follow on question.

Assume some user requested to execute some non-blocking variant of efi_rts and 
the kernel hasn't called efi_call_virt() yet, but was scheduled out. IOW, even though 
user requests for non-blocking efi call, we might still block. Am I right?

With efi_rts_wq, I think, I have increased the window of getting blocked. With efi_rts_wq, 
kernel should explicitly call schedule() to run firmware and the chances of getting blocked 
are much more.

Expect this increased window, I think firmware should be executed as before.

So, can you please explain me the difference between blocking and non-blocking variants
from kernel perspective?
(the way we get locks are different down_interruptible() vs down_trylock())

> - If the non-blocking set_variable() does not use the work queue, can we just call
> it from efi_delete_dummy_variable(), and keep the calls where they are?

Yes, I think we can do that (if we don't use efi_rts_wq for non-blocking variants).

Regards,
Sai