linux-kernel - Re: [PATCH V2 2/3] efi: Introduce efi_rts_workqueue and some infrastructure to invoke all efi_runtime

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Date:   Fri, 9 Mar 2018 12:11:57 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     "Prakhya, Sai Praneeth" <sai.praneeth.prakhya@...el.com>
Cc:     "linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Chun-Yi Lee <jlee@...e.com>,
        "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>,
        "Neri, Ricardo" <ricardo.neri@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        "Zijlstra, Peter" <peter.zijlstra@...el.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        "Williams, Dan J" <dan.j.williams@...el.com>
Subject: Re: [PATCH V2 2/3] efi: Introduce efi_rts_workqueue and some
 infrastructure to invoke all efi_runtime_services()

On Fri, Mar 09, 2018 at 02:37:59AM +0000, Prakhya, Sai Praneeth wrote:
> But, I guess, we have some downsides with this design:
> 1. We are doing this to have "no exceptions to use efi_rts_wq", but we will be making
> the common case complicated. i.e. When a user requests to write some efi variable,

So if the pstore use case is so important and special, I think we should
make the EFI path as fast as possible as getting that data to the pstore
is a priority.

> Alternatively, instead of playing around with in_atomic(), we could have wrapper
> functions like efi_write_var_non_wq() which will only be used by pstore. This function
> will not use efi_rts_wq and directly invoke efi_runtime_service. Just an attempt to
> make the code not look messy.

I guess.

If the write-to-pstore case is such a critical one, I guess the
exception is justified.

> That's true! AFAIK, we don't have any issues handling NMI while in efi_pgd.
> We might have issues only when, we are already in efi_pgd, NMI comes along

Can you trigger this? Or is it something hypothetical?

> and NMI handler tries to touch the regions that are not mapped in efi_pgd

If it is not hypothetical, the NMI handler should learn to look at CR3
first and return if CR3 has the efi pgd.

-- 
Regards/Gruss,
    Boris.

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