linux-kernel - Re: [PATCH v3 7/7] firmware: coreboot: Request table region for exclusive access

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Message-ID: <153386966793.37448.12640092735399834661@swboyd.mtv.corp.google.com>
Date:   Thu, 09 Aug 2018 19:54:27 -0700
From:   Stephen Boyd <swboyd@...omium.org>
To:     Julius Werner <jwerner@...omium.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Wei-Ning Huang <wnhuang@...omium.org>,
        Brian Norris <briannorris@...omium.org>, samuel@...lland.org
Subject: Re: [PATCH v3 7/7] firmware: coreboot: Request table region for exclusive
 access

What's with the top posting? ;-)

Quoting Julius Werner (2018-08-09 16:44:43)
> Actually, looking at what IO_STRICT_DEVMEM really does, would it
> really prevent userspace accesses to these areas? Because it seems
> that it only prevents accesses to areas marked as IORESOURCE_BUSY, and
> while I can't fully follow how the kernel assigns that, comments
> suggest that this is only set when "Driver has marked this resource
> busy".

Requesting the memory region as is done in this patch marks it as busy.

> 
> So after you make the change to the other patch where we immediately
> unmap the coreboot table again at the end of the probe() function,
> shouldn't it become available to userspace again even with
> IO_STRICT_DEVMEM set?

Yes, if we unmap the region immediately after devices are populated then
this whole point is moot with the assumption that this code isn't
running at the same time as the cbmem utility. I've done this already
and it is working on arm. I still need to build/boot/test on an x86
platform which I should be able to do tomorrow.