linux-kernel - Re: [PATCH v2 3/3] arm64: remove the rest of asm-uaccess.h

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Date:   Wed, 27 Nov 2019 16:03:43 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Pavel Tatashin <pasha.tatashin@...een.com>
Cc:     James Morris <jmorris@...ei.org>, Sasha Levin <sashal@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, steve.capper@....com,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Marc Zyngier <marc.zyngier@....com>,
        James Morse <james.morse@....com>,
        Vladimir Murzin <vladimir.murzin@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        allison@...utok.net, info@...ux.net, alexios.zavras@...el.com,
        Stefano Stabellini <sstabellini@...nel.org>,
        boris.ostrovsky@...cle.com, jgross@...e.com,
        Stefan Agner <stefan@...er.ch>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        xen-devel@...ts.xenproject.org,
        Russell King - ARM Linux admin <linux@...linux.org.uk>
Subject: Re: [PATCH v2 3/3] arm64: remove the rest of asm-uaccess.h

On Wed, Nov 27, 2019 at 10:31:54AM -0500, Pavel Tatashin wrote:
> On Wed, Nov 27, 2019 at 10:12 AM Mark Rutland <mark.rutland@....com> wrote:
> >
> > On Thu, Nov 21, 2019 at 09:24:06PM -0500, Pavel Tatashin wrote:
> > > The __uaccess_ttbr0_disable and __uaccess_ttbr0_enable,
> > > are the last two macros defined in asm-uaccess.h.
> > >
> > > Replace them with C wrappers and call C functions from
> > > kernel_entry and kernel_exit.
> >
> > For now, please leave those as-is.
> >
> > I don't think we want to have out-of-line C wrappers in the middle of
> > the entry assembly where we don't have a complete kernel environment.
> > The use in entry code can also assume non-preemptibility, while the C
> > functions have to explcitily disable that.
> 
> I do not understand, if C function is called form non-preemptible
> context it stays non-preemptible. kernel_exit already may call C
> functions around the time __uaccess_ttbr0_enable is called (it may
> call post_ttbr_update_workaround), and that C functions does not do
> explicit preempt disable:

Sorry, I meant that IRQs are disabled here.

The C wrapper calls __uaccess_ttbr0_enable(), which calls
local_irq_save() and local_irq_restore(). Those are pointless in the
bowels of the entry code, and potentially expensive if IRQ prio masking
is in use.

I'd rather not add more out-of-line C code calls here right now as I'd
prefer to factor out the logic to C in a better way.

> > We can certainly remove the includes of <asm/asm-uaccess.h> elsewhere,
> > and maybe fold the macros into entry.S if it's not too crowded.
> 
> I can do this as a separate patch.

That sounds fine to me,

Thanks,
Mark.