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

From: Pavel Tatashin <pasha.tatashin@soleen.com>
To: Mark Rutland <mark.rutland@arm.com>
Cc: James Morris <jmorris@namei.org>, Sasha Levin <sashal@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	steve.capper@arm.com,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Marc Zyngier <marc.zyngier@arm.com>,
	James Morse <james.morse@arm.com>,
	Vladimir Murzin <vladimir.murzin@arm.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	allison@lohutok.net, info@metux.net, alexios.zavras@intel.com,
	Stefano Stabellini <sstabellini@kernel.org>,
	boris.ostrovsky@oracle.com, jgross@suse.com,
	Stefan Agner <stefan@agner.ch>,
	Masahiro Yamada <yamada.masahiro@socionext.com>,
	xen-devel@lists.xenproject.org,
	Russell King - ARM Linux admin <linux@armlinux.org.uk>
Subject: Re: [PATCH v2 3/3] arm64: remove the rest of asm-uaccess.h
Date: Wed, 27 Nov 2019 11:09:35 -0500	[thread overview]
Message-ID: <CA+CK2bBszdMYbneQ1UiYxSndN8zmoVwbTVJ20NeajYPehT_X5Q@mail.gmail.com> (raw)
In-Reply-To: <20191127160342.GF51937@lakrids.cambridge.arm.com>

On Wed, Nov 27, 2019 at 11:03 AM Mark Rutland <mark.rutland@arm.com> wrote:
>
> 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@arm.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.

Ah, yes, this makes sense. I could certainly factor out C calls in a
better way, or is this something you want to work on?

Without removing these assembly macros I do not think we want to
address this suggestion from Kees Cook:
https://lore.kernel.org/lkml/CA+CK2bCBS2fKOTmTFm13iv3u5TBPwpoCsYeeP352DVE-gs9GJw@mail.gmail.com/

Thank you,
Pasha