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From: David Laight <David.Laight@ACULAB.COM>
To: 'Mark Rutland' <mark.rutland@arm.com>, Ard Biesheuvel <ardb@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Frederic Weisbecker <frederic@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	James Morse <james.morse@arm.com>,
	Quentin Perret <qperret@google.com>,
	Christophe Leroy <christophe.leroy@csgroup.eu>
Subject: RE: [PATCH 2/4] arm64: implement support for static call trampolines
Date: Sat, 25 Sep 2021 17:46:23 +0000	[thread overview]
Message-ID: <944ef479f1104c4a97d0e3f629a9b765@AcuMS.aculab.com> (raw)
In-Reply-To: <20210921162804.GD35846@C02TD0UTHF1T.local>

From: Mark Rutland
> Sent: 21 September 2021 17:28
> 
> On Tue, Sep 21, 2021 at 05:55:11PM +0200, Ard Biesheuvel wrote:
> > On Tue, 21 Sept 2021 at 17:33, Mark Rutland <mark.rutland@arm.com> wrote:
> > >
> > > On Tue, Sep 21, 2021 at 04:44:56PM +0200, Ard Biesheuvel wrote:
> > > > On Tue, 21 Sept 2021 at 09:10, Peter Zijlstra <peterz@infradead.org> wrote:
> > ...
...
> > >
> > > I think so, yes. We can do sligntly better with an inline literal pool
> > > and a PC-relative LDR to fold the ADRP+LDR, e.g.
> > >
> > >         .align 3
> > > tramp:
> > >         BTI     C
> > >         {B <func> | RET | NOP}
> > >         LDR     X16, 1f
> > >         BR      X16
> > > 1:      .quad   <literal>
> > >
> > > Since that's in the .text, it's RO for regular accesses anyway.
> > >
> >
> > I tried to keep the literal in .rodata to avoid inadvertent gadgets
> > and/or anticipate exec-only mappings of .text, but that may be a bit
> > overzealous.
> 
> I think that in practice the risk of gadgetisation is minimal, and
> having it inline means we only need to record a single address per
> trampoline, so there's less risk that we get the patching wrong.

But doesn't that mean that it is almost certainly a data cache miss?
You really want an instruction that reads the constant from the I-cache.
Or at least be able to 'bunch together' the constants so they
stand a chance of sharing a D-cache line.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)


  reply	other threads:[~2021-09-25 17:46 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-20 23:32 [PATCH 0/4] arm64: Support dynamic preemption Frederic Weisbecker
2021-09-20 23:32 ` [PATCH 1/4] sched/preempt: Prepare for supporting !CONFIG_GENERIC_ENTRY " Frederic Weisbecker
2021-09-21  7:10   ` Peter Zijlstra
2021-09-21 13:50     ` Mark Rutland
2021-09-20 23:32 ` [PATCH 2/4] arm64: implement support for static call trampolines Frederic Weisbecker
2021-09-21  7:09   ` Peter Zijlstra
2021-09-21 14:44     ` Ard Biesheuvel
2021-09-21 15:08       ` Peter Zijlstra
2021-09-21 15:33       ` Mark Rutland
2021-09-21 15:55         ` Ard Biesheuvel
2021-09-21 16:28           ` Mark Rutland
2021-09-25 17:46             ` David Laight [this message]
2021-09-27  8:58               ` Mark Rutland
2021-09-21 16:10   ` Ard Biesheuvel
2021-09-20 23:32 ` [PATCH 3/4] arm64: Implement IRQ exit preemption static call for dynamic preemption Frederic Weisbecker
2021-09-20 23:32 ` [PATCH 4/4] arm64: Implement HAVE_PREEMPT_DYNAMIC Frederic Weisbecker
2021-10-25 12:20 [PATCH 0/4] arm64: Support dynamic preemption v2 Frederic Weisbecker
2021-10-25 12:21 ` [PATCH 2/4] arm64: implement support for static call trampolines Frederic Weisbecker
2021-10-25 13:56   ` Peter Zijlstra
2021-10-25 14:08     ` Ard Biesheuvel
2021-10-25 14:19       ` Peter Zijlstra
2021-10-25 14:44         ` Peter Zijlstra
2021-10-25 14:55           ` Ard Biesheuvel
2021-10-25 15:03             ` Peter Zijlstra
2021-10-25 15:10               ` Ard Biesheuvel
2021-10-26 10:36                 ` Mark Rutland
2021-10-26 10:45                   ` Peter Zijlstra
2021-10-26 11:06                   ` David Laight
2021-10-27 12:47                     ` Mark Rutland
2021-10-25 15:03             ` David Laight
2021-10-25 14:25   ` David Laight
2021-10-25 14:31     ` Ard Biesheuvel
2021-10-25 14:38       ` David Laight

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