linux-kernel - Re: [PATCH 0/5] powerpc: Implement masked user access

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Message-ID: <20250624175001.148a768f@pumpkin>
Date: Tue, 24 Jun 2025 17:50:01 +0100
From: David Laight <david.laight.linux@...il.com>
To: Segher Boessenkool <segher@...nel.crashing.org>
Cc: Christophe Leroy <christophe.leroy@...roup.eu>, Michael Ellerman
 <mpe@...erman.id.au>, Nicholas Piggin <npiggin@...il.com>, Naveen N Rao
 <naveen@...nel.org>, Madhavan Srinivasan <maddy@...ux.ibm.com>, Alexander
 Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan
 Kara <jack@...e.cz>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar
 <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, Darren Hart
 <dvhart@...radead.org>, Davidlohr Bueso <dave@...olabs.net>, Andre Almeida
 <andrealmeid@...lia.com>, Andrew Morton <akpm@...ux-foundation.org>, Dave
 Hansen <dave.hansen@...ux.intel.com>, Linus Torvalds
 <torvalds@...ux-foundation.org>, linux-kernel@...r.kernel.org,
 linuxppc-dev@...ts.ozlabs.org, linux-fsdevel@...r.kernel.org,
 linux-mm@...ck.org
Subject: Re: [PATCH 0/5] powerpc: Implement masked user access

On Tue, 24 Jun 2025 08:17:14 -0500
Segher Boessenkool <segher@...nel.crashing.org> wrote:

> On Tue, Jun 24, 2025 at 07:27:47AM +0200, Christophe Leroy wrote:
> > Ah ok, I overlooked that, I didn't know the cmove instruction, seem 
> > similar to the isel instruction on powerpc e500.  
> 
> cmove does a move (register or memory) when some condition is true.

The destination of x86 'cmov' is always a register (only the source can be
memory - an is probably always read).
It is a also a computational instruction.
It may well always do the register write - hard to detect.

There is a planned new instruction that would do a conditional write
to memory - but not on any cpu yet.

> isel (which is base PowerPC, not something "e500" only) is a
> computational instruction, it copies one of two registers to a third,
> which of the two is decided by any bit in the condition register.

Does that mean it could be used for all the ppc cpu variants?

> But sure, seen from very far off both isel and cmove can be used to
> implement the ternary operator ("?:"), are similar in that way :-)

Which is exactly what you want to avoid speculation.

	David

> 
> 
> Segher