linux-kernel - Re: [PATCH tip] x86: vdso: Compute timens page offset statically

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Message-ID: <ZzqkEzxJ69dBSps_@zx2c4.com>
Date: Mon, 18 Nov 2024 03:18:59 +0100
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
	x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH tip] x86: vdso: Compute timens page offset statically

On Tue, Sep 10, 2024 at 02:20:17PM +0200, Jason A. Donenfeld wrote:
> On Tue, Sep 10, 2024 at 02:08:11PM +0200, Thomas Gleixner wrote:
> > On Mon, Sep 09 2024 at 17:44, Jason A. Donenfeld wrote:
> > > On Fri, Sep 06, 2024 at 09:06:55PM +0200, Jason A. Donenfeld wrote:
> > >> The expression `((void *)&__timens_vdso_data - (void *)&__vdso_data)`
> > >> seems harmless, but it actually results in quite a bit of code and two
> > >> jumps, in a place that's supposed to be somewhat lean on code. The value
> > >> of that calculation is always 3*PAGE_SIZE, as it turns out. Changing it
> > >> to that results in a more modest cmov instruction being emitted. It also
> > >> makes it a bit more clear what's happening.
> > >> 
> > >> To accomplish this, define offset macros in vvar.h, which can be shared
> > >> by C code and by the linker script that decides where these pages will
> > >> actually go.
> > >
> > > I noticed we've only got a week left til the merge window opens, so I
> > > thought I should poke you about this, if you want to take this through
> > > tip. I can also take it through my random.git tree with your ack, if
> > > that's easier for you. (Assuming, of course, that this patch is actually
> > > correct.) Let me know.
> > 
> > It's not the end of the world if this does not go in now. It's in my
> > back log and that VDSO stuff needs more care than this particular thing
> > as the recent discussion about vdso random on other architectures show.
> 
> Okay, that makes sense. And thanks for taking a look.

Thought I'd poke again about this. It looks like this still applies to
tip.

Jason