linux-kernel - Re: [PATCH v2 03/10] um: vdso: Implement __vdso

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Message-ID: <1568f254-7963-4015-91ed-7630d5d87881@t-8ch.de>
Date: Sun, 21 Sep 2025 22:00:41 +0200
From: Thomas Weißschuh <linux@...ssschuh.net>
To: Johannes Berg <johannes@...solutions.net>
Cc: Tiwei Bie <tiwei.bie@...ux.dev>, richard@....at, 
	anton.ivanov@...bridgegreys.com, linux-um@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	benjamin@...solutions.net, arnd@...db.de, tiwei.btw@...group.com
Subject: Re: [PATCH v2 03/10] um: vdso: Implement __vdso_getcpu() via syscall

On 2025-09-10 13:59:02+0200, Johannes Berg wrote:
> On Sun, 2025-08-10 at 13:51 +0800, Tiwei Bie wrote:
> > From: Tiwei Bie <tiwei.btw@...group.com>
> > 
> > We are going to support SMP in UML, so we can not hard code
> > the CPU and NUMA node in __vdso_getcpu() anymore.
> 
> Correct. But does that mean we actually have to implement it via syscall
> in the VDSO? That seems a bit odd? ARM doesn't seem to have getcpu in
> the VDSO at all, for example, so could we do the same and just remove
> it?

It is my understanding that the UM VDSO exists to cope with old versions
of glibc which would fall back to the old vsyscall mechanism if no VDSO
was present. That could fall through to the host kernels vsyscalls.
See commit f1c2bb8b9964 ("um: implement a x86_64 vDSO").

If this is not necessary anymore, the whole VDSO on UM can probably go
away.


Thomas