Re: [PATCH v2 03/10] um: vdso: Implement __vdso_getcpu() via syscall

["Thomas Weißschuh" <linux@weissschuh.net>]

On 2025-09-22 14:12:52+0200, Johannes Berg wrote:
> On Mon, 2025-09-22 at 14:05 +0200, Thomas Weißschuh wrote:
> > > The original issue could now be considered resolved. So in v3, we no
> > > longer turn __vdso_getcpu into a syscall wrapper; we simply removed it.
> > > Perhaps we could remove the whole VDSO before we implement the "real"
> > > VDSO. However, its implementation is clean, so keeping it wouldn't hurt
> > > and it could serve as a useful starting point for the "real" VDSO.
> > 
> > A "real" vDSO would require quite some more infrastructure.
> > 
> 
> What's not "real" about the vDSO now? Yes it just implement syscalls
> after the getcpu removal, but ... it's still a vDSO? I _have_ played
> with getting data into it for the time-travel case, at least.

Right now it does not provide any advantage over a regular syscall.
Essentially it is just overhead. That said, if you do want to make a
real vDSO out of it, I'd be happy to help in that.
(I did most of the recent work on the generic vDSO infrastructure)

> > And it is not even clear if such a vDSO will make a difference on UML.
> 
> Syscall overhead is _huge_ in UML, if it does anything but syscalls it
> will _certainly_ make a difference.

Ack.

> > In my
> > opinion if __vdso_getcpu() gets removed, the whole vDSO should go with
> > it. The code can still be easily restored from git.
> 
> I mean ... on the one hand, sure, it doesn't really do much after this,
> but OTOH it lets userspace actually use that path? So might be useful.

What advantage does userspace have from it?

> > Also the functionality to map the host vDSO and vsyscall page into UML
> > userspace looks very weird and error-prone. Maybe it can also go away.
> 
> Surely host vDSO etc. is never mapped into UML userspace and never is,
> not sure what you're thinking of, but clearly that's wrong as written.

This is how I understand the 32bit implementation using
ARCH_REUSE_HOST_VSYSCALL_AREA and NEW_AUX_ENT(AT_SYSINFO_EHDR, vsyscall_ehdr)
where vsyscall_ehdr comes from the hosts getauxval(AT_SYSINFO_EHDR).
But I didn't actually test this. I'll look at it again, but currently
I'm travelling.


Thomas