Re: [RFC PATCH v1 7/9] x86: Add unsafe_copy_from_user()

[Yury Norov <ynorov@xxxxxxxxxx>]

On Mon, Apr 27, 2026 at 02:52:05PM -0700, Linus Torvalds wrote:
> On Mon, 27 Apr 2026 at 12:19, Yury Norov <ynorov@xxxxxxxxxx> wrote:
> >
> > This is what Linus said when added x86 implementation for copy_from_user()
> > in c512c69187197:
> 
> Note that some things have happily changed in the six+ years since...
> 
> >   That's partly because we have no current users of it, but also partly
> >   because the copy_from_user() case is slightly different and cannot
> >   efficiently be implemented in terms of a unsafe_get_user() loop (because
> >   gcc can't do asm goto with outputs).
> 
> now everybody can do asm goto with outputs.
> 
> Yes, it's disabled on older versions, so it's not *always* available,
> but all modern versions do it. And if you care about performance, you
> won't be using an old compiler.

The minimal GCC version is 8.1, and asm goto with outputs is supported
since GCC-11. That would brake the build, if we just switch to using it
without "CC_IS_GCC && (GCC_VERSION >= 110100)" guard.

Is it worth to maintain 2 version of the function? I don't know...

Thanks,
Yury