svn commit: r350764 - head/sys/arm64/arm64

[Rodney W. Grimes]

> On Fri, Aug 09, 2019 at 10:01:31AM -0600, Warner Losh wrote:
> > On Fri, Aug 9, 2019 at 12:57 AM Konstantin Belousov <kostikbel at gmail.com>
> > wrote:
> > 
> > > On Thu, Aug 08, 2019 at 07:38:28PM -0600, Warner Losh wrote:
> > > > On Thu, Aug 8, 2019, 4:59 PM Gleb Smirnoff <glebius at freebsd.org> wrote:
> > > >
> > > > >   Hi,
> > > > >
> > > > > why do we need COMPAT_43 for arm64 at all? I can't imagine an
> > > > > application that would require this compatibility.
> > > > >
> > > > > A more general question is how far in the future are we going
> > > > > to carry COMPAT_43 for i386/amd64?
> > > > >
> > > >
> > > > COMPAT_43 is a weird option. It's a combo of both sys calls and kernel
> > > > behavior modifications. Before we thinned the ABIs we supported, it was
> > > > necessary for them as well. The biggest behavior change is around
> > > signals.
> > > > It is weird to sort out and nobody has done the deep analysis to see what
> > > > is truly unused and what is there for compat with Linux and other SysV
> > > > systems...
> > > I am not aware of any changes that COMPAT_43 provides for the signal
> > > handling semantic, except a minor adjustment for interpretation of
> > > zero-sized stack for sigaltstack(2).
> > >
> > 
> > The onstack stuff was what I was thinking about, but we also have code in
> > sys_getpid() that returns the ppid in the second retval register, and
> > similar things for getuid and getgid,  It also allows ioctl numbers that
> > have IOC_IN set, but size == 0 (these would otherwise return ENOTTY). It
> > also turns on the COMPAT_OLDSOCK code which generally only kicks in when
> > compat bits are set, but in one place it allows a shorter unix domain
> > socket path length to be compatible unconditionally. The compatibility TTY
> > stuff, at least is under COMPAT_43TTY, but that's purely ioctl translation
> > code.
> I only reacted to the note about changing the signals syscalls behavior.
> But the point is valid, we should not change the syscalls ABI for new
> binaries when COMPAT_43 is enabled.  I propose the following
> https://reviews.freebsd.org/D21200
> 
> WRT ioctl code for no IOC_OUT and size == 0, I believe that this is in
> fact cannot be changed. It is enabled also under COMPAT_FREEBSD4 and
> 5, and we always enable these for GENERIC. So effectively this ioctl
> permissive mode is always there.
> 
> > 
> > The COMPAT_43 option indeed enables lcall 7,0 syscall entry emulation,
> > > on both i386 and amd64.  We are able to run FreeBSD 1.1.8 (i386) on amd64
> > > kernel in chroot this way.  Since sometimes I get bug reports about this
> > > stuff, there are some users of it.  I believe it is important to be able
> > > to run any FreeBSD binary for PR purposes, to wave the flag of excellent
> > > binary compatibility we offer.
> > >
> > > COMPAT_43 is there to stay as far as there are people willing to maintain
> > > it.  There are more than one.
> > >
> > 
> > I think it's safe to retain on i386. amd64 is less clear to me, but I'd
> > lean yes.
> I believe amd64 is required since you have less and less chances to usefully
> run i386 kernel on modern hardware.

Would this also be required for running i386 binaries on amd64 using lib32
that expect the COMPAT_43 behavior?

> > All the other platforms I'd agree with gleb: why do we need it in
> > the kernels by default (and maybe why do we need to support it at all)?
> > 
> > Warner

-- 
Rod Grimes                                                 rgrimes at freebsd.org