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

[Konstantin Belousov]

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.

> 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