From: hshoexer <hshoexer@yerbouti.franken.de>
Subject: Re: Sysupgrade on i386 vmm to current snapshot hangs on boot into upgrade kernel
To: tech@openbsd.org
Date: Tue, 7 Apr 2026 11:46:34 +0200

Hi,

On Sun, Apr 05, 2026 at 01:08:51PM -0500, Brent Cook wrote:
> On Sun, Apr 05, 2026 at 04:58:11AM -0500, Brent Cook wrote:
> > On Tue, Mar 31, 2026 at 1:35???PM Alexander Bluhm <bluhm@openbsd.org> wrote:
> > 
> > > On Tue, Mar 31, 2026 at 09:30:30AM -0700, Mike Larkin wrote:
> > > > rebuild the host kernel with HZ=1000 and see if running that makes the
> > > problem
> > > > go away.
> > >
> > > I have option HZ=1000 in host's sys/conf/GENERIC now.  It does not help.
> > > Guest's GENERIC is unmodified.
> > >
> > > When the i386 guest boots, its GENERIC either says
> > > cpu0: AMD EPYC 73F3 16-Core Processor ("AuthenticAMD" 686-class, 512KB L2
> > > cache) 3.50 GHz, 19-01-01
> > > or
> > > cpu0: AMD EPYC 73F3 16-Core Processor ("AuthenticAMD" 686-class, 512KB L2
> > > cache) 74 MHz, 19-01-01
> > >
> > >
> > Same thing here, but this is a clue why RAMDISK_CD hangs but not GENERIC
> > are hanging, at least for me.
> > 
> > Adding pvclock0 allows RAMDISK_CD to boot 100% of the time in my local vmm
> > machine.
> > 
> > diff --git a/sys/arch/i386/conf/RAMDISK_CD b/sys/arch/i386/conf/RAMDISK_CD
> > index c00f6dcbbb4..24bd4e465a9 100644
> > --- a/sys/arch/i386/conf/RAMDISK_CD
> > +++ b/sys/arch/i386/conf/RAMDISK_CD
> > @@ -28,6 +28,7 @@ config                bsd root on rd0a swap on rd0b and
> > wd0b and sd0b
> >  mainbus0       at root
> > 
> >  pvbus0         at mainbus0     # Paravirtual device bus
> > +pvclock0       at pvbus?       # KVM/VMD paravirtual clock
> > 
> >  acpi0          at bios?
> >  #acpitimer*    at acpi?
> > 
> > 
> 
> Ugh, that last paste was ugly, sorry. Switching mail clients:
> 
> Would anyone be able to give this diff a try? It's working locally; I
> was able reliably get the i8254 to calibrate to 4GHz, and see no hangs
> either in RAMDISK_CD or when switching sysctl
> kern.timecounter.hardware=i8254. Now my i386 VMM machine can
> sysupgrade reliably

diff works for me with openbsd i386 and amd64 with the RAMDISK_CD
and switching timecounter from pvclock0 to i8254 back and forth.

> The main change here is to set the timestamp right in the vcpu thread
> instead of later in the event loop in i8253_reset. Otherwise, a latch
> command issued before i8253_reset runs will be using a new start value,
> but an old stale ts value. Maybe this makes the clock_gettime in
> i8253_reset unnecessary, but I left it in.
> 
> This also adjusts a couple of the register states to match that of
> Bhyve when the mode is updated or there is a latch command:
> 
> https://github.com/freebsd/freebsd-src/blob/main/sys/amd64/vmm/io/vatpit.c#L239
> https://github.com/freebsd/freebsd-src/blob/main/sys/amd64/vmm/io/vatpit.c#L330
> 
> Hopefully this makes sense, or if it's the wrong approach, at least
> inspires the correct fix.
> 
> diff --git a/usr.sbin/vmd/i8253.c b/usr.sbin/vmd/i8253.c
> index 9e32d9382b6..091716a2f32 100644
> --- a/usr.sbin/vmd/i8253.c
> +++ b/usr.sbin/vmd/i8253.c
> @@ -262,12 +262,14 @@ vcpu_exit_i8253(struct vm_run_params *vrp)
>  					    ticks % i8253_channel[sel].start;
>  				} else
>  					i8253_channel[sel].olatch = 0;
> +				i8253_channel[sel].last_r = 2;
>  				goto ret;
>  			} else if (rw != TIMER_16BIT) {
>  				log_warnx("%s: i8253 PIT: unsupported counter "
>  				    "%d rw mode 0x%x selected", __func__,
>  				    sel, (rw & TIMER_16BIT));
>  			}
> +			i8253_channel[sel].last_w = 0;
>  			i8253_channel[sel].mode = (out_data & 0xe) >> 1;
>  
>  			goto ret;
> @@ -293,6 +295,9 @@ vcpu_exit_i8253(struct vm_run_params *vrp)
>  				if (i8253_channel[sel].start == 0)
>  					i8253_channel[sel].start = 0xffff;
>  
> +				clock_gettime(CLOCK_MONOTONIC,
> +				    &i8253_channel[sel].ts);
> +
>  				DPRINTF("%s: channel %d reset, mode=%d, "
>  				    "start=%d\n", __func__,
>  				    sel, i8253_channel[sel].mode,
>