Re: 014_amdlfence.patch breaks OpenBSD VMs on AMD systems

On Wed, Aug 01, 2018 at 11:27:26AM -0400, Bryan Steele wrote:
> On Wed, Aug 01, 2018 at 03:46:25PM +0200, Elmer Skjødt Henriksen wrote:
> > After installing the 014_amdlfence patch released yesterday for 6.3, my
> > OpenBSD VM crashes on boot. It's running under KVM on a Linux box (Ubuntu
> > 18.04 w/ kernel 4.15) on an AMD Ryzen 7 1700 (microcode 0x8001137).
> > I suppose this would also happen on vmm(4) and bhyve, however I don't have
> > any such AMD hosts available for testing.
>
> Hi Elmer,
>
> This was tested in vmm(4), which does work, unfortunately there was not
> extensive testing by in other virtualization software. The MSR that is
> being set here is only mentioned in AMDs whitepaper and I had no reason
> to believe any special consideration was needed for guest VMs on AMD
> processors.
>
> > It occurs both using libvirt's "EPYC" CPU model and using "host-passthrough"
> > (i.e. no virtual CPU model), but the "core2duo" CPU model works fine.
> >
> > I guess not many people are running OpenBSD as a VM, and even less on AMD
> > hardware. But still, a syspatch leaving the system unable to boot is
> > probably not a good thing. :)
> >
>
> Even so, I would like to apologize. This situation is unfortunate, and
> I'll try to work with other developers to find the best way forward.
> But, I regret I am only but an amateur magician.
>
> -Bryan.

Actually, it looks like this is at least partially a KVM/QEMU bug. In
the meantime I guess the solution would be to do as you suggested and
set a different CPU model for now until Linux distros include a fix for
this.

https://lkml.org/lkml/2018/2/21/1202

Afterwards, on the OpenBSD side, it looks like one small change may be
required in addition..

-Bryan.

Index: sys/arch/amd64/amd64/identcpu.c
===================================================================
RCS file: /cvs/src/sys/arch/amd64/amd64/identcpu.c,v
retrieving revision 1.95.2.2
diff -u -p -u -r1.95.2.2 identcpu.c
--- sys/arch/amd64/amd64/identcpu.c 30 Jul 2018 14:45:05 -0000 1.95.2.2
+++ sys/arch/amd64/amd64/identcpu.c 1 Aug 2018 16:09:50 -0000
@@ -650,8 +650,10 @@ identifycpu(struct cpu_info *ci)

msr = rdmsr(MSR_DE_CFG);
#define DE_CFG_SERIALIZE_LFENCE (1 << 1)
- msr |= DE_CFG_SERIALIZE_LFENCE;
- wrmsr(MSR_DE_CFG, msr);
+ if ((msr & DE_CFG_SERIALIZE_LFENCE) == 0) {
+ msr |= DE_CFG_SERIALIZE_LFENCE;
+ wrmsr(MSR_DE_CFG, msr);
+ }
}
}