success: netbsd guest works again on Ubuntu 17 with pv-linear-pt=true

[Pierre-Philipp Braun <pbraun%nethence.com@localhost>]

Hi there.

as discussed in another thread, I also confirm the `pv-linear-pt=true` boot argument to the xen micro-kernel did the trick: NetBSD/amd64 7.1 guest is starting fine on both, Ubuntu 17.04/amd64 with XEN 4.6 and 17.10/amd64 with XEN 4.9, where it was failing.  This is a relief.

	vi /etc/default/grub

	GRUB_CMDLINE_XEN="pv-linear-pt=true"

	update-grub
	grep linear /boot/grub/grub.cfg
	reboot # or shutdown -r now to avoid some misuses on BSD

Ref. https://xenbits.xen.org/xsa/advisory-240.txt

For the record, this is the recurrent error I used to get (some parts of the HEX changes, some don't, see pastebin below).

`xl dmesg`

(XEN) d1v0 Unhandled bkpt fault/trap [#3, ec=ffffffff]
(XEN) domain_crash_sync called from entry.S: fault at ffff82d080345308 entry.o#create_bounce_frame+0x135/0x14d
(XEN) Domain 1 (vcpu#0) crashed on cpu#0:
(XEN) ----[ Xen-4.9.0  x86_64  debug=n   Not tainted ]----
(XEN) CPU:    0
(XEN) RIP:    e033:[<ffffffff8012e5bd>]
(XEN) RFLAGS: 0000000000000246   EM: 1   CONTEXT: pv guest (d1v0)
(XEN) rax: 0000000000000001   rbx: 0000000000000104   rcx: 0000000000000000
(XEN) rdx: 0000000000000000   rsi: 0000000000000005   rdi: 0000000000000000
(XEN) rbp: ffffffff805f2f48   rsp: ffffffff805f2f08   r8:  ffffffff805f2ef8
(XEN) r9:  ffffffff80df6000   r10: 00000000ffffffea   r11: 0000000000000073
(XEN) r12: ffffffff80457411   r13: ffffffff805f2f80   r14: ffffffff80dd1000
(XEN) r15: 0000000000dd2000   cr0: 000000008005003b   cr4: 00000000000026e4
(XEN) cr3: 0000000040dac000   cr2: 0000000000000000
(XEN) ds: 0000   es: 0000   fs: 0000   gs: 0000   ss: e02b   cs: e033
(XEN) Guest stack trace from rsp=ffffffff805f2f08:
(XEN)    0000000000000000 0000000000000073 ffffffff8012e5bd 000000010000e030
(XEN)    0000000000000046 ffffffff805f2f48 000000000000e02b 0000000000000104
(XEN)    ffffffff805f2f70 ffffffff80348a99 0000000000040dd1 0000000000000003
(XEN)    0000000000006e88 ffffffff805f2fc8 ffffffff80348b5d 0000000000000008
(XEN)    ffffffff805f2fd8 ffffffff805f2f98 ffffffff803e0040 0000000080000000
(XEN)    0000000000000000 0000000000000002 0000000000dd1000 ffffffff80df6000
(XEN)    ffffffff805f3000 ffffffff803e033f 0000000000000003 0000000000040dd1
(XEN)    0000000000000000 ffffffff80df6000 ffffffff80dac000 ffffffff805f3078
(XEN)    ffffffff803e088f 0000000000006ea0 0000000000006e98 0000000b00000000
(XEN)    ffffffff80dd3000 ffffffff80dd2000 ffffffff80df6000 0000000000dd1000
(XEN)    ffffffff80dd4000 ffffffff80dd1000 0000000000000021 ffffffff80dac000
(XEN)    ffffffff80dd1000 0000000000000025 ffffffff805f30b0 ffffffff803e10c3
(XEN)    00000000756e6547 0000000000000000 0000000000000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 ffffffff80100069 0000000000000000
(XEN)    0000000000000000 0000000000000000 0000000000000000 000000000000007f
(XEN)    ffffffff805f3100 0000000000000000 0000000000000000 ffffffff804499be
(XEN)    000000a0ffffffff 0000000000000001 0000000000000000 ffffffff80452bfe
(XEN)    000000a1ffffffff 0000000000000001 0000000000000000 ffffffff80448a3d
(XEN)    000000a3000000a3 0000000000000002 0000000000000004 ffffffff80448a41
(XEN)    000000a4ffffffff 0000000000000000 0000000000000000 ffffffff80448a45

also https://pastebin.com/kw101MeG

-- 
Pierre-Philipp