Bug 2084566

Summary: Disable 5-level page tables when using -cpu max
Product: Red Hat Enterprise Linux 8 Reporter: Richard W.M. Jones <rjones>
Component: libguestfsAssignee: Richard W.M. Jones <rjones>
Status: CLOSED ERRATA QA Contact: YongkuiGuo <yoguo>
Severity: unspecified Docs Contact:
Priority: unspecified    
Version: 8.7CC: qzhang, rjones, virt-maint, ymao, yoguo
Target Milestone: rcKeywords: Triaged
Target Release: ---   
Hardware: Unspecified   
OS: Unspecified   
Whiteboard:
Fixed In Version: libguestfs-1.44.0-7.module+el8.7.0+15229+b143b6f1 Doc Type: If docs needed, set a value
Doc Text:
Story Points: ---
Clone Of: Environment:
Last Closed: 2022-11-08 09:19:55 UTC Type: Bug
Regression: --- Mount Type: ---
Documentation: --- CRM:
Verified Versions: Category: ---
oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
Cloudforms Team: --- Target Upstream Version:
Embargoed:
Bug Depends On: 2084719    
Bug Blocks: 2075424, 2082806    

Description Richard W.M. Jones 2022-05-12 12:42:25 UTC 
Description of problem:

In https://bugzilla.redhat.com/show_bug.cgi?id=2082806 we are
tracking an insidious qemu bug which intermittently prevents the
libguestfs appliance from starting.  The symptoms are that SeaBIOS
starts and displays its messages, but the kernel isn't reached.  We
found that the kernel does in fact start, but when it tries to set up
page tables and jump to protected mode it gets a triple fault which
causes the emulated CPU in qemu to reset (qemu exits).

This seems to only affect TCG (not KVM).

This is caused by using -cpu max which enables the "la57" feature
(5-level page tables[0]), and that we can make the problem go away
using -cpu max,la57=off.

Note this is only a workaround for bug 2082806.  We may in future
fix that bug properly (in qemu).  However the proposed workaround
for libguestfs should not have any negative effects.

This bug affects all versions of libguestfs that run qemu with
-cpu max or the libvirt equivalent <cpu mode="maximum"/>, which
includes RHEL 8.7 (not 8.6), and RHEL 9.0 and 9.1.

Version-Release number of selected component (if applicable):

libguestfs-1.44.0-6.el8

How reproducible:

100%

Steps to Reproduce:

These two commands test the libvirt and direct paths (which are
fixed separately):

while LIBGUESTFS_BACKEND_SETTINGS=force_tcg ./run libguestfs-test-tool  >&/tmp/log ; do echo -n . ; done

while LIBGUESTFS_BACKEND=direct LIBGUESTFS_BACKEND_SETTINGS=force_tcg ./run libguestfs-test-tool  >&/tmp/log ; do echo -n . ; done

Upstream bug fix:
https://listman.redhat.com/archives/libguestfs/2022-May/028853.html
Comment 4 YongkuiGuo 2022-05-18 04:15:28 UTC 
Verified with the following packages on RHEL8.7 host:

kernel-4.18.0-390.el8.x86_64
libguestfs-1.44.0-7.module+el8.7.0+15229+b143b6f1.x86_64
libvirt-8.0.0-7.module+el8.7.0+15262+04e62783.x86_64
qemu-kvm-6.2.0-13.module+el8.7.0+15131+941fbd8d.x86_64
seabios-bin-1.16.0-1.module+el8.7.0+15006+42c9ed21.noarch



# while LIBGUESTFS_BACKEND_SETTINGS=force_tcg libguestfs-test-tool  >&/tmp/log ; do echo -n . ; done
.................................................

# while LIBGUESTFS_BACKEND=direct LIBGUESTFS_BACKEND_SETTINGS=force_tcg libguestfs-test-tool  >&/tmp/log ; do echo -n . ; done
.............................................

The above two commands worked fine.
Comment 6 errata-xmlrpc 2022-11-08 09:19:55 UTC 
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory (Low: virt:rhel and virt-devel:rhel security, bug fix, and enhancement update), and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHSA-2022:7472