Comment 7 for bug 1921754

As mentioned in LP#1921880 I get an unchecked MSR access when running a focal guest with the EPYC-Rome model on a Milan CPU, but it did not crash. So if you have access to a P620, which is pretty much a Milan CPU, you should be able to reproduce this. It does not happen, if the guest has just the EPYC model.
Anyway the MSR error got me on the path that this is related to the missing IBRS flag, which after trying it with the patch, the MSR error was gone. So yes, this is related to LP#1915063 and as you already concluded with this backport, it would be resolved as well.

I don't think only the kernel commit will resolve this. I have seen a similar/same MSR error without IBRS on a Fedora 33 guest which has kernel 5.11.x.

So I if I have time, I test the mainline ppa kernel to confirm this.

For reference, this is the MSR error on the focal guest with 5.4.0-70 on both host and guest:

unchecked MSR access error: WRMSR to 0x48 (tried to write 0x0000000000000006) at rIP: 0xffffffff89a73594 (native_write_msr+0x4/0x30)
Call Trace:
 ? __switch_to_xtra+0x1ae/0x5e0
 ? __switch_to_asm+0x34/0x70
 ? __switch_to_asm+0x40/0x70
 ? __switch_to_asm+0x34/0x70
 ? __switch_to_asm+0x40/0x70
 ? __switch_to_asm+0x34/0x70
 __switch_to+0x3b0/0x470
 ? __switch_to_asm+0x40/0x70
 ? __switch_to_asm+0x34/0x70
 __schedule+0x2e3/0x740
 preempt_schedule_common+0x18/0x30
 _cond_resched+0x22/0x30
 stop_one_cpu+0x69/0xa0
 ? sched_ttwu_pending+0xe0/0xe0
 sched_exec+0x92/0xc0
 __do_execve_file.isra.0+0x1fc/0x840
 ? strncpy_from_user+0x4c/0x150
 __x64_sys_execve+0x39/0x50
 do_syscall_64+0x57/0x190
 entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x7f31e09ef2fb
Code: 41 89 01 eb da 66 2e 0f 1f 84 00 00 00 00 00 f7 d8 64 41 89 01 eb d6 0f 1f 84 00 00 00 00 00 f3 0f 1e fa b8 3b 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 65 4b 10 00 f7 d8 64 89 01 48
RSP: 002b:00007fff1cfd3b48 EFLAGS: 00000246 ORIG_RAX: 000000000000003b
RAX: ffffffffffffffda RBX: 000055713f000370 RCX: 00007f31e09ef2fb
RDX: 000055713f0d5010 RSI: 000055713f069690 RDI: 000055713f006070
RBP: 00007fff1cfd3d50 R08: 000055713f057cd0 R09: 0000000000000000
R10: 000055713efe9980 R11: 0000000000000246 R12: 0000000000000000
R13: 000055713f0d0f50 R14: 0000000000000000 R15: 000055713f069690