Re: [PATCH v3 0/1] xen: fix HVM kexec kernel panic

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 2/25/22 8:17 PM, Dongli Zhang wrote:
> Hi Boris,
>
> On 2/25/22 2:39 PM, Boris Ostrovsky wrote:
> > On 2/24/22 4:50 PM, Dongli Zhang wrote:
> > > This is the v3 of the patch to fix xen kexec kernel panic issue when the
> > > kexec is triggered on VCPU >= 32.
> > >
> > > PANIC: early exception 0x0e IP 10:ffffffffa96679b6 error 0 cr2 0x20
> > > [    0.000000] CPU: 0 PID: 0 Comm: swapper Not tainted
> > > 5.17.0-rc4xen-00054-gf71077a4d84b-dirty #1
> > > [    0.000000] Hardware name: Xen HVM domU, BIOS 4.4.4OVM 12/15/2020
> > > [    0.000000] RIP: 0010:pvclock_clocksource_read+0x6/0xb0
> > > ... ...
> > > [    0.000000] RSP: 0000:ffffffffaae03e10 EFLAGS: 00010082 ORIG_RAX:
> > > 0000000000000000
> > > [    0.000000] RAX: 0000000000000000 RBX: 0000000000010000 RCX: 0000000000000002
> > > [    0.000000] RDX: 0000000000000003 RSI: ffffffffaac37515 RDI: 0000000000000020
> > > [    0.000000] RBP: 0000000000011000 R08: 0000000000000000 R09: 0000000000000001
> > > [    0.000000] R10: ffffffffaae03df8 R11: ffffffffaae03c68 R12: 0000000040000004
> > > [    0.000000] R13: ffffffffaae03e50 R14: 0000000000000000 R15: 0000000000000000
> > > [    0.000000] FS:  0000000000000000(0000) GS:ffffffffab588000(0000)
> > > knlGS:0000000000000000
> > > [    0.000000] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > [    0.000000] CR2: 0000000000000020 CR3: 00000000ea410000 CR4: 00000000000406a0
> > > [    0.000000] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > > [    0.000000] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > > [    0.000000] Call Trace:
> > > [    0.000000]  <TASK>
> > > [    0.000000]  ? xen_clocksource_read+0x24/0x40
> >
> > This is done to set xen_sched_clock_offset which I think will not be used for a
> > while, until sched_clock is called (and the other two uses are for 
> > suspend/resume)
> >
> >
> > Can we simply defer 'xen_sched_clock_offset = xen_clocksource_read();' until
> > after all vcpu areas are properly set? Or are there other uses of
> > xen_clocksource_read() before ?
> I have tested that below patch will panic kdump kernel.


Oh well, so much for that then. Yes, sched_clock() is at least called from 
printk path.


I guess we will have to go with v2 then, we don't want to start seeing time 
going back, even if only with older hypervisors. The only thing I might ask is 
that you roll the logic inside xen_hvm_init_time_ops(). Something like


xen_hvm_init_time_ops()
{
        /*
         * Wait until per_cpu(xen_vcpu, 0) is initialized which may happen
         * later (e.g. when kdump kernel runs on >=MAX_VIRT_CPUS vcpu)
         */
        if (__this_cpu_read(xen_vcpu_nr(0)) == NULL)
                return;

        ...

-boris