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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

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 ?


-boris


> [    0.000000]  ? xen_init_time_common+0x5/0x49
> [    0.000000]  ? xen_hvm_init_time_ops+0x23/0x45
> [    0.000000]  ? xen_hvm_guest_init+0x221/0x25c
> [    0.000000]  ? 0xffffffffa9600000
> [    0.000000]  ? setup_arch+0x440/0xbd6
> [    0.000000]  ? start_kernel+0x6c/0x695
> [    0.000000]  ? secondary_startup_64_no_verify+0xd5/0xdb
> [    0.000000]  </TASK>
>
>
> Changed since v1:
>    - Add commit message to explain why xen_hvm_init_time_ops() is delayed
>      for any vcpus. (Suggested by Boris Ostrovsky)
>    - Add a comment in xen_hvm_smp_prepare_boot_cpu() referencing the related
>      code in xen_hvm_guest_init(). (suggested by Juergen Gross)
> Changed since v2:
>    - Delay for all VCPUs. (Suggested by Boris Ostrovsky)
>    - Add commit message that why PVM is not supported by this patch
>    - Test if kexec/kdump works with mainline xen (HVM and PVM)
>
>
> I have delayed the xen_hvm_init_time_ops() for all VCPUs. Unfortunately,
> now I am able to reproduce the clock backward as shown below on some old
> versions of xen. I am not able to reproduce on most recent mainline xen.
>
> [    0.359687] pcpu-alloc: [0] 16 17 18 19 20 21 22 23 [0] 24 25 26 27 28 29 30 
> 31
> [    0.359694] pcpu-alloc: [0] 32 33 34 35 36 37 38 39 [0] 40 41 42 43 44 45 46 
> 47
> [    0.359701] pcpu-alloc: [0] 48 49 50 51 52 53 54 55 [0] 56 57 58 59 60 61 62 
> 63
>
> ... clock backward after the clocksource is switched from native to xen...
>
> [    0.000004] Fallback order for Node 0: 0
> [    0.002967] Built 1 zonelists, mobility grouping on.  Total pages: 3527744
> [    0.007129] Policy zone: Normal
> [    0.008937] Kernel command line: 
> BOOT_IMAGE=/boot/vmlinuz-5.17.0-rc4xen-00054-gf71077a4d84b-dirty 
> root=UUID=2a5975ab-a059-4697-9aee-7a53ddfeea21 ro text console=ttyS0,115200n8 
> console=tty1 earlyprintk=tty    S0,115200n8 loglevel=7 no_timer_check 
> reboot=s32 splash crashkernel=512M-:192M vt.handoff=1
> [    0.023880] Unknown kernel command line parameters "text splash 
> BOOT_IMAGE=/boot/vmlinuz-5.17.0-rc4xen-00054-gf71077a4d84b-dirty", will be passed to 
> user space.
> [    0.032647] printk: log_buf_len individual max cpu contribution: 4096 bytes
> [    0.036828] printk: log_buf_len total cpu_extra contributions: 258048 bytes
> [    0.041049] printk: log_buf_len min size: 262144 bytes
> [    0.044481] printk: log_buf_len: 524288 bytes
>
>
> Since now I am able to reproduce the clock backward on old xen version,
> please let me know if I should re-use the v2 of this patch, as it has been
> running well in our env well for very long time.
>
> https://lore.kernel.org/all/20211028012543.8776-1-dongli.zhang@xxxxxxxxxx/
>
>
> BTW, I have tested that 'soft_reset' does not work with mainline xen, even
> when I directly trigger kexec with below commands.
>
> # kexec -l /boot/vmlinuz-5.17.0-rc4xen-00054-gf71077a4d84b-dirty \
>          --initrd=/boot/initrd.img-5.17.0-rc4xen-00054-gf71077a4d84b-dirty \
>          --reuse-cmdline
> # kexec -e
>
>
> Thank you very much!
>
> Dongli Zhang