Re: [PATCH v2 1/1] xen: delay xen_hvm_init_time_ops() if kdump is boot on vcpu>=32

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 10/27/21 9:25 PM, Dongli Zhang wrote:
> The sched_clock() can be used very early since
> commit 857baa87b642 ("sched/clock: Enable sched clock early"). In addition,
> with commit 38669ba205d1 ("x86/xen/time: Output xen sched_clock time from
> 0"), kdump kernel in Xen HVM guest may panic at very early stage when
> accessing &__this_cpu_read(xen_vcpu)->time as in below:
>
> setup_arch()
>   -> init_hypervisor_platform()
>       -> x86_init.hyper.init_platform = xen_hvm_guest_init()
>           -> xen_hvm_init_time_ops()
>               -> xen_clocksource_read()
>                   -> src = &__this_cpu_read(xen_vcpu)->time;
>
> This is because Xen HVM supports at most MAX_VIRT_CPUS=32 'vcpu_info'
> embedded inside 'shared_info' during early stage until xen_vcpu_setup() is
> used to allocate/relocate 'vcpu_info' for boot cpu at arbitrary address.
>
> However, when Xen HVM guest panic on vcpu >= 32, since
> xen_vcpu_info_reset(0) would set per_cpu(xen_vcpu, cpu) = NULL when
> vcpu >= 32, xen_clocksource_read() on vcpu >= 32 would panic.
>
> This patch delays xen_hvm_init_time_ops() to later in
> xen_hvm_smp_prepare_boot_cpu() after the 'vcpu_info' for boot vcpu is
> registered when the boot vcpu is >= 32.
>
> Another option is to always delay xen_hvm_init_time_ops() for any vcpus
> (including vcpu=0). Since to delay xen_hvm_init_time_ops() may lead to
> clock backward issue,

This is referring to 
https://lists.xenproject.org/archives/html/xen-devel/2021-10/msg01516.html I 
assume?


>   it is preferred to avoid that for regular boot (The
> pv_sched_clock=native_sched_clock() is used at the very beginning until
> xen_sched_clock() is registered). That requires to adjust
> xen_sched_clock_offset. That's why we only delay xen_hvm_init_time_ops()
> for vcpu>=32.

We delay only on VCPU>=32 because we want to avoid the clock going backwards 
due to hypervisor problem pointed to be the link above, not because we need to 
adjust xen_sched_clock_offset (which we could if we wanted).


> This issue can be reproduced on purpose via below command at the guest
> side when kdump/kexec is enabled:
>
> "taskset -c 33 echo c > /proc/sysrq-trigger"
>
> Reference:
> https://lists.xenproject.org/archives/html/xen-devel/2021-10/msg00571.html
> Cc: Joe Jin <joe.jin@xxxxxxxxxx>
> Signed-off-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
> ---
> 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)
>
>   arch/x86/xen/enlighten_hvm.c | 20 +++++++++++++++++++-
>   arch/x86/xen/smp_hvm.c       |  8 ++++++++
>   2 files changed, 27 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/xen/enlighten_hvm.c b/arch/x86/xen/enlighten_hvm.c
> index e68ea5f4ad1c..7734dec52794 100644
> --- a/arch/x86/xen/enlighten_hvm.c
> +++ b/arch/x86/xen/enlighten_hvm.c
> @@ -216,7 +216,25 @@ static void __init xen_hvm_guest_init(void)
>         WARN_ON(xen_cpuhp_setup(xen_cpu_up_prepare_hvm, xen_cpu_dead_hvm));
>         xen_unplug_emulated_devices();
>         x86_init.irqs.intr_init = xen_init_IRQ;
> -       xen_hvm_init_time_ops();
> +
> +       /*
> +        * Only MAX_VIRT_CPUS 'vcpu_info' are embedded inside 'shared_info'
> +        * and the VM would use them until xen_vcpu_setup() is used to
> +        * allocate/relocate them at arbitrary address.
> +        *
> +        * However, when Xen HVM guest panic on vcpu >= MAX_VIRT_CPUS,
> +        * per_cpu(xen_vcpu, cpu) is still NULL at this stage. To access
> +        * per_cpu(xen_vcpu, cpu) via xen_clocksource_read() would panic.
> +        *
> +        * Therefore we delay xen_hvm_init_time_ops() to
> +        * xen_hvm_smp_prepare_boot_cpu() when boot vcpu is >= MAX_VIRT_CPUS.
> +        */
> +       if (xen_vcpu_nr(0) >= MAX_VIRT_CPUS)
> +               pr_info("Delay xen_hvm_init_time_ops() as kernel is running on 
> vcpu=%d\n",
> +                       xen_vcpu_nr(0));
> +       else
> +               xen_hvm_init_time_ops();
> +
>         xen_hvm_init_mmu_ops();
>   
>   #ifdef CONFIG_KEXEC_CORE
> diff --git a/arch/x86/xen/smp_hvm.c b/arch/x86/xen/smp_hvm.c
> index 6ff3c887e0b9..f99043df8bb5 100644
> --- a/arch/x86/xen/smp_hvm.c
> +++ b/arch/x86/xen/smp_hvm.c
> @@ -19,6 +19,14 @@ static void __init xen_hvm_smp_prepare_boot_cpu(void)
>          */
>         xen_vcpu_setup(0);
>   
> +	/*
> +        * The xen_hvm_init_time_ops() is delayed from
> +        * xen_hvm_guest_init() to here to avoid panic when the kernel
> +        * boots from vcpu>=MAX_VIRT_CPUS (32).
> +        */

How about

  /* Deferred call to xen_hvm_init_time_ops(). See comment in 
xen_hvm_guest_init() */


-boris



> +       if (xen_vcpu_nr(0) >= MAX_VIRT_CPUS)
> +               xen_hvm_init_time_ops();
> +
>         /*
>          * The alternative logic (which patches the unlock/lock) runs before
>          * the smp bootup up code is activated. Hence we need to set this up