linux-kernel - Re: [PATCH linux 1/2] xen: delay xen_hvm_init_time

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Date:   Tue, 12 Oct 2021 10:48:44 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Dongli Zhang <dongli.zhang@...cle.com>,
        xen-devel@...ts.xenproject.org
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        boris.ostrovsky@...cle.com, sstabellini@...nel.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        andrew.cooper3@...rix.com, george.dunlap@...rix.com,
        iwj@...project.org, jbeulich@...e.com, julien@....org, wl@....org,
        joe.jin@...cle.com
Subject: Re: [PATCH linux 1/2] xen: delay xen_hvm_init_time_ops() if kdump is
 boot on vcpu>=32

On 12.10.21 09:24, Dongli Zhang wrote:
> The sched_clock() can be used very early since upstream
> commit 857baa87b642 ("sched/clock: Enable sched clock early"). In addition,
> with upstream 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.
> 
> 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"
> 
> Cc: Joe Jin <joe.jin@...cle.com>
> Signed-off-by: Dongli Zhang <dongli.zhang@...cle.com>
> ---
>   arch/x86/xen/enlighten_hvm.c | 20 +++++++++++++++++++-
>   arch/x86/xen/smp_hvm.c       |  3 +++
>   2 files changed, 22 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/xen/enlighten_hvm.c b/arch/x86/xen/enlighten_hvm.c
> index e68ea5f4ad1c..152279416d9a 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..60cd4fafd188 100644
> --- a/arch/x86/xen/smp_hvm.c
> +++ b/arch/x86/xen/smp_hvm.c
> @@ -19,6 +19,9 @@ static void __init xen_hvm_smp_prepare_boot_cpu(void)
>   	 */
>   	xen_vcpu_setup(0);
>   
> +	if (xen_vcpu_nr(0) >= MAX_VIRT_CPUS)
> +		xen_hvm_init_time_ops();
> +

Please add a comment referencing the related code in
xen_hvm_guest_init().


Juergen

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