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Date:   Tue, 12 Oct 2021 13:17:41 -0400
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Subject: Re: [PATCH linux 1/2] xen: delay xen_hvm_init_time_ops() if kdump is
 boot on vcpu>=32
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To:     Dongli Zhang <dongli.zhang@oracle.com>,
        xen-devel@lists.xenproject.org
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org, jgross@suse.com,
        sstabellini@kernel.org, tglx@linutronix.de, mingo@redhat.com,
        bp@alien8.de, hpa@zytor.com, andrew.cooper3@citrix.com,
        george.dunlap@citrix.com, iwj@xenproject.org, jbeulich@suse.com,
        julien@xen.org, wl@xen.org, joe.jin@oracle.com
References: <20211012072428.2569-1-dongli.zhang@oracle.com>
 <20211012072428.2569-2-dongli.zhang@oracle.com>
From:   Boris Ostrovsky <boris.ostrovsky@oracle.com>
In-Reply-To: <20211012072428.2569-2-dongli.zhang@oracle.com>
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On 10/12/21 3:24 AM, 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:


Please drop "upstream". It's always upstream here.


> +
> +	/*
> +	 * 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)


What about always deferring this when panicing? Would that work?


Deciding whether to defer based on cpu number feels a bit awkward.


-boris


> +		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();
>