linux-kernel - Re: [PATCH v4 2/2] xen: delay xen_hvm_init_time

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Date:   Fri, 11 Mar 2022 09:19:46 -0500
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Dongli Zhang <dongli.zhang@...cle.com>,
        xen-devel@...ts.xenproject.org, x86@...nel.org
Cc:     linux-kernel@...r.kernel.org, jgross@...e.com,
        sstabellini@...nel.org, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, joe.jin@...cle.com
Subject: Re: [PATCH v4 2/2] xen: delay xen_hvm_init_time_ops() if kdump is
 boot on vcpu>=32


On 3/2/22 11:40 AM, 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 calls xen_hvm_init_time_ops() again 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"
>
> The bugfix for PVM is not implemented due to the lack of testing
> environment.
>
> Cc: Joe Jin <joe.jin@...cle.com>
> Signed-off-by: Dongli Zhang <dongli.zhang@...cle.com>


Reviewed-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>


Applied to for-linus-5.18 (with return path adjusted)