linux-kernel - Re: [PATCH] x86/panic/reboot: Flush processor caches during panic/reboot

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Date:	Thu, 23 Jul 2015 10:40:48 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	Toshi Kani <toshi.kani@...com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, X86 ML <x86@...nel.org>,
	Borislav Petkov <bp@...en8.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>
Subject: Re: [PATCH] x86/panic/reboot: Flush processor caches during panic/reboot

On Thu, Jul 23, 2015 at 10:33 AM, Toshi Kani <toshi.kani@...com> wrote:
> During power failure, Asynchronous DRAM Refresh (ADR) flushes
> the write buffer in memory controllers into NVDIMM, but does not
> flush processor caches.  While the kernel and application code
> need to take care of processor cache flush, they may not be able
> to do so during panic or reboot.
>
> Add processor cache flush (wbinvd) to the stop-CPUs interfaces,
> native_stop_other_cpus() and nmi_shootdown_cpus(), which are
> called during panic and reboot as follows.  These wbinvd()s are
> called on each CPU after its irq/APIC is disabled.
>
>   - panic()
>     + smp_send_stop()
>        o native_stop_other_cpus()
>           o stop_this_cpu()
>
>   - native_machine_restart()
>   - native_machine_halt()
>   - native_machine_power_off()
>      + native_machine_shutdown()
>         + stop_other_cpus()
>            o native_stop_other_cpus()
>               o stop_this_cpu()
>
>   - native_machine_crash_shutdown()
>      + kdump_nmi_shootdown_cpus()
>         o nmi_shootdown_cpus()
>            o crash_nmi_callback()
>
> Note, the cpu offline path, mwait_play_dead(), already calls
> wbinvd().
>

If the application is already prepared for surprise power loss what
additional benefit is there to flushing caches on panic?  In other
words, if the application needs this for correctness then it is broken
with respect to surprise power loss, otherwise these flushes are not
necessary.
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