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[Xen-devel] Re: [PATCH] Allocate vmcs pages when system booting

To: "Jiang, Yunhong" <yunhong.jiang@xxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: [Xen-devel] Re: [PATCH] Allocate vmcs pages when system booting
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Thu, 12 Nov 2009 11:22:44 +0000
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Thread-topic: [PATCH] Allocate vmcs pages when system booting
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On 12/11/2009 10:52, "Jiang, Yunhong" <yunhong.jiang@xxxxxxxxx> wrote:

> Currently the VMCS page is allocated when the CPU is brought-up and
> identified, and then spin_debug_enable() is called.
> This does not work for cpu hot-add. When hot-added CPU is brought-up and try
> to allocate the vmcs page, it will hit check_lock, because irq is disabled
> still.
> This patch allocate the vmcs pages for all possible pages when system booting,
> so that we don't allocate vmcs anymore when secondary CPU is up.
> Signed-off-by: Jiang, Yunhong <yunhong.jiang@xxxxxxxxx>

A general point: using cpu_possible_map is not a good idea any longer, since
it is going to be all-ones as soon as your physical cpu hotplug patches go
in (I don't intend to make that a build-time option). Hence we could
potentially massively over-allocate pages with this approach.

The good news is that, when bringing a CPU online, we don't need to worry
about acquiring IRQ-unsafe locks with IRQS disabled until the CPU is added
to the cpu_online_map. This is because the race we are trying to avoid with
the spin-debug checks involves rendezvous of CPUs via IPIs, and a CPU not in
cpu_online_map will never get waited on by others.

So, your better fix would be to spin_debug_disable() at the top of
start_secondary(), and spin_debug_enable() just before

Can you try this alternative fix please?

 -- Keir

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