linux-kernel - Re: [PATCH] cpu/hotplug: Allow the CPU in CPU_UP

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Message-ID: <20211206112555.ubscfwwxnn5bnyte@linutronix.de>
Date:   Mon, 6 Dec 2021 12:25:55 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "Longpeng (Mike, Cloud Infrastructure Service Product Dept.)" 
        <longpeng2@...wei.com>, linux-kernel@...r.kernel.org,
        "Gonglei (Arei)" <arei.gonglei@...wei.com>, x86@...nel.org,
        xen-devel@...ts.xenproject.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] cpu/hotplug: Allow the CPU in CPU_UP_PREPARE state to be
 brought up again.

On 2021-11-24 21:17:34 [-0500], Boris Ostrovsky wrote:
> 
> On 11/24/21 5:54 PM, Thomas Gleixner wrote:
> > Any comment from XEN folks?
> 
> 
> If memory allocation in cpu_initialize_context() fails we will not be
> able to bring up the VCPU because xen_cpu_initialized_map bit at the
> top of that routine will already have been set. We will BUG in
> xen_pv_cpu_up() on second (presumably successful) attempt because
> nothing for that VCPU would be initialized. This can in principle be
> fixed by moving allocation to the top of the routine and freeing
> context if the bit in the bitmap is already set.
> 
> 
> Having said that, allocation really should not fail: for PV guests we
> first bring max number of VCPUs up and then offline them down to
> however many need to run. I think if we fail allocation during boot we
> are going to have a really bad day anyway.
> 

So can we keep the patch as-is or are some changes needed?

> -boris

Sebastian