linux-kernel - Re: [PATCH v7 06/20] x86/virt/tdx: Shut down TDX module in case of error

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Message-ID: <Y30dujuXC8wlLwoQ@hirez.programming.kicks-ass.net>
Date:   Tue, 22 Nov 2022 20:06:34 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Kai Huang <kai.huang@...el.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, linux-mm@...ck.org, seanjc@...gle.com,
        pbonzini@...hat.com, dave.hansen@...el.com,
        dan.j.williams@...el.com, rafael.j.wysocki@...el.com,
        kirill.shutemov@...ux.intel.com, ying.huang@...el.com,
        reinette.chatre@...el.com, len.brown@...el.com,
        tony.luck@...el.com, ak@...ux.intel.com, isaku.yamahata@...el.com,
        chao.gao@...el.com, sathyanarayanan.kuppuswamy@...ux.intel.com,
        bagasdotme@...il.com, sagis@...gle.com, imammedo@...hat.com
Subject: Re: [PATCH v7 06/20] x86/virt/tdx: Shut down TDX module in case of
 error

On Tue, Nov 22, 2022 at 04:06:25PM +0100, Thomas Gleixner wrote:
> On Tue, Nov 22 2022 at 10:20, Peter Zijlstra wrote:
> 
> > On Mon, Nov 21, 2022 at 01:26:28PM +1300, Kai Huang wrote:
> >
> >> Shutting down the TDX module requires calling TDH.SYS.LP.SHUTDOWN on all
> >> BIOS-enabled CPUs, and the SEMACALL can run concurrently on different
> >> CPUs.  Implement a mechanism to run SEAMCALL concurrently on all online
> >> CPUs and use it to shut down the module.  Later logical-cpu scope module
> >> initialization will use it too.
> >
> > Uhh, those requirements ^ are not met by this:
> 
>   Can run concurrently != Must run concurrently
>  
> The documentation clearly says "can run concurrently" as quoted above.

The next sentense says: "Implement a mechanism to run SEAMCALL
concurrently" -- it does not.

Anyway, since we're all in agreement there is no such requirement at
all, a schedule_on_each_cpu() might be more appropriate, there is no
reason to use IPIs and spin-waiting for any of this.

That said; perhaps we should grow:

  schedule_on_cpu(struct cpumask *cpus, work_func_t func);

to only disturb a given mask of CPUs.