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

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Date:   Tue, 22 Nov 2022 10:20:53 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kai Huang <kai.huang@...el.com>
Cc:     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 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:

> +static void seamcall_on_each_cpu(struct seamcall_ctx *sc)
> +{
> +	on_each_cpu(seamcall_smp_call_function, sc, true);
> +}

Consider:

	CPU0			CPU1			CPU2

	local_irq_disable()
	...
				seamcall_on_each_cpu()
				send-IPIs to 0 and 2
							<IPI>
				runs local seamcall
				(seamcall done)
				waits for 0 and 2
							<has an NMI delay things>
							runs seamcall
							clears CSD_LOCK
							</IPI>
				... spinning ...

	local_irq_enable()
	<IPI>
	  runs seamcall
	  clears CSD_LOCK
				*FINALLY* observes CSD_LOCK cleared on
				all CPU and continues
	</IPI>

IOW, they all 3 run seamcall at different times.

Either the Changelog is broken or this TDX crud is worse crap than I
thought possible, because the only way to actually meet that requirement
as stated is stop_machine().