linux-kernel - Re: [PATCH 10/13] x86/tdx: Convert shared memory back to private on kexec

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Date:   Fri, 6 Oct 2023 07:58:03 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        kexec@...ts.infradead.org, linux-coco@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 10/13] x86/tdx: Convert shared memory back to private on kexec

On Thu, Oct 05, 2023, Kirill A. Shutemov wrote:
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 7368d254d01f..b5acf9fb4c70 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -884,6 +884,7 @@ config INTEL_TDX_GUEST
>  	select X86_MEM_ENCRYPT
>  	select X86_MCE
>  	select UNACCEPTED_MEMORY
> +	select EMERGENCY_VIRT_CALLBACK
>  	help
>  	  Support running as a guest under Intel TDX.  Without this support,
>  	  the guest kernel can not boot or run under TDX.

...

>  void __init tdx_early_init(void)
>  {
>  	struct tdx_module_args args = {
> @@ -882,6 +1007,14 @@ void __init tdx_early_init(void)
>  	 */
>  	x86_cpuinit.parallel_bringup = false;
>  
> +	machine_ops.shutdown = tdx_shutdown;
> +
> +	/*
> +	 * KVM overrides machine_ops.crash_shutdown, use emergency

This is going to be super confusing.  KVM utilizes the emergency virt callback.
The KVM paravirt guest code uses .crash_shutdown().  People that are passingly
familiar with virt and know what KVM is, but don't already know the difference
between the two are going to be all kinds of confused.

I also feel like you're playing with fire, e.g. what's to stop the hypervisor
specific paravirt guest support from using .shutdown() in the future?

And the callback is invoked for far more than just kexec().  I don't see how the
host can emulate a reboot without destroying and rebuilding the VM, e.g. it can't
stuff register state to emulate INIT or RESET.  Unless I'm missing something,
converting shared memory back to private for a shutdown or reboot is undesirable
as adds one more thing that can go wrong and prevent the system from cleanly
shutting down ASAP (for some definitions of "cleanly").

Lastly, doesn't SEV need similar behavior?  This seems like core functionality
for any guest with cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT).  Why not make the
"unshare on kexec" code common and gate it with CC_ATTR_GUEST_MEM_ENCRYPT?