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 17:15:37 -0500
From:   "Kalra, Ashish" <ashish.kalra@....com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.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 10/6/2023 10:11 AM, Kirill A. Shutemov wrote:
> On Fri, Oct 06, 2023 at 07:58:03AM -0700, Sean Christopherson wrote:
>> 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").
> 
> Okay, fair enough. I will look for better way to hookup into kexec
> process. That was the best fit I found so far, but yes it is not ideal.
> 
>> 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?
> 
> I don't know SEV specifics. I am open to collaboration on this.
> 
> Tom, Ashish, let me know if you need this in generic code. I can arrange
> that.
> 

Yes, some kind of a generic interface like unshare_on_kexec() gated with 
CC_ATTR_GUEST_MEM_ENCRYPT is needed, we can then add SNP specific kexec 
functionality as part of this.

Thanks,
Ashish