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

From: "Kalra, Ashish" <ashish.kalra@amd.com>
To: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Sean Christopherson <seanjc@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	x86@kernel.org, "Rafael J. Wysocki" <rafael@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Adrian Hunter <adrian.hunter@intel.com>,
	Kuppuswamy Sathyanarayanan
	<sathyanarayanan.kuppuswamy@linux.intel.com>,
	Elena Reshetova <elena.reshetova@intel.com>,
	Jun Nakajima <jun.nakajima@intel.com>,
	Rick Edgecombe <rick.p.edgecombe@intel.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	kexec@lists.infradead.org, linux-coco@lists.linux.dev,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 10/13] x86/tdx: Convert shared memory back to private on kexec
Date: Fri, 6 Oct 2023 17:15:37 -0500	[thread overview]
Message-ID: <ef24ca9a-383e-196d-fcfb-9b4f2e62d60a@amd.com> (raw)
In-Reply-To: <20231006151151.2m6t7fcu36hlrtzj@box.shutemov.name>

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

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