linux-kernel - Re: [PATCH v2 3/3] x86/sev: Add SEV-SNP CipherTextHiding support

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Message-ID: <Z1eZmXmC9oZ5RyPc@google.com>
Date: Mon, 9 Dec 2024 17:30:01 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Ashish Kalra <ashish.kalra@....com>
Cc: Tom Lendacky <thomas.lendacky@....com>, Peter Gonda <pgonda@...gle.com>, pbonzini@...hat.com, 
	tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, 
	dave.hansen@...ux.intel.com, hpa@...or.com, herbert@...dor.apana.org.au, 
	x86@...nel.org, john.allen@....com, davem@...emloft.net, michael.roth@....com, 
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-crypto@...r.kernel.org
Subject: Re: [PATCH v2 3/3] x86/sev: Add SEV-SNP CipherTextHiding support

On Fri, Dec 06, 2024, Ashish Kalra wrote:
> On 12/6/2024 4:30 PM, Sean Christopherson wrote:
> >> This can reuse the current support (in KVM) to do SEV INIT implicitly when
> >> the first SEV VM is run: sev_guest_init() -> sev_platform_init() 
> > 
> > I don't love the implicit behavior, but assuming hotloading firmware can't be done
> > after SEV_CMD_INIT{_EX}, that does seem like the least awful solution.
> > 
> > To summarize, if the above assumptions hold:
> > 
> >  1. Initialize SNP when kvm-amd.ko is loaded.
> >  2. Define CipherTextHiding and ASID params kvm-amd.ko.
> >  3. Initialize SEV+ at first use.
> 
> Yes, the above summary is correct except for (3).

Heh, that wasn't a statement of fast, it was a suggestion for a possible
implementation.

> The initial set of patches will initialize SNP and SEV both at kvm-amd.ko module load,
> similar to PSP module load/probe time.

Why?  If SEV+ is initialized at kvm-amd.ko load, doesn't that prevent firmware
hotloading?

> For backward compatibility, the PSP module parameter psp_init_on_probe will still be
> supported, i believe it is used for INIT_EX support.

Again, why?  If the only use of psp_init_on_probe is to _disable_ that behavior,
and we make the code never init-on-probe, then the param is unnecessary, no?

> > Just to triple check: that will allow firmware hotloading even if kvm-amd.ko is
> > built-in, correct?  I.e. doesn't requires deferring kvm-amd.ko load until after
> > firmware hotloading.
> 
> Yes, this should work, for supporting firmware hotloading, the PSP driver's
> psp_init_on_probe parameter will need to be set to false, which will ensure
> that SEV INIT is not done during SEV/SNP platform initialization at KVM module
> probe time and instead it will be done implicitly at first SEV/SEV-ES VM launch.

Please no.  I really, really don't want gunk like this in KVM:

	init_args.probe = false;
	ret = sev_platform_init(&init_args);

That's inscrutable without a verbose comment, and all kinds of ugly.  Why can't
we simply separate SNP initialization from SEV+ initialization?