linux-kernel - Re: [PATCH v4 5/5] KVM: SEV: Add SEV-SNP CipherTextHiding support

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Message-ID: <f76327dd-b505-4a24-938c-5b917da9aff2@intel.com>
Date: Fri, 23 May 2025 10:29:48 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Ashish Kalra <Ashish.Kalra@....com>, corbet@....net, seanjc@...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,
 akpm@...ux-foundation.org, paulmck@...nel.org, rostedt@...dmis.org
Cc: x86@...nel.org, thuth@...hat.com, ardb@...nel.org,
 gregkh@...uxfoundation.org, john.allen@....com, davem@...emloft.net,
 thomas.lendacky@....com, michael.roth@....com, kvm@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
 linux-doc@...r.kernel.org
Subject: Re: [PATCH v4 5/5] KVM: SEV: Add SEV-SNP CipherTextHiding support

On 5/19/25 17:02, Ashish Kalra wrote:
> +	kvm-amd.ciphertext_hiding_nr_asids=
> +			[KVM,AMD] Enables SEV-SNP CipherTextHiding feature and
> +			controls show many ASIDs are available for SEV-SNP guests.
> +			The ASID space is basically split into legacy SEV and
> +			SEV-ES+. CipherTextHiding feature further splits the
> +			SEV-ES+ ASID space into SEV-ES and SEV-SNP.
> +			If the value is -1, then it is used as an auto flag
> +			and splits the ASID space equally between SEV-ES and
> +			SEV-SNP ASIDs.

This help text isn't great.

It doesn't come out and say what the connection between CipherTextHiding
and SEV-ES+. It's also impossible to choose a good number without
knowing how large the ASID space is in the first place.

Why use "-1"? Why not just take "auto" as a parameter?

It also needs to say what CipherTextHiding is in the first place and be
more clear about what the tradeoffs are from enabling this.