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

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Message-ID: <6532fa2e-53e7-454f-80a6-c88b70182a6e@amd.com>
Date: Mon, 2 Jun 2025 15:32:31 -0500
From: "Kalra, Ashish" <ashish.kalra@....com>
To: Dave Hansen <dave.hansen@...el.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 5/23/2025 12:29 PM, Dave Hansen wrote:
> 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.
> 
> 

Sure, i will work on improving the documentation.

Sean, looking forward to your review and feedback on these patch-series, especially the KVM side of patches.

Thanks,
Ashish