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Message-ID: <74421510-ac2f-4094-b388-566a5c55b07f@amd.com>
Date: Thu, 5 Jun 2025 14:18:09 -0500
From: "Kalra, Ashish" <ashish.kalra@....com>
To: Alexey Kardashevskiy <aik@....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 6/5/2025 1:32 AM, Alexey Kardashevskiy wrote:
> On 20/5/25 10:02, Ashish Kalra wrote:
>> From: Ashish Kalra <ashish.kalra@....com>
>>
>> Ciphertext hiding prevents host accesses from reading the ciphertext of
>> SNP guest private memory. Instead of reading ciphertext, the host reads
>> will see constant default values (0xff).
>>
>> The SEV ASID space is basically split into legacy SEV and SEV-ES+.
>> CipherTextHiding further partitions the SEV-ES+ ASID space into SEV-ES
>> and SEV-SNP.
>>
>> Add new module parameter to the KVM module to enable CipherTextHiding
>> support and a user configurable system-wide maximum SNP ASID value. If
>> the module parameter value is -1 then the ASID space is equally
>> divided between SEV-SNP and SEV-ES guests.
>>
>> Suggested-by: Sean Christopherson <seanjc@...gle.com>
>> Signed-off-by: Ashish Kalra <ashish.kalra@....com>
>> ---
>> .../admin-guide/kernel-parameters.txt | 10 ++++++
>> arch/x86/kvm/svm/sev.c | 31 +++++++++++++++++++
>> 2 files changed, 41 insertions(+)
>>
>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>> index 1e5e76bba9da..2cddb2b5c59d 100644
>> --- a/Documentation/admin-guide/kernel-parameters.txt
>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>> @@ -2891,6 +2891,16 @@
>> (enabled). Disable by KVM if hardware lacks support
>> for NPT.
>> + 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.
>
>
> Why in halves? 0 or max would make sense and I'd think the user wants all SEV-ES+ VMs be hidden by default so I'd name the parameter as no_hiding_nr_asids and make the default value of zero mean "every SEV-ES+ is hidden".
>
Actually 'max' sounds nice, but we still want to name it ciphertext_hiding_asids or something and keep the value relative to it, so probably it can be
a number or 'max'.
Thanks,
Ashish
> Or there is a downside of hiding all VMs?
>
>
>> +
>> kvm-arm.mode=
>> [KVM,ARM,EARLY] Select one of KVM/arm64's modes of
>> operation.
>> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
>> index 383db1da8699..68dcb13d98f2 100644
>> --- a/arch/x86/kvm/svm/sev.c
>> +++ b/arch/x86/kvm/svm/sev.c
>> @@ -59,6 +59,10 @@ static bool sev_es_debug_swap_enabled = true;
>> module_param_named(debug_swap, sev_es_debug_swap_enabled, bool, 0444);
>> static u64 sev_supported_vmsa_features;
>> +static int ciphertext_hiding_nr_asids;
>> +module_param(ciphertext_hiding_nr_asids, int, 0444);
>> +MODULE_PARM_DESC(max_snp_asid, " Number of ASIDs available for SEV-SNP guests when CipherTextHiding is enabled");
>> +
>> #define AP_RESET_HOLD_NONE 0
>> #define AP_RESET_HOLD_NAE_EVENT 1
>> #define AP_RESET_HOLD_MSR_PROTO 2
>> @@ -200,6 +204,9 @@ static int sev_asid_new(struct kvm_sev_info *sev, unsigned long vm_type)
>> /*
>> * The min ASID can end up larger than the max if basic SEV support is
>> * effectively disabled by disallowing use of ASIDs for SEV guests.
>> + * Similarly for SEV-ES guests the min ASID can end up larger than the
>> + * max when CipherTextHiding is enabled, effectively disabling SEV-ES
>> + * support.
>> */
>> if (min_asid > max_asid)
>> @@ -2955,6 +2962,7 @@ void __init sev_hardware_setup(void)
>> {
>> unsigned int eax, ebx, ecx, edx, sev_asid_count, sev_es_asid_count;
>> struct sev_platform_init_args init_args = {0};
>> + bool snp_cipher_text_hiding = false;
>> bool sev_snp_supported = false;
>> bool sev_es_supported = false;
>> bool sev_supported = false;
>> @@ -3052,6 +3060,27 @@ void __init sev_hardware_setup(void)
>> if (min_sev_asid == 1)
>> goto out;
>> + /*
>> + * The ASID space is basically split into legacy SEV and SEV-ES+.
>> + * CipherTextHiding feature further partitions the SEV-ES+ ASID space
>> + * into ASIDs for SEV-ES and SEV-SNP guests.
>> + */
>> + if (ciphertext_hiding_nr_asids && sev_is_snp_ciphertext_hiding_supported()) {
>> + /* Do sanity checks on user-defined ciphertext_hiding_nr_asids */
>> + if (ciphertext_hiding_nr_asids != -1 &&
>> + ciphertext_hiding_nr_asids >= min_sev_asid) {
>> + pr_info("ciphertext_hiding_nr_asids module parameter invalid, limiting SEV-SNP ASIDs to %d\n",
>> + min_sev_asid);
>> + ciphertext_hiding_nr_asids = min_sev_asid - 1;
>> + }
>> +
>> + min_sev_es_asid = ciphertext_hiding_nr_asids == -1 ? (min_sev_asid - 1) / 2 :
>> + ciphertext_hiding_nr_asids + 1;
>> + max_snp_asid = min_sev_es_asid - 1;
>> + snp_cipher_text_hiding = true;
>> + pr_info("SEV-SNP CipherTextHiding feature support enabled\n");
>
>
> Can do "init_args.snp_max_snp_asid = max_snp_asid;" here (as max_snp_asid seems to not change between here and next hunk) and drop snp_cipher_text_hiding. Thanks,
>
>> + }
>> +
>> sev_es_asid_count = min_sev_asid - 1;
>> WARN_ON_ONCE(misc_cg_set_capacity(MISC_CG_RES_SEV_ES, sev_es_asid_count));
>> sev_es_supported = true;
>> @@ -3092,6 +3121,8 @@ void __init sev_hardware_setup(void)
>> * Do both SNP and SEV initialization at KVM module load.
>> */
>> init_args.probe = true;
>> + if (snp_cipher_text_hiding)
>> + init_args.snp_max_snp_asid = max_snp_asid;
>> sev_platform_init(&init_args);
>> }
>>
>
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