On 7/1/25 15:16, Ashish Kalra wrote:
> From: Ashish Kalra <ashish.kalra@xxxxxxx>
>
> 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+.
s/basically//
s/legacy//
s|SEV-ES+.|SEV-ES/SEV-SNP ASID ranges.|
> Ciphertext hiding further partitions the SEV-ES+ ASID space into SEV-ES
> and SEV-SNP.
Enabling ciphertext hiding further splits the SEV-ES/SEV-SNP ASID space
into separate ASID ranges for SEV-ES and SEV-SNP guests.
>
> Add new module parameter to the KVM module to enable Ciphertext hiding
s/Ciphertext/ciphertext/
> support and a user configurable system-wide maximum SNP ASID value. If
> the module parameter value is "max" then the complete SEV-ES+ ASID
s|SEV-ES+|SEV-ES/SEV-SNP|
> space is allocated to SEV-SNP guests.
>
> Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Ashish Kalra <ashish.kalra@xxxxxxx>
> ---
> .../admin-guide/kernel-parameters.txt | 19 ++++++
> arch/x86/kvm/svm/sev.c | 58 ++++++++++++++++++-
> 2 files changed, 76 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index ee0735c6b8e2..05e50c37969e 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -2942,6 +2942,25 @@
> (enabled). Disable by KVM if hardware lacks support
> for NPT.
>
> + kvm-amd.ciphertext_hiding_asids=
> + [KVM,AMD] Ciphertext hiding prevents host accesses from reading
> + the ciphertext of SNP guest private memory. Instead of reading
> + ciphertext, the host will see constant default values (0xff).
> + The SEV ASID space is split into legacy SEV and joint
s/legacy//
> + SEV-ES and SEV-SNP ASID space. Ciphertext hiding further
> + partitions the joint SEV-ES/SEV-SNP ASID space into separate
> + SEV-ES and SEV-SNP ASID ranges with the SEV-SNP ASID range
> + starting at 1. For SEV-ES/SEV-SNP guests the maximum ASID
> + available is MIN_SEV_ASID - 1 and MIN_SEV_ASID value is
s/and/where/
> + discovered by CPUID Fn8000_001F[EDX].
> +
> + Format: { <unsigned int> | "max" }
> + A non-zero value enables SEV-SNP CipherTextHiding feature and sets
s/CipherTextHiding feature/ciphertext hiding/
> + how many ASIDs are available for SEV-SNP guests.
the ASID range available for SEV-SNP guests.
> + A Value of "max" assigns all ASIDs available in the joint SEV-ES
> + and SEV-SNP ASID range to SNP guests and also effectively disables
s/and also effectively disables/, effectively disabling/
> + SEV-ES.
> +
> 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 89ce9e298201..16723b8e0e37 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -59,6 +59,11 @@ 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 char ciphertext_hiding_asids[16];
> +module_param_string(ciphertext_hiding_asids, ciphertext_hiding_asids,
> + sizeof(ciphertext_hiding_asids), 0444);
> +MODULE_PARM_DESC(ciphertext_hiding_asids, " Enable ciphertext hiding for SEV-SNP guests and set the number of ASIDs to use ('max' to use all available SEV-SNP ASIDs");
This reads a bit awkward, but I don't really have a suggestion for a
short, concise message.
> +
> #define AP_RESET_HOLD_NONE 0
> #define AP_RESET_HOLD_NAE_EVENT 1
> #define AP_RESET_HOLD_MSR_PROTO 2
> @@ -200,6 +205,9 @@ 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 ciphertext hiding is enabled, effectively disabling SEV-ES
> + * support.
> */
> if (min_asid > max_asid)
> return -ENOTTY;
> @@ -2913,10 +2921,46 @@ static bool is_sev_snp_initialized(void)
> return initialized;
> }
>
> +static bool check_and_enable_sev_snp_ciphertext_hiding(void)
> +{
> + unsigned int ciphertext_hiding_asid_nr = 0;
> +
> + if (!sev_is_snp_ciphertext_hiding_supported()) {
> + pr_warn("Module parameter ciphertext_hiding_asids specified but ciphertext hiding not supported or enabled\n");
s/or enabled//
> + return false;
> + }
> +
> + if (isdigit(ciphertext_hiding_asids[0])) {
> + if (kstrtoint(ciphertext_hiding_asids, 10, &ciphertext_hiding_asid_nr)) {
> + pr_warn("Module parameter ciphertext_hiding_asids (%s) invalid\n",
> + ciphertext_hiding_asids);
> + return false;
> + }
Add a blank line.
> + /* Do sanity checks on user-defined ciphertext_hiding_asids */
s/checks/check/
> + if (ciphertext_hiding_asid_nr >= min_sev_asid) {
> + pr_warn("Requested ciphertext hiding ASIDs (%u) exceeds or equals minimum SEV ASID (%u)\n",
Module parameter ciphertext_hiding_asids (%u) ...
> + ciphertext_hiding_asid_nr, min_sev_asid);
> + return false;
> + }
> + } else if (!strcmp(ciphertext_hiding_asids, "max")) {
> + ciphertext_hiding_asid_nr = min_sev_asid - 1;
> + } else {
> + pr_warn("Module parameter ciphertext_hiding_asids (%s) invalid\n",
> + ciphertext_hiding_asids);
> + return false;
> + }
> +
> + max_snp_asid = ciphertext_hiding_asid_nr;
> + min_sev_es_asid = max_snp_asid + 1;
> + pr_info("SEV-SNP ciphertext hiding enabled\n");
Add a blank line.
Thanks,
Tom
> + return true;
> +}
> +
> 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_ciphertext_hiding_enabled = false;
> bool sev_snp_supported = false;
> bool sev_es_supported = false;
> bool sev_supported = false;
> @@ -3014,6 +3058,14 @@ void __init sev_hardware_setup(void)
> min_sev_es_asid = min_snp_asid = 1;
> max_sev_es_asid = max_snp_asid = min_sev_asid - 1;
>
> + /*
> + * The ciphertext hiding feature partitions the joint SEV-ES/SEV-SNP
> + * ASID range into separate SEV-ES and SEV-SNP ASID ranges with
> + * the SEV-SNP ASID starting at 1.
> + */
> + if (ciphertext_hiding_asids[0])
> + snp_ciphertext_hiding_enabled = check_and_enable_sev_snp_ciphertext_hiding();
> +
> 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;
> @@ -3022,6 +3074,8 @@ void __init sev_hardware_setup(void)
> out:
> if (sev_enabled) {
> init_args.probe = true;
> + if (snp_ciphertext_hiding_enabled)
> + init_args.max_snp_asid = max_snp_asid;
> if (sev_platform_init(&init_args))
> sev_supported = sev_es_supported = sev_snp_supported = false;
> else if (sev_snp_supported)
> @@ -3036,7 +3090,9 @@ void __init sev_hardware_setup(void)
> min_sev_asid, max_sev_asid);
> if (boot_cpu_has(X86_FEATURE_SEV_ES))
> pr_info("SEV-ES %s (ASIDs %u - %u)\n",
> - str_enabled_disabled(sev_es_supported),
> + sev_es_supported ? min_sev_es_asid < min_sev_asid ? "enabled" :
> + "unusable" :
> + "disabled",
> min_sev_es_asid, max_sev_es_asid);
> if (boot_cpu_has(X86_FEATURE_SEV_SNP))
> pr_info("SEV-SNP %s (ASIDs %u - %u)\n",
