Re: [PATCH v3] x86/sev: Add support for allowing zero SEV ASIDs.

[Tom Lendacky <thomas.lendacky@amd.com>]

On 1/4/24 13:05, Ashish Kalra wrote:
> From: Ashish Kalra <ashish.kalra@amd.com>
> 
> Some BIOSes allow the end user to set the minimum SEV ASID value
> (CPUID 0x8000001F_EDX) to be greater than the maximum number of
> encrypted guests, or maximum SEV ASID value (CPUID 0x8000001F_ECX)
> in order to dedicate all the SEV ASIDs to SEV-ES or SEV-SNP.
> 
> The SEV support, as coded, does not handle the case where the minimum
> SEV ASID value can be greater than the maximum SEV ASID value.
> As a result, the following confusing message is issued:
> 
> [   30.715724] kvm_amd: SEV enabled (ASIDs 1007 - 1006)
> 
> Fix the support to properly handle this case.
> 
> Fixes: 916391a2d1dc ("KVM: SVM: Add support for SEV-ES capability in KVM")
> Suggested-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Ashish Kalra <ashish.kalra@amd.com>
> Cc: stable@vger.kernel.org

One minor comment below that could maybe be done when merging vs sending a 
another version? Otherwise...

Acked-by: Tom Lendacky <thomas.lendacky@amd.com>

> ---
>   arch/x86/kvm/svm/sev.c | 40 ++++++++++++++++++++++++----------------
>   1 file changed, 24 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 4900c078045a..2112c94bac76 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -143,8 +143,20 @@ static void sev_misc_cg_uncharge(struct kvm_sev_info *sev)
>   
>   static int sev_asid_new(struct kvm_sev_info *sev)
>   {
> -	int asid, min_asid, max_asid, ret;
> +	/*
> +	 * SEV-enabled guests must use asid from min_sev_asid to max_sev_asid.
> +	 * SEV-ES-enabled guest can use from 1 to min_sev_asid - 1.
> +	 * Note: min ASID can end up larger than the max if basic SEV support is
> +	 * effectively disabled by disallowing use of ASIDs for SEV guests.
> +	 */
> +	unsigned int min_asid = sev->es_active ? 1 : min_sev_asid;
> +	unsigned int max_asid = sev->es_active ? min_sev_asid - 1 : max_sev_asid;
> +	unsigned int asid;
>   	bool retry = true;
> +	int ret;
> +
> +	if (min_asid > max_asid)
> +		return -ENOTTY;
>   
>   	WARN_ON(sev->misc_cg);
>   	sev->misc_cg = get_current_misc_cg();
> @@ -157,12 +169,6 @@ static int sev_asid_new(struct kvm_sev_info *sev)
>   
>   	mutex_lock(&sev_bitmap_lock);
>   
> -	/*
> -	 * SEV-enabled guests must use asid from min_sev_asid to max_sev_asid.
> -	 * SEV-ES-enabled guest can use from 1 to min_sev_asid - 1.
> -	 */
> -	min_asid = sev->es_active ? 1 : min_sev_asid;
> -	max_asid = sev->es_active ? min_sev_asid - 1 : max_sev_asid;
>   again:
>   	asid = find_next_zero_bit(sev_asid_bitmap, max_asid + 1, min_asid);
>   	if (asid > max_asid) {
> @@ -246,21 +252,20 @@ static void sev_unbind_asid(struct kvm *kvm, unsigned int handle)
>   static int sev_guest_init(struct kvm *kvm, struct kvm_sev_cmd *argp)
>   {
>   	struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
> -	int asid, ret;
> +	int ret;
>   
>   	if (kvm->created_vcpus)
>   		return -EINVAL;
>   
> -	ret = -EBUSY;
>   	if (unlikely(sev->active))
> -		return ret;
> +		return -EINVAL;
>   
>   	sev->active = true;
>   	sev->es_active = argp->id == KVM_SEV_ES_INIT;
> -	asid = sev_asid_new(sev);
> -	if (asid < 0)
> +	ret = sev_asid_new(sev);
> +	if (ret < 0)
>   		goto e_no_asid;
> -	sev->asid = asid;
> +	sev->asid = ret;
>   
>   	ret = sev_platform_init(&argp->error);
>   	if (ret)
> @@ -2229,8 +2234,10 @@ void __init sev_hardware_setup(void)
>   		goto out;
>   	}
>   
> -	sev_asid_count = max_sev_asid - min_sev_asid + 1;
> -	WARN_ON_ONCE(misc_cg_set_capacity(MISC_CG_RES_SEV, sev_asid_count));
> +	if (min_sev_asid <= max_sev_asid) {
> +		sev_asid_count = max_sev_asid - min_sev_asid + 1;
> +		WARN_ON_ONCE(misc_cg_set_capacity(MISC_CG_RES_SEV, sev_asid_count));
> +	}
>   	sev_supported = true;
>   
>   	/* SEV-ES support requested? */
> @@ -2261,7 +2268,8 @@ void __init sev_hardware_setup(void)
>   out:
>   	if (boot_cpu_has(X86_FEATURE_SEV))
>   		pr_info("SEV %s (ASIDs %u - %u)\n",
> -			sev_supported ? "enabled" : "disabled",
> +			sev_supported ? (min_sev_asid <= max_sev_asid ?  "enabled" : "unusable")
> +			: "disabled",

Just a nit with the alignment, it would look better if the ":" was lined 
up under the first "?".

Thanks,
Tom

>   			min_sev_asid, max_sev_asid);
>   	if (boot_cpu_has(X86_FEATURE_SEV_ES))
>   		pr_info("SEV-ES %s (ASIDs %u - %u)\n",