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Date: Wed, 3 Jan 2024 14:41:39 -0600
From: "Kalra, Ashish" <ashish.kalra@....com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: pbonzini@...hat.com, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
thomas.lendacky@....com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
joro@...tes.org
Subject: Re: [PATCH] x86/sev: Add support for allowing zero SEV ASIDs.
Hello Sean,
On 1/2/2024 6:30 PM, Sean Christopherson wrote:
> On Tue, Jan 02, 2024, Ashish Kalra wrote:
>> @@ -2172,8 +2176,10 @@ void sev_vm_destroy(struct kvm *kvm)
>>
>> void __init sev_set_cpu_caps(void)
>> {
>> - if (!sev_enabled)
>> + if (!sev_guests_enabled) {
> Ugh, what a mess. The module param will show sev_enabled=false, but the caps
> and CPUID will show SEV=true.
>
> And this is doubly silly because "sev_enabled" is never actually checked, e.g.
> if misc cgroup support is disabled, KVM_SEV_INIT will try to reclaim ASIDs and
> eventually fail with -EBUSY, which is super confusing to users.
But this is what we expect that KVM_SEV_INIT will fail. In this case,
sev_asid_new() will not actually
try to reclaim any ASIDs as sev_misc_cg_try_charge() will fail before
any ASID bitmap walking/reclamation and
return an error which will eventually return -EBUSY to the user.
>
> The other weirdness is that KVM can cause sev_enabled=false && sev_es_enabled=true,
> but if *userspace* sets sev_enabled=false then sev_es_enabled is also forced off.
But that is already the behavior without this patch applied.
>
> In other words, the least awful option seems to be to keep sev_enabled true :-(
>
>> kvm_cpu_cap_clear(X86_FEATURE_SEV);
>> + return;
> This is blatantly wrong, as it can result in KVM advertising SEV-ES if SEV is
> disabled by the user.
No, this ensures that we don't advertise any SEV capability if neither
SEV/SEV-ES or in future SNP is enabled.
>
>> + }
>> if (!sev_es_enabled)
>> kvm_cpu_cap_clear(X86_FEATURE_SEV_ES);
>> }
>> @@ -2229,9 +2235,11 @@ 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));
>> - sev_supported = true;
>> + 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? */
>> if (!sev_es_enabled)
>> @@ -2262,7 +2270,8 @@ void __init sev_hardware_setup(void)
>> if (boot_cpu_has(X86_FEATURE_SEV))
>> pr_info("SEV %s (ASIDs %u - %u)\n",
>> sev_supported ? "enabled" : "disabled",
>> - min_sev_asid, max_sev_asid);
>> + sev_supported ? min_sev_asid : 0,
>> + sev_supported ? max_sev_asid : 0);
> I honestly think we should print the "garbage" values. The whole point of
> printing the min/max SEV ASIDs was to help users understand why SEV is disabled,
> i.e. printing zeroes is counterproductive.
>
>> if (boot_cpu_has(X86_FEATURE_SEV_ES))
>> pr_info("SEV-ES %s (ASIDs %u - %u)\n",
>> sev_es_supported ? "enabled" : "disabled",
> It's all a bit gross, but I think we want something like this (I'm definitely
> open to suggestions though):
>
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index d0c580607f00..bfac6d17462a 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, 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.
> + */
> + 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;
>
This will still return -EBUSY to user. This check here or the failure
return from sev_misc_cg_try_charge() are quite similar in that sense.
My point is that the same is achieved quite cleanly with
sev_misc_cg_try_charge() too.
> 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) {
> @@ -2232,8 +2238,10 @@ 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? */
> @@ -2264,8 +2272,9 @@ 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",
> - min_sev_asid, max_sev_asid);
> + sev_supported ? (min_sev_asid <= max_sev_asid ? "enabled" : "unusable") : "disabled",
> + sev_supported ? min_sev_asid : 0,
> + sev_supported ? max_sev_asid : 0);
We are not showing min and max ASIDs for SEV as {0,0} with this patch as
sev_supported is true ?
Thanks, Ashish
> if (boot_cpu_has(X86_FEATURE_SEV_ES))
> pr_info("SEV-ES %s (ASIDs %u - %u)\n",
> sev_es_supported ? "enabled" : "disabled",
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