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

["Kalra, Ashish" <ashish.kalra@amd.com>]

On 1/3/2024 3:10 PM, Sean Christopherson wrote:

> On Wed, Jan 03, 2024, Ashish Kalra wrote:
>> 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.
> Please read what I wrote.  "if misc cgroup support is disabled", i.e. if
> CONFIG_CGROUP_MISC=n, then sev_misc_cg_try_charge() is a nop.
>
>>> 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.
> No, it does not.  There is an early return statement here that prevents KVM from
> invoking kvm_cpu_cap_clear() for X86_FEATURE_SEV_ES.  Do I think userspace will
> actually be tripped up by seeing SEV_ES without SEV?  No.  Is it unnecessarily
> confusing?  Yes.
>
>>>> +	}
>>>>    	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.
> Huh?  The above is obviously -ENOTTY, and I don't see anything in the call stack
> that will convert it to -EBUSY.

Actually, sev_asid_new() returning failure to sev_guest_init() will 
cause it to return -EBUSY to user.

Thanks, Ashish

>> 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.
> "Without additional effort" is not synonymous with "cleanly".  Relying on an
> accounting restriction that is completely orthogonal to basic functionality is
> not "clean".
>
>>>           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 ?
> Yes, and that is deliberate.  See this from above:
>
>   : 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.