Re: [PATCH v5 7/7] KVM: SEV: Add SEV-SNP CipherTextHiding support

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

On 7/2/2025 5:43 PM, Kalra, Ashish wrote:
> Hello Kim,
> 
> On 7/2/2025 4:46 PM, Kim Phillips wrote:
>> Hi Ashish,
>>
>> I can confirm that this v5 series fixes v4's __sev_do_cmd_locked
>> assertion failure problem, thanks.  More comments inline:
>>
>> On 7/1/25 3:16 PM, Ashish Kalra wrote:
>>> From: Ashish Kalra <ashish.kalra@amd.com>
>>
>> Extra From: line not necessary.
>>
>>> @@ -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");
>>> +        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;
>>> +        }
>>> +        /* Do sanity checks on user-defined ciphertext_hiding_asids */
>>> +        if (ciphertext_hiding_asid_nr >= min_sev_asid) {
>>> +            pr_warn("Requested ciphertext hiding ASIDs (%u) exceeds or equals minimum SEV ASID (%u)\n",
>>> +                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;
>>> +    }
>>
>> This code can be made much simpler if all the invalid
>> cases were combined to emit a single pr_warn().
>>
> 
> There definitely has to be a different pr_warn() for the sanity check case and invalid parameter cases and sanity check has to be done if the
> specified parameter is an unsigned int, so the check needs to be done separately.
> 
> I can definitely add a branch just for the invalid cases.
> 
>>> @@ -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",
>>
>> If I set ciphertext_hiding_asids=99, I get the new 'unusable':
>>
>> kvm_amd: SEV-SNP ciphertext hiding enabled
>> ...
>> kvm_amd: SEV enabled (ASIDs 100 - 1006)
>> kvm_amd: SEV-ES unusable (ASIDs 100 - 99)
>> kvm_amd: SEV-SNP enabled (ASIDs 1 - 99)
>>
>> Ok.
> 
> Which is correct. 
> 
> This is similar to the SEV case where min_sev_asid can be greater than max_sev_asid and that also emits similarly : 
> SEV unusable (ASIDs 1007 - 1006) (this is an example of that case).
> 

Also do note that the message above is printing the exact values of min_sev_es_asid and max_sev_es_asid, as they have been computed.

And it adds that SEV-ES is now unusable as now min_sev_es_asid > max_sev_es_asid.

>>
>> Now, if I set ciphertext_hiding_asids=0, I get:
>>
>> kvm_amd: SEV-SNP ciphertext hiding enabled
>> ...
>> kvm_amd: SEV enabled (ASIDs 100 - 1006)
>> kvm_amd: SEV-ES enabled (ASIDs 1 - 99)
>> kvm_amd: SEV-SNP enabled (ASIDs 1 - 0)
>>
>> ..where SNP is unusable this time, yet it's not flagged as such.
>>
> 
> Actually SNP still needs to be usable/enabled in this case, as specifying ciphertext_hiding_asids=0 is same as specifying that ciphertext hiding feature should
> not be enabled, so code-wise this is behaving correctly, but messaging needs to be fixed, which i will fix.
> 

And i do need to fix this case for ciphertext_hiding_asids==0, i.e., ciphertext hiding feature is not enabled, as the above is not functioning correctly.

Thanks,
Ashish

> 
>> If there's no difference between "unusable" and not enabled, then
>> I think it's better to keep the not enabled messaging behaviour
>> and just not emit the line at all:  It's confusing to see the
>> invalid "100 - 99" and "1 - 0" ranges.
>>
>> Thanks,
>>
>> Kim
>