Re: [PATCH v5 6/7] KVM: SVM: Add support to initialize SEV/SNP functionality in KVM

Kalra, Ashish
2025-03-03T21:39:11Z

On 3/3/2025 2:49 PM, Sean Christopherson wrote:
> On Mon, Mar 03, 2025, Ashish Kalra wrote:
>> On 2/28/2025 4:32 PM, Sean Christopherson wrote:
>>> On Fri, Feb 28, 2025, Ashish Kalra wrote:
>>>> And the other consideration is that runtime setup of especially SEV-ES VMs will not
>>>> work if/when first SEV-ES VM is launched, if SEV INIT has not been issued at
>>>> KVM setup time.
>>>>
>>>> This is because qemu has a check for SEV INIT to have been done (via SEV platform
>>>> status command) prior to launching SEV-ES VMs via KVM_SEV_INIT2 ioctl.
>>>>
>>>> So effectively, __sev_guest_init() does not get invoked in case of launching
>>>> SEV_ES VMs, if sev_platform_init() has not been done to issue SEV INIT in
>>>> sev_hardware_setup().
>>>>
>>>> In other words the deferred initialization only works for SEV VMs and not SEV-ES VMs.
>>>
>>> In that case, I vote to kill off deferred initialization entirely, and commit to
>>> enabling all of SEV+ when KVM loads (which we should have done from day one).
>>> Assuming we can do that in a way that's compatible with the /dev/sev ioctls.
>>
>> Yes, that's what seems to be the right approach to enabling all SEV+ when KVM loads.
>>
>> For SEV firmware hotloading we will do implicit SEV Shutdown prior to DLFW_EX
>> and SEV (re)INIT after that to ensure that SEV is in UNINIT state before
>> DLFW_EX.
>>
>> We still probably want to keep the deferred initialization for SEV in
>> __sev_guest_init() by calling sev_platform_init() to support the SEV INIT_EX
>> case.
>
> Refresh me, how does INIT_EX fit into all of this? I.e. why does it need special
> casing?

For SEV INIT_EX, we need the filesystem to be up and running as the user-supplied
SEV related persistent data is read from a regular file and provided to the
INIT_EX command.

Now, with the modified SEV/SNP init flow, when SEV/SNP initialization is
performed during KVM module load, then as i believe the filesystem will be
mounted before KVM module loads, so SEV INIT_EX can be supported without
any issues.

Therefore, we don't need deferred initialization support for SEV INIT_EX
in case of KVM being loaded as a module.

But if KVM module is built-in, then filesystem will not be mounted when
SEV/SNP initialization is done during KVM initialization and in that case
SEV INIT_EX cannot be supported.

Therefore to support SEV INIT_EX when KVM module is built-in, the following
will need to be done:

- Boot kernel with psp_init_on_probe=false command line.
- This ensures that during KVM initialization, only SNP INIT is done.
- Later at runtime, when filesystem has already been mounted,
SEV VM launch will trigger deferred SEV (INIT_EX) initialization
(via the __sev_guest_init() -> sev_platform_init() code path).

NOTE: psp_init_on_probe module parameter and deferred SEV initialization
during SEV VM launch (__sev_guest_init()->sev_platform_init()) was added
specifically to support SEV INIT_EX case.

Thanks,
Ashish