Re: [RFC PATCH v2 04/12] i386/sev: initialize SNP context

[Dov Murik]

On 05/09/2021 17:05, Brijesh Singh wrote:
> 
> On 9/5/21 4:19 AM, Dov Murik wrote:
>>
>> On 27/08/2021 1:26, Michael Roth wrote:
>>> From: Brijesh Singh <brijesh.singh@amd.com>
>>>
>>> When SEV-SNP is enabled, the KVM_SNP_INIT command is used to initialize
>>> the platform. The command checks whether SNP is enabled in the KVM, if
>>> enabled then it allocates a new ASID from the SNP pool and calls the
>>> firmware to initialize the all the resources.
>>>
>>
>> From the KVM code ("[PATCH Part2 v5 24/45] KVM: SVM: Add
>> KVM_SEV_SNP_LAUNCH_START command") it seems that KVM_SNP_INIT does *not*
>> allocate the ASID; actually this is done in KVM_SEV_SNP_LAUNCH_START.
> 
> Actually, the KVM_SNP_INIT does allocate the ASID. If you look at the
> driver code then in switch state, the SNP_INIT fallthrough to SEV_INIT
> which will call sev_guest_init(). The sev_guest_init() allocates a new
> ASID.
> https://github.com/AMDESE/linux/blob/bb9ba49cd9b749d5551aae295c091d8757153dd7/arch/x86/kvm/svm/sev.c#L255
> 
> The LAUNCH_START simply binds the ASID to a guest.

OK thank you for clearing this up.  So the kernel is choosing the new
ASID during the KVM_SNP_INIT ioctl, but doesn't "tell" the firmware
about it.  Then later in SNP_LAUNCH_START that integer (saved in the
kernel sev structure) is given to the firmware as an argument of the
SNP_LAUNCH_START (binding?).  Is this description correct?


-Dov