Re: [PATCH RFC v1 0/9] KVM: SVM: Defer page pinning for SEV guests

["Nikunj A. Dadhania" <nikunj@amd.com>]

On 4/1/2022 12:30 AM, Sean Christopherson wrote:
> On Thu, Mar 31, 2022, Peter Gonda wrote:
>> On Wed, Mar 30, 2022 at 10:48 PM Nikunj A. Dadhania <nikunj@amd.com> wrote:
>>> On 3/31/2022 1:17 AM, Sean Christopherson wrote:
>>>> On Wed, Mar 30, 2022, Nikunj A. Dadhania wrote:
>>>>> On 3/29/2022 2:30 AM, Sean Christopherson wrote:
>>>>>> Let me preface this by saying I generally like the idea and especially the
>>>>>> performance, but...
>>>>>>
>>>>>> I think we should abandon this approach in favor of committing all our resources
>>>>>> to fd-based private memory[*], which (if done right) will provide on-demand pinning
>>>>>> for "free".
>>>>>
>>>>> I will give this a try for SEV, was on my todo list.
>>>>>
>>>>>> I would much rather get that support merged sooner than later, and use
>>>>>> it as a carrot for legacy SEV to get users to move over to its new APIs, with a long
>>>>>> term goal of deprecating and disallowing SEV/SEV-ES guests without fd-based private
>>>>>> memory.
>>>>>
>>>>>> That would require guest kernel support to communicate private vs. shared,
>>>>>
>>>>> Could you explain this in more detail? This is required for punching hole for shared pages?
>>>>
>>>> Unlike SEV-SNP, which enumerates private vs. shared in the error code, SEV and SEV-ES
>>>> don't provide private vs. shared information to the host (KVM) on page fault.  And
>>>> it's even more fundamental then that, as SEV/SEV-ES won't even fault if the guest
>>>> accesses the "wrong" GPA variant, they'll silent consume/corrupt data.
>>>>
>>>> That means KVM can't support implicit conversions for SEV/SEV-ES, and so an explicit
>>>> hypercall is mandatory.  SEV doesn't even have a vendor-agnostic guest/host paravirt
>>>> ABI, and IIRC SEV-ES doesn't provide a conversion/map hypercall in the GHCB spec, so
>>>> running a SEV/SEV-ES guest under UPM would require the guest firmware+kernel to be
>>>> properly enlightened beyond what is required architecturally.
>>>>
>>>
>>> So with guest supporting KVM_FEATURE_HC_MAP_GPA_RANGE and host (KVM) supporting
>>> KVM_HC_MAP_GPA_RANGE hypercall, SEV/SEV-ES guest should communicate private/shared
>>> pages to the hypervisor, this information can be used to mark page shared/private.
>>
>> One concern here may be that the VMM doesn't know which guests have
>> KVM_FEATURE_HC_MAP_GPA_RANGE support and which don't. Only once the
>> guest boots does the guest tell KVM that it supports
>> KVM_FEATURE_HC_MAP_GPA_RANGE. If the guest doesn't we need to pin all
>> the memory before we run the guest to be safe to be safe.
> 
> Yep, that's a big reason why I view purging the existing SEV memory management as
> a long term goal.  The other being that userspace obviously needs to be updated to
> support UPM[*].   I suspect the only feasible way to enable this for SEV/SEV-ES
> would be to restrict it to new VM types that have a disclaimer regarding additional
> requirements.

For SEV/SEV-ES could we base demand pinning on my first RFC[*]. Those patches does not touch 
the core KVM flow. Moreover, it does not expect any guest/firmware changes.

[*] https://lore.kernel.org/kvm/20220118110621.62462-1-nikunj@amd.com/