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@xxxxxxx> 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@xxxxxxx/
