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

From: "Nikunj A. Dadhania" <nikunj@amd.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	Brijesh Singh <brijesh.singh@amd.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Peter Gonda <pgonda@google.com>, Bharata B Rao <bharata@amd.com>,
	"Maciej S . Szmigiero" <mail@maciej.szmigiero.name>,
	Mingwei Zhang <mizhang@google.com>,
	David Hildenbrand <david@redhat.com>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH RFC v1 0/9] KVM: SVM: Defer page pinning for SEV guests
Date: Thu, 31 Mar 2022 10:18:33 +0530	[thread overview]
Message-ID: <5567f4ec-bbcf-4caf-16c1-3621b77a1779@amd.com> (raw)
In-Reply-To: <YkSz1R3YuFszcZrY@google.com>

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.

Regards,
Nikunj