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

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Date:   Thu, 31 Mar 2022 10:18:33 +0530
From:   "Nikunj A. Dadhania" <nikunj@....com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Brijesh Singh <brijesh.singh@....com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Peter Gonda <pgonda@...gle.com>,
        Bharata B Rao <bharata@....com>,
        "Maciej S . Szmigiero" <mail@...iej.szmigiero.name>,
        Mingwei Zhang <mizhang@...gle.com>,
        David Hildenbrand <david@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC v1 0/9] KVM: SVM: Defer page pinning for SEV guests



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