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[35.185.214.157]) by smtp.gmail.com with ESMTPSA id ij17-20020a17090af81100b001c67c964d93sm8557230pjb.2.2022.03.30.12.47.36 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 30 Mar 2022 12:47:36 -0700 (PDT) Date: Wed, 30 Mar 2022 19:47:33 +0000 From: Sean Christopherson To: "Nikunj A. Dadhania" Cc: Paolo Bonzini , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , Brijesh Singh , Tom Lendacky , Peter Gonda , Bharata B Rao , "Maciej S . Szmigiero" , Mingwei Zhang , David Hildenbrand , kvm@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH RFC v1 0/9] KVM: SVM: Defer page pinning for SEV guests Message-ID: References: <20220308043857.13652-1-nikunj@amd.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org 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.