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AJvYcCUXcbONzyoqOoDXV6Q7Y52329xny7f23yiU7qkSHkJql3F5irNR7VTHXslaDNVNUTjfh2Le7NmjOdxPSKxilEEpRCyT X-Gm-Message-State: AOJu0Yy02lc621wPfQIkgEgO+aYRuGk2NgA0MpIB6kId76U7RnQwy18V L1oF5lGrEEg5CFiuqN3tNVBvNTrn3a5FvdwoEFP5g5ota3dGC0EjxKZ1ADmzE7Ls2cVGzdPAIsQ qnw== X-Google-Smtp-Source: AGHT+IG/2rDr9URUPDl9FM8oMDrvH3jCXr+cSppLGOe0KQE9X5dKiZqG3sFVtvXlrLbhADZaZ6WgRYBXjd8= X-Received: from zagreus.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:5c37]) (user=seanjc job=sendgmr) by 2002:a0d:d147:0:b0:651:2eea:4dfe with SMTP id 00721157ae682-6a9df63c480mr655687b3.0.1723597839045; Tue, 13 Aug 2024 18:10:39 -0700 (PDT) Date: Tue, 13 Aug 2024 18:10:37 -0700 In-Reply-To: Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20240812224820.34826-1-rick.p.edgecombe@intel.com> <20240812224820.34826-26-rick.p.edgecombe@intel.com> Message-ID: Subject: Re: [PATCH 25/25] KVM: x86: Add CPUID bits missing from KVM_GET_SUPPORTED_CPUID From: Sean Christopherson To: Rick P Edgecombe Cc: Chao Gao , Kai Huang , Xiaoyao Li , "isaku.yamahata@gmail.com" , "linux-kernel@vger.kernel.org" , "tony.lindgren@linux.intel.com" , "kvm@vger.kernel.org" , "pbonzini@redhat.com" Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable On Tue, Aug 13, 2024, Rick P Edgecombe wrote: > On Tue, 2024-08-13 at 19:34 +0800, Chao Gao wrote: > > Mandating that all fixed-1 bits be supported by KVM would be a burden f= or both > > KVM and the TDX module: the TDX module couldn't add any fixed-1 bits un= til KVM > > supports them, and=C2=A0 >=20 > > KVM shouldn't drop any feature that was ever a fixed-1 bit > > in any TDX module. >=20 > Honest question...can/does this happen for normal VMs? KVM dropping suppo= rt for > features? Almost never. KVM still supports Intel CPUs without virtual NMI support, w= hich IIRC was something like one SKU of Yonah that was 32-bit only. Keeping bac= kwards compability is annoying from time to time, but it's generally not that much= of a maintenance burden. The only CPUs I really wish had never existed are thos= e that have EPT without A/D bits. Other than that, maintaining support for old CP= Us doesn't hinder us too much. > I think I recall even MPX getting limped along for backward compatibility= reasons. Yep, KVM still supports virtualizing MPX.