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Tue, 14 Oct 2025 06:37:33 -0700 (PDT) X-Google-Smtp-Source: AGHT+IGahZnaCHbiP1ZAhAAulCTNUjphIuxxm7ZG6Z7vq8wAX20/lhsmcyI17LvVpgh6mHqGUJ2oVw== X-Received: by 2002:a05:600d:416a:b0:46e:3f75:da49 with SMTP id 5b1f17b1804b1-46fa9b11794mr176638175e9.37.1760449052742; Tue, 14 Oct 2025 06:37:32 -0700 (PDT) From: Paolo Bonzini To: qemu-devel@nongnu.org Cc: Richard Henderson , Jon Kohler , Pawan Gupta , Sean Christopherson , qemu-stable@nongnu.org Subject: [PULL 08/28] i386/kvm: Expose ARCH_CAP_FB_CLEAR when invulnerable to MDS Date: Tue, 14 Oct 2025 15:36:53 +0200 Message-ID: <20251014133713.1103695-8-pbonzini@redhat.com> X-Mailer: git-send-email 2.51.0 In-Reply-To: <20251014133540.1103268-1-pbonzini@redhat.com> References: <20251014133540.1103268-1-pbonzini@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=170.10.129.124; envelope-from=pbonzini@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @redhat.com) X-ZM-MESSAGEID: 1760449184384158500 Content-Type: text/plain; charset="utf-8" From: Jon Kohler Newer Intel hardware (Sapphire Rapids and higher) sets multiple MDS immunity bits in MSR_IA32_ARCH_CAPABILITIES but lacks the hardware-level MSR_ARCH_CAP_FB_CLEAR (bit 17): ARCH_CAP_MDS_NO ARCH_CAP_TAA_NO ARCH_CAP_PSDP_NO ARCH_CAP_FBSDP_NO ARCH_CAP_SBDR_SSDP_NO This prevents VMs with fb-clear=3Don from migrating from older hardware (Cascade Lake, Ice Lake) to newer hardware, limiting live migration capabilities. Note fb-clear was first introduced in v8.1.0 [1]. Expose MSR_ARCH_CAP_FB_CLEAR for MDS-invulnerable systems to enable seamless migration between hardware generations. Note: There is no impact when a guest migrates to newer hardware as the existing bit combinations already mark the host as MMIO-immune and disable FB_CLEAR operations in the kernel (see Linux's arch_cap_mmio_immune() and vmx_update_fb_clear_dis()). See kernel side discussion for [2] for additional context. [1] 22e1094ca82 ("target/i386: add support for FB_CLEAR feature") [2] https://patchwork.kernel.org/project/kvm/patch/20250401044931.793203-1-= jon@nutanix.com/ Cc: Pawan Gupta Suggested-by: Sean Christopherson Signed-off-by: Jon Kohler Link: https://lore.kernel.org/r/20251008202557.4141285-1-jon@nutanix.com Cc: qemu-stable@nongnu.org Signed-off-by: Paolo Bonzini --- target/i386/kvm/kvm.c | 17 +++++++++++++++++ 1 file changed, 17 insertions(+) diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c index db40caa3412..e40e374b046 100644 --- a/target/i386/kvm/kvm.c +++ b/target/i386/kvm/kvm.c @@ -653,6 +653,23 @@ uint64_t kvm_arch_get_supported_msr_feature(KVMState *= s, uint32_t index) must_be_one =3D (uint32_t)value; can_be_one =3D (uint32_t)(value >> 32); return can_be_one & ~must_be_one; + case MSR_IA32_ARCH_CAPABILITIES: + /* + * Special handling for fb-clear bit in ARCH_CAPABILITIES MSR. + * KVM will only report the bit if it is enabled in the host, + * but, for live migration capability purposes, we want to + * expose the bit to the guest even if it is disabled in the + * host, as long as the host itself is not vulnerable to + * the issue that the fb-clear bit is meant to mitigate. + */ + if ((value & MSR_ARCH_CAP_MDS_NO) && + (value & MSR_ARCH_CAP_TAA_NO) && + (value & MSR_ARCH_CAP_SBDR_SSDP_NO) && + (value & MSR_ARCH_CAP_FBSDP_NO) && + (value & MSR_ARCH_CAP_PSDP_NO)) { + value |=3D MSR_ARCH_CAP_FB_CLEAR; + } + return value; =20 default: return value; --=20 2.51.0