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d="scan'208";a="117916674" From: Andrew Cooper To: Xen-devel CC: Andrew Cooper , Jan Beulich , Jan Beulich , =?UTF-8?q?Roger=20Pau=20Monn=C3=A9?= , Wei Liu Subject: [PATCH v2] x86/cpu-policy: Advertise MSR_ARCH_CAPS to guests by default Date: Wed, 26 Jul 2023 10:39:18 +0100 Message-ID: <20230726093918.2418025-1-andrew.cooper3@citrix.com> X-Mailer: git-send-email 2.30.2 In-Reply-To: <4025d346-848b-730e-38a3-de7763edcf39@suse.com> References: <4025d346-848b-730e-38a3-de7763edcf39@suse.com> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable X-ZohoMail-DKIM: pass (identity @citrix.com) X-ZM-MESSAGEID: 1690364397813100001 With xl/libxl now able to control the policy bits for MSR_ARCH_CAPS, it is safe to advertise to guests by default. In turn, we don't need the special case to expose details to dom0. This advertises MSR_ARCH_CAPS to guests on *all* Intel hardware, even if the register content ends up being empty. - Advertising ARCH_CAPS and not RSBA signals "retpoline is safe here and everywhere you might migrate to". This is important because it avoids = the guest kernel needing to rely on model checks. - Alternatively, levelling for safety across the Broadwell/Skylake divide requires advertising ARCH_CAPS and RSBA, meaning "retpoline not safe on some hardware you might migrate to". On Cascade Lake and later hardware, guests can now see RDCL_NO (not vulnera= ble to Meltdown) amongst others. This causes substantial performance improvements, as guests are no longer applying software mitigations in cases where they don't need to. Signed-off-by: Andrew Cooper Reviewed-by: Jan Beulich --- CC: Jan Beulich CC: Roger Pau Monn=C3=A9 CC: Wei Liu Not to go in before Roger's libxl changes... v2: * Extend the commit message. --- xen/arch/x86/cpu-policy.c | 11 ----------- xen/include/public/arch-x86/cpufeatureset.h | 2 +- 2 files changed, 1 insertion(+), 12 deletions(-) diff --git a/xen/arch/x86/cpu-policy.c b/xen/arch/x86/cpu-policy.c index f40eeb8be8dc..1f954d4e5940 100644 --- a/xen/arch/x86/cpu-policy.c +++ b/xen/arch/x86/cpu-policy.c @@ -888,17 +888,6 @@ void __init init_dom0_cpuid_policy(struct domain *d) if ( cpu_has_itsc ) p->extd.itsc =3D true; =20 - /* - * Expose the "hardware speculation behaviour" bits of ARCH_CAPS to do= m0, - * so dom0 can turn off workarounds as appropriate. Temporary, until = the - * domain policy logic gains a better understanding of MSRs. - */ - if ( is_hardware_domain(d) && cpu_has_arch_caps ) - { - p->feat.arch_caps =3D true; - p->arch_caps.raw =3D host_cpu_policy.arch_caps.raw; - } - /* Apply dom0-cpuid=3D command line settings, if provided. */ if ( dom0_cpuid_cmdline ) { diff --git a/xen/include/public/arch-x86/cpufeatureset.h b/xen/include/publ= ic/arch-x86/cpufeatureset.h index ce7407d6a10c..6d20810cb9d1 100644 --- a/xen/include/public/arch-x86/cpufeatureset.h +++ b/xen/include/public/arch-x86/cpufeatureset.h @@ -271,7 +271,7 @@ XEN_CPUFEATURE(AVX512_FP16, 9*32+23) /*A AVX512 FP16= instructions */ XEN_CPUFEATURE(IBRSB, 9*32+26) /*A IBRS and IBPB support (used by= Intel) */ XEN_CPUFEATURE(STIBP, 9*32+27) /*A STIBP */ XEN_CPUFEATURE(L1D_FLUSH, 9*32+28) /*S MSR_FLUSH_CMD and L1D flush. */ -XEN_CPUFEATURE(ARCH_CAPS, 9*32+29) /*!a IA32_ARCH_CAPABILITIES MSR */ +XEN_CPUFEATURE(ARCH_CAPS, 9*32+29) /*!A IA32_ARCH_CAPABILITIES MSR */ XEN_CPUFEATURE(CORE_CAPS, 9*32+30) /* IA32_CORE_CAPABILITIES MSR */ XEN_CPUFEATURE(SSBD, 9*32+31) /*A MSR_SPEC_CTRL.SSBD available */ =20 --=20 2.30.2