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charset="utf-8" A VMM may not expose ITS_NO or BHI_CTL, so guests cannot rely on those bits to determine whether they might be migrated to ITS-affected hardware. Rather than depending on a control that may be absent, detect ITS-unaffected hosts via a CPU feature that is exclusive to Sapphire Rapids and newer processors. Use X86_FEATURE_BUS_LOCK_DETECT as the canary: it is present on Sapphire Rapids+ parts and provides a reliable indicator that the guest won't be moved to ITS-affected hardware. This avoids false negatives caused by VMMs that omit ITS_NO or BHI_CTL. For example, QEMU added bhi-ctrl only in v9.2.0 [1], well after adding the Sapphire Rapids model in v8.0.0 [2]. [1] 10eaf9c0fb7 ("target/i386: Add more features enumerated by CPUID.7.2.ED= X") [2] 7eb061b06e9 ("i386: Add new CPU model SapphireRapids") Cc: Pawan Gupta Fixes: 159013a7ca18 ("x86/its: Enumerate Indirect Target Selection (ITS) bu= g") Signed-off-by: Jon Kohler --- v1->v2: Fix logic typo and checkpatch warning for Fixes line. .../admin-guide/hw-vuln/indirect-target-selection.rst | 5 +++-- arch/x86/kernel/cpu/common.c | 6 ++++-- 2 files changed, 7 insertions(+), 4 deletions(-) diff --git a/Documentation/admin-guide/hw-vuln/indirect-target-selection.rs= t b/Documentation/admin-guide/hw-vuln/indirect-target-selection.rst index d9ca64108d23..3cfe4b9f9bd0 100644 --- a/Documentation/admin-guide/hw-vuln/indirect-target-selection.rst +++ b/Documentation/admin-guide/hw-vuln/indirect-target-selection.rst @@ -98,8 +98,9 @@ Mitigation in guests ^^^^^^^^^^^^^^^^^^^^ All guests deploy ITS mitigation by default, irrespective of eIBRS enumera= tion and Family/Model of the guest. This is because eIBRS feature could be hidd= en -from a guest. One exception to this is when a guest enumerates BHI_DIS_S, = which -indicates that the guest is running on an unaffected host. +from a guest. One exception to this is when a guest enumerates BHI_DIS_S or +BUS_LOCK_DETECT, either of which indicates that the guest is running on an +unaffected host and would not be migratable to an affected host. =20 To prevent guests from unnecessarily deploying the mitigation on unaffected platforms, Intel has defined ITS_NO bit(62) in MSR IA32_ARCH_CAPABILITIES.= When diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c index c7d3512914ca..60fbfeba92e9 100644 --- a/arch/x86/kernel/cpu/common.c +++ b/arch/x86/kernel/cpu/common.c @@ -1361,9 +1361,11 @@ static bool __init vulnerable_to_its(u64 x86_arch_ca= p_msr) /* * If a VMM did not expose ITS_NO, assume that a guest could * be running on a vulnerable hardware or may migrate to such - * hardware. + * hardware, except in the situation where the guest is presented + * with a feature that only exists in non-vulnerable hardware. */ - if (boot_cpu_has(X86_FEATURE_HYPERVISOR)) + if (boot_cpu_has(X86_FEATURE_HYPERVISOR) && + !boot_cpu_has(X86_FEATURE_BUS_LOCK_DETECT)) return true; =20 if (cpu_matches(cpu_vuln_blacklist, ITS)) --=20 2.43.0