Merge tag 'x86_misc_for_v5.10' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull misc x86 fixes fromm Borislav Petkov:

 - Ratelimit the message about writes to unrecognized MSRs so that they
   don't spam the console log (Chris Down)

 - Document how the /proc/cpuinfo machinery works for future reference
   (Kyung Min Park, Ricardo Neri and Dave Hansen)

 - Correct the current NMI's duration calculation (Libing Zhou)

* tag 'x86_misc_for_v5.10' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/nmi: Fix nmi_handle() duration miscalculation
  Documentation/x86: Add documentation for /proc/cpuinfo feature flags
  x86/msr: Make source of unrecognised MSR writes unambiguous
  x86/msr: Prevent userspace MSR access from dominating the console

Documentation/x86/cpuinfo.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+=================
+x86 Feature Flags
+=================
+
+Introduction
+============
+
+On x86, flags appearing in /proc/cpuinfo have an X86_FEATURE definition
+in arch/x86/include/asm/cpufeatures.h. If the kernel cares about a feature
+or KVM want to expose the feature to a KVM guest, it can and should have
+an X86_FEATURE_* defined. These flags represent hardware features as
+well as software features.
+
+If users want to know if a feature is available on a given system, they
+try to find the flag in /proc/cpuinfo. If a given flag is present, it
+means that the kernel supports it and is currently making it available.
+If such flag represents a hardware feature, it also means that the
+hardware supports it.
+
+If the expected flag does not appear in /proc/cpuinfo, things are murkier.
+Users need to find out the reason why the flag is missing and find the way
+how to enable it, which is not always easy. There are several factors that
+can explain missing flags: the expected feature failed to enable, the feature
+is missing in hardware, platform firmware did not enable it, the feature is
+disabled at build or run time, an old kernel is in use, or the kernel does
+not support the feature and thus has not enabled it. In general, /proc/cpuinfo
+shows features which the kernel supports. For a full list of CPUID flags
+which the CPU supports, use tools/arch/x86/kcpuid.
+
+How are feature flags created?
+==============================
+
+a: Feature flags can be derived from the contents of CPUID leaves.
+------------------------------------------------------------------
+These feature definitions are organized mirroring the layout of CPUID
+leaves and grouped in words with offsets as mapped in enum cpuid_leafs
+in cpufeatures.h (see arch/x86/include/asm/cpufeatures.h for details).
+If a feature is defined with a X86_FEATURE_<name> definition in
+cpufeatures.h, and if it is detected at run time, the flags will be
+displayed accordingly in /proc/cpuinfo. For example, the flag "avx2"
+comes from X86_FEATURE_AVX2 in cpufeatures.h.
+
+b: Flags can be from scattered CPUID-based features.
+----------------------------------------------------
+Hardware features enumerated in sparsely populated CPUID leaves get
+software-defined values. Still, CPUID needs to be queried to determine
+if a given feature is present. This is done in init_scattered_cpuid_features().
+For instance, X86_FEATURE_CQM_LLC is defined as 11*32 + 0 and its presence is
+checked at runtime in the respective CPUID leaf [EAX=f, ECX=0] bit EDX[1].
+
+The intent of scattering CPUID leaves is to not bloat struct
+cpuinfo_x86.x86_capability[] unnecessarily. For instance, the CPUID leaf
+[EAX=7, ECX=0] has 30 features and is dense, but the CPUID leaf [EAX=7, EAX=1]
+has only one feature and would waste 31 bits of space in the x86_capability[]
+array. Since there is a struct cpuinfo_x86 for each possible CPU, the wasted
+memory is not trivial.
+
+c: Flags can be created synthetically under certain conditions for hardware features.
+-------------------------------------------------------------------------------------
+Examples of conditions include whether certain features are present in
+MSR_IA32_CORE_CAPS or specific CPU models are identified. If the needed
+conditions are met, the features are enabled by the set_cpu_cap or
+setup_force_cpu_cap macros. For example, if bit 5 is set in MSR_IA32_CORE_CAPS,
+the feature X86_FEATURE_SPLIT_LOCK_DETECT will be enabled and
+"split_lock_detect" will be displayed. The flag "ring3mwait" will be
+displayed only when running on INTEL_FAM6_XEON_PHI_[KNL|KNM] processors.
+
+d: Flags can represent purely software features.
+------------------------------------------------
+These flags do not represent hardware features. Instead, they represent a
+software feature implemented in the kernel. For example, Kernel Page Table
+Isolation is purely software feature and its feature flag X86_FEATURE_PTI is
+also defined in cpufeatures.h.
+
+Naming of Flags
+===============
+
+The script arch/x86/kernel/cpu/mkcapflags.sh processes the
+#define X86_FEATURE_<name> from cpufeatures.h and generates the
+x86_cap/bug_flags[] arrays in kernel/cpu/capflags.c. The names in the
+resulting x86_cap/bug_flags[] are used to populate /proc/cpuinfo. The naming
+of flags in the x86_cap/bug_flags[] are as follows:
+
+a: The name of the flag is from the string in X86_FEATURE_<name> by default.
+----------------------------------------------------------------------------
+By default, the flag <name> in /proc/cpuinfo is extracted from the respective
+X86_FEATURE_<name> in cpufeatures.h. For example, the flag "avx2" is from
+X86_FEATURE_AVX2.
+
+b: The naming can be overridden.
+--------------------------------
+If the comment on the line for the #define X86_FEATURE_* starts with a
+double-quote character (""), the string inside the double-quote characters
+will be the name of the flags. For example, the flag "sse4_1" comes from
+the comment "sse4_1" following the X86_FEATURE_XMM4_1 definition.
+
+There are situations in which overriding the displayed name of the flag is
+needed. For instance, /proc/cpuinfo is a userspace interface and must remain
+constant. If, for some reason, the naming of X86_FEATURE_<name> changes, one
+shall override the new naming with the name already used in /proc/cpuinfo.
+
+c: The naming override can be "", which means it will not appear in /proc/cpuinfo.
+----------------------------------------------------------------------------------
+The feature shall be omitted from /proc/cpuinfo if it does not make sense for
+the feature to be exposed to userspace. For example, X86_FEATURE_ALWAYS is
+defined in cpufeatures.h but that flag is an internal kernel feature used
+in the alternative runtime patching functionality. So, its name is overridden
+with "". Its flag will not appear in /proc/cpuinfo.
+
+Flags are missing when one or more of these happen
+==================================================
+
+a: The hardware does not enumerate support for it.
+--------------------------------------------------
+For example, when a new kernel is running on old hardware or the feature is
+not enabled by boot firmware. Even if the hardware is new, there might be a
+problem enabling the feature at run time, the flag will not be displayed.
+
+b: The kernel does not know about the flag.
+-------------------------------------------
+For example, when an old kernel is running on new hardware.
+
+c: The kernel disabled support for it at compile-time.
+------------------------------------------------------
+For example, if 5-level-paging is not enabled when building (i.e.,
+CONFIG_X86_5LEVEL is not selected) the flag "la57" will not show up [#f1]_.
+Even though the feature will still be detected via CPUID, the kernel disables
+it by clearing via setup_clear_cpu_cap(X86_FEATURE_LA57).
+
+d: The feature is disabled at boot-time.
+----------------------------------------
+A feature can be disabled either using a command-line parameter or because
+it failed to be enabled. The command-line parameter clearcpuid= can be used
+to disable features using the feature number as defined in
+/arch/x86/include/asm/cpufeatures.h. For instance, User Mode Instruction
+Protection can be disabled using clearcpuid=514. The number 514 is calculated
+from #define X86_FEATURE_UMIP (16*32 + 2).
+
+In addition, there exists a variety of custom command-line parameters that
+disable specific features. The list of parameters includes, but is not limited
+to, nofsgsbase, nosmap, and nosmep. 5-level paging can also be disabled using
+"no5lvl". SMAP and SMEP are disabled with the aforementioned parameters,
+respectively.
+
+e: The feature was known to be non-functional.
+----------------------------------------------
+The feature was known to be non-functional because a dependency was
+missing at runtime. For example, AVX flags will not show up if XSAVE feature
+is disabled since they depend on XSAVE feature. Another example would be broken
+CPUs and them missing microcode patches. Due to that, the kernel decides not to
+enable a feature.
+
+.. [#f1] 5-level paging uses linear address of 57 bits.

Documentation/x86/index.rst

@@ -9,6 +9,7 @@ x86-specific Documentation
    :numbered:
 
    boot
+   cpuinfo
    topology
    exception-tables
    kernel-stacks

arch/x86/kernel/msr.c

@@ -80,18 +80,30 @@ static ssize_t msr_read(struct file *file, char __user *buf,
 
 static int filter_write(u32 reg)
 {
+	/*
+	 * MSRs writes usually happen all at once, and can easily saturate kmsg.
+	 * Only allow one message every 30 seconds.
+	 *
+	 * It's possible to be smarter here and do it (for example) per-MSR, but
+	 * it would certainly be more complex, and this is enough at least to
+	 * avoid saturating the ring buffer.
+	 */
+	static DEFINE_RATELIMIT_STATE(fw_rs, 30 * HZ, 1);
+
 	switch (allow_writes) {
 	case MSR_WRITES_ON:  return 0;
 	case MSR_WRITES_OFF: return -EPERM;
 	default: break;
 	}
 
+	if (!__ratelimit(&fw_rs))
+		return 0;
+
 	if (reg == MSR_IA32_ENERGY_PERF_BIAS)
 		return 0;
 
-	pr_err_ratelimited("Write to unrecognized MSR 0x%x by %s\n"
-			   "Please report to x86@kernel.org\n",
-			   reg, current->comm);
+	pr_err("Write to unrecognized MSR 0x%x by %s (pid: %d). Please report to x86@kernel.org.\n",
+	       reg, current->comm, current->pid);
 
 	return 0;
 }

arch/x86/kernel/nmi.c

@@ -102,7 +102,6 @@ fs_initcall(nmi_warning_debugfs);
 
 static void nmi_check_duration(struct nmiaction *action, u64 duration)
 {
-	u64 whole_msecs = READ_ONCE(action->max_duration);
 	int remainder_ns, decimal_msecs;
 
 	if (duration < nmi_longest_ns || duration < action->max_duration)
@@ -110,12 +109,12 @@ static void nmi_check_duration(struct nmiaction *action, u64 duration)
 
 	action->max_duration = duration;
 
-	remainder_ns = do_div(whole_msecs, (1000 * 1000));
+	remainder_ns = do_div(duration, (1000 * 1000));
 	decimal_msecs = remainder_ns / 1000;
 
 	printk_ratelimited(KERN_INFO
 		"INFO: NMI handler (%ps) took too long to run: %lld.%03d msecs\n",
-		action->handler, whole_msecs, decimal_msecs);
+		action->handler, duration, decimal_msecs);
 }
 
 static int nmi_handle(unsigned int type, struct pt_regs *regs)