File xsa435-0-08.patch of Package xen.33138

From 5bd2b82df28cb7390f5ffb00fac635d0b9e36674 Mon Sep 17 00:00:00 2001
From: Andrew Cooper <andrew.cooper3@citrix.com>
Date: Tue, 14 Dec 2021 16:53:36 +0000
Subject: x86/cpuid: Introduce dom0-cpuid command line option

Specifically, this lets the user opt in to non-default features.

Collect all dom0 settings together in dom0_{en,dis}able_feat[], and apply it
to dom0's policy when other tweaks are being made.

As recalculate_cpuid_policy() is an expensive action, and dom0-cpuid= is
likely to only be used by the x86 maintainers for development purposes, forgo
the recalculation in the general case.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -717,6 +717,22 @@ Controls for how dom0 is constructed on
     restore the pre-4.13 behaviour.  If specifying `no-cpuid-faulting` fixes
     an issue in dom0, please report a bug.
 
+### dom0-cpuid
+    = List of comma separated booleans
+
+    Applicability: x86
+
+This option allows for fine tuning of the facilities dom0 will use, after
+accounting for hardware capabilities and Xen settings as enumerated via CPUID.
+
+Options are accepted in positive and negative form, to enable or disable
+specific features.  All selections via this mechanism are subject to normal
+CPU Policy safety and dependency logic.
+
+This option is intended for developers to opt dom0 into non-default features,
+and is not intended for use in production circumstances.  If using this option
+is necessary to fix an issue, please report a bug.
+
 ### dom0-iommu
     = List of [ passthrough=<bool>, strict=<bool>, map-inclusive=<bool>,
                 map-reserved=<bool>, none ]
--- a/xen/arch/x86/cpuid.c
+++ b/xen/arch/x86/cpuid.c
@@ -108,6 +108,24 @@ static int __init parse_xen_cpuid(const
 }
 custom_param("cpuid", parse_xen_cpuid);
 
+static bool __initdata dom0_cpuid_cmdline;
+static uint32_t __initdata dom0_enable_feat[FSCAPINTS];
+static uint32_t __initdata dom0_disable_feat[FSCAPINTS];
+
+static void __init _parse_dom0_cpuid(unsigned int feat, bool val)
+{
+    __set_bit  (feat, val ? dom0_enable_feat  : dom0_disable_feat);
+    __clear_bit(feat, val ? dom0_disable_feat : dom0_enable_feat );
+}
+
+static int __init parse_dom0_cpuid(const char *s)
+{
+    dom0_cpuid_cmdline = true;
+
+    return parse_cpuid(s, _parse_dom0_cpuid);
+}
+custom_param("dom0-cpuid", parse_dom0_cpuid);
+
 #define EMPTY_LEAF ((struct cpuid_leaf){})
 static void zero_leaves(struct cpuid_leaf *l,
                         unsigned int first, unsigned int last)
@@ -682,6 +700,25 @@ void __init init_dom0_cpuid_policy(struc
      */
     if ( cpu_has_arch_caps )
         p->feat.arch_caps = true;
+
+    /* Apply dom0-cpuid= command line settings, if provided. */
+    if ( dom0_cpuid_cmdline )
+    {
+        uint32_t fs[FSCAPINTS];
+        unsigned int i;
+
+        cpuid_policy_to_featureset(p, fs);
+
+        for ( i = 0; i < ARRAY_SIZE(fs); ++i )
+        {
+            fs[i] |=  dom0_enable_feat [i];
+            fs[i] &= ~dom0_disable_feat[i];
+        }
+
+        cpuid_featureset_to_policy(fs, p);
+
+        recalculate_cpuid_policy(d);
+    }
 }
 
 void guest_cpuid(const struct vcpu *v, uint32_t leaf,