[RFC PATCH 01/22] x86/msr: MSR_PLATFORM_INFO shouldn't claim that turbo is programmable

[Edwin Török <edwin.torok@xxxxxxxxx>]

From: Edwin Török <edvin.torok@xxxxxxxxxx>

Xen forbids writes to the various turbo control MSRs, however MSR_PLATFORM_INFO 
claims that these MSRs are writable.
Override MSR_PLATFORM_INFO bits to indicate lack of support.

See Intel SDM Volume 4, 2.17.6 "MSRs Introduced in the Intel Xeon Scaslable 
Processor Family",
which describes that MSR_PLATFORM_INFO.[28] = 1 implies that 
MSR_TURBO_RATIO_LIMIT is R/W,
and similarly bit 29 for TDP control, and bit 30 for MSR_TEMPERATURE_TARGET.

These bits were not all present on earlier processors, however where missing 
the bits were reserved,
and when present they are always present in the same bits.

(Curiously bit 31 that Xen uses is not documented anywhere in this manual but a 
separate one).

Backport: 4.0+

Signed-off-by: Edwin Török <edvin.torok@xxxxxxxxxx>
---
 xen/arch/x86/cpu-policy.c            | 8 ++++++++
 xen/include/xen/lib/x86/cpu-policy.h | 5 ++++-
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/cpu-policy.c b/xen/arch/x86/cpu-policy.c
index 81e574390f..64c8857a61 100644
--- a/xen/arch/x86/cpu-policy.c
+++ b/xen/arch/x86/cpu-policy.c
@@ -407,6 +407,14 @@ static void __init calculate_host_policy(void)
     /* 0x000000ce  MSR_INTEL_PLATFORM_INFO */
     /* probe_cpuid_faulting() sanity checks presence of MISC_FEATURES_ENABLES 
*/
     p->platform_info.cpuid_faulting = cpu_has_cpuid_faulting;
+
+    /* Xen denies write access to turbo control MSRs, however natively the CPU 
may support them
+       and advertise those MSRs as writable by having bits 28 to 30 set to 1 
in MSR_PLATFORM_INFO.
+       Set these bits to 0 to avoid confusing guests on the availability of 
turbo controls.
+    */
+    p->platform_info.programmable_ratio_turbo = 0;
+    p->platform_info.programmable_tdp_turbo = 0;
+    p->platform_info.programmable_tj_offset = 0;
 }
 
 static void __init guest_common_max_feature_adjustments(uint32_t *fs)
diff --git a/xen/include/xen/lib/x86/cpu-policy.h 
b/xen/include/xen/lib/x86/cpu-policy.h
index bab3eecda6..70479689f2 100644
--- a/xen/include/xen/lib/x86/cpu-policy.h
+++ b/xen/include/xen/lib/x86/cpu-policy.h
@@ -339,7 +339,10 @@ struct cpu_policy
     union {
         uint32_t raw;
         struct {
-            uint32_t :31;
+            uint32_t :28;
+            bool programmable_ratio_turbo:1;
+            bool programmable_tdp_turbo:1;
+            bool programmable_tj_offset:1;
             bool cpuid_faulting:1;
         };
     } platform_info;
-- 
2.41.0