x86/platform: Adjust temperature sensors MSRs

Temperature sensors MSR were previously assumed to be available when "DTS"
CPUID bit is set. This is not really the case :
 * MSR_IA32_THERM_STATUS is gated behind ACPI CPUID bit, only DTS-related bits
of this MSR are gated behind the DTS CPUID
 * MSR_PACKAGE_THERM_STATUS is gated behind "PTM" CPUID

Also adjust the MSR_TEMPERATURE_TARGET which is not architectural, but stable
in practice, and required to be exposed for reliably querying CPU temperature.

Fixes: 615c9f3f82 ("x86/platform: Expose DTS sensors MSR")
Reported-by: Tu Dinh <ngoc-tu.dinh@vates.tech>
Signed-off-by: Teddy Astie <teddy.astie@vates.tech>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/platform_hypercall.c

@@ -89,9 +89,26 @@ static bool msr_read_allowed(unsigned int msr)
         return cpu_has_srbds_ctrl;
 
     case MSR_IA32_THERM_STATUS:
+        return host_cpu_policy.basic.acpi;
+
+    /*
+     * This MSR is present on most Intel Core-family CPUs since Nehalem but is not an
+     * architectural MSR. No CPUID bit enumerates this MSR.
+     *
+     * This MSR exposes "temperature target" that is needed to compute the CPU
+     * temperature. The "temperature target" is a model specific value, and this MSR is
+     * the only known method of getting the one used for the CPU. On some CPU models with
+     * Intel SST-PP, the "temperature target" can vary over time.
+     *
+     * We assume all Intel CPUs with DTS may support this MSR; but reads can fail in case
+     * the platform doesn't actually support this MSR.
+     */
     case MSR_TEMPERATURE_TARGET:
+        return boot_cpu_data.x86_vendor == X86_VENDOR_INTEL &&
+               host_cpu_policy.basic.digital_temp_sensor;
+
     case MSR_PACKAGE_THERM_STATUS:
-        return host_cpu_policy.basic.digital_temp_sensor;
+        return host_cpu_policy.basic.pkg_therm_mgmt;
     }
 
     if ( ppin_msr && msr == ppin_msr )

xen/include/xen/lib/x86/cpu-policy.h

@@ -132,7 +132,7 @@ struct cpu_policy
                         :1,
                         :1,
                         :1,
-                        :1,
+                        pkg_therm_mgmt:1,
                         hwp:1,
                         hwp_interrupt:1,
                         hwp_activity_window:1,