[PATCH] x86/PAT: have pat_enabled() properly reflect state when running on e.g. Xen

[Jan Beulich <jbeulich@xxxxxxxx>]

The latest with commit bdd8b6c98239 ("drm/i915: replace X86_FEATURE_PAT
with pat_enabled()") pat_enabled() returning false (because of PAT
initialization being suppressed in the absence of MTRRs being announced
to be available) has become a problem: The i915 driver now fails to
initialize when running PV on Xen (i915_gem_object_pin_map() is where I
located the induced failure), and its error handling is flaky enough to
(at least sometimes) result in a hung system.

Yet even beyond that problem the keying of the use of WC mappings to
pat_enabled() (see arch_can_pci_mmap_wc()) means that in particular
graphics frame buffer accesses would have been quite a bit less
performant than possible.

Arrange for the function to return true in such environments, without
undermining the rest of PAT MSR management logic considering PAT to be
disabled: Specifically, no writes to the PAT MSR should occur.

For the new boolean to live in .init.data, init_cache_modes() also needs
moving to .init.text (where it could/should have lived already before).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
On the system where I observed the issue, a knock-on effect of driver
initialization failing was that the SATA-controller also started to
report failures.

--- a/arch/x86/mm/pat/memtype.c
+++ b/arch/x86/mm/pat/memtype.c
@@ -62,6 +62,7 @@
 
 static bool __read_mostly pat_bp_initialized;
 static bool __read_mostly pat_disabled = !IS_ENABLED(CONFIG_X86_PAT);
+static bool __initdata pat_force_disabled = !IS_ENABLED(CONFIG_X86_PAT);
 static bool __read_mostly pat_bp_enabled;
 static bool __read_mostly pat_cm_initialized;
 
@@ -86,6 +87,7 @@ void pat_disable(const char *msg_reason)
 static int __init nopat(char *str)
 {
        pat_disable("PAT support disabled via boot option.");
+       pat_force_disabled = true;
        return 0;
 }
 early_param("nopat", nopat);
@@ -272,7 +274,7 @@ static void pat_ap_init(u64 pat)
        wrmsrl(MSR_IA32_CR_PAT, pat);
 }
 
-void init_cache_modes(void)
+void __init init_cache_modes(void)
 {
        u64 pat = 0;
 
@@ -313,6 +315,13 @@ void init_cache_modes(void)
                 */
                pat = PAT(0, WB) | PAT(1, WT) | PAT(2, UC_MINUS) | PAT(3, UC) |
                      PAT(4, WB) | PAT(5, WT) | PAT(6, UC_MINUS) | PAT(7, UC);
+       } else if (!pat_force_disabled &&
+                  boot_cpu_has(X86_FEATURE_HYPERVISOR)) {
+               /*
+                * Clearly PAT is enabled underneath. Allow pat_enabled() to
+                * reflect this.
+                */
+               pat_bp_enabled = true;
        }
 
        __init_cache_modes(pat);