Commit 72cbc8f0 authored Apr 28, 2022 by Jan Beulich Committed by Borislav Petkov Aug 15, 2022

x86/PAT: Have pat_enabled() properly reflect state when running on Xen



After commit ID in the Fixes: tag, pat_enabled() returns false (because
of PAT initialization being suppressed in the absence of MTRRs being
announced to be available).

This 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 optimal
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).

  [ bp: This is the "small fix" variant for stable. It'll get replaced
    with a proper PAT and MTRR detection split upstream but that is too
    involved for a stable backport.
    - additional touchups to commit msg. Use cpu_feature_enabled(). ]

Fixes: bdd8b6c9 ("drm/i915: replace X86_FEATURE_PAT with pat_enabled()")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Ingo Molnar <mingo@kernel.org>
Cc: <stable@vger.kernel.org>
Cc: Juergen Gross <jgross@suse.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/9385fa60-fa5d-f559-a137-6608408f88b0@suse.com

parent 568035b0

arch/x86/mm/pat/memtype.c

+9 −1

Original line number	Original line	Diff line number	Diff line
	@@ -62,6 +62,7 @@

	static bool __read_mostly pat_bp_initialized;		static bool __read_mostly pat_bp_initialized;
	static bool __read_mostly pat_disabled = !IS_ENABLED(CONFIG_X86_PAT);		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_bp_enabled;
	static bool __read_mostly pat_cm_initialized;		static bool __read_mostly pat_cm_initialized;

	@@ -86,6 +87,7 @@ void pat_disable(const char *msg_reason)
	static int __init nopat(char *str)		static int __init nopat(char *str)
	{		{
	pat_disable("PAT support disabled via boot option.");		pat_disable("PAT support disabled via boot option.");
			pat_force_disabled = true;
	return 0;		return 0;
	}		}
	early_param("nopat", nopat);		early_param("nopat", nopat);
	@@ -272,7 +274,7 @@ static void pat_ap_init(u64 pat)
	wrmsrl(MSR_IA32_CR_PAT, pat);		wrmsrl(MSR_IA32_CR_PAT, pat);
	}		}

	void init_cache_modes(void)		void __init init_cache_modes(void)
	{		{
	u64 pat = 0;		u64 pat = 0;

	@@ -313,6 +315,12 @@ void init_cache_modes(void)
	*/		*/
	pat = PAT(0, WB) \| PAT(1, WT) \| PAT(2, UC_MINUS) \| PAT(3, UC) \|		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);		PAT(4, WB) \| PAT(5, WT) \| PAT(6, UC_MINUS) \| PAT(7, UC);
			} else if (!pat_force_disabled && cpu_feature_enabled(X86_FEATURE_HYPERVISOR)) {
			/*
			* Clearly PAT is enabled underneath. Allow pat_enabled() to
			* reflect this.
			*/
			pat_bp_enabled = true;
	}		}

	__init_cache_modes(pat);		__init_cache_modes(pat);