[PATCH v2 2/8] x86/mtrr: support setting MTRR state for software defined MTRRs

[Juergen Gross]

When running virtualized, MTRR access can be reduced (e.g. in Xen PV
guests or when running as a SEV-SNP guest under Hyper-V). Typically
the hypervisor will reset the MTRR feature in cpuid data, resulting
in no MTRR memory type information being available for the kernel.

This has turned out to result in problems:

- Hyper-V SEV-SNP guests using uncached mappings where they shouldn't
- Xen PV dom0 mapping memory as WB which should be UC- instead

Solve those problems by supporting to set a fixed MTRR state,
overwriting the empty state used today. In case such a state has been
set, don't call get_mtrr_state() in mtrr_bp_init(). The set state
will only be used by mtrr_type_lookup(), as in all other cases
mtrr_enabled() is being checked, which will return false. Accept the
overwrite call only in case of MTRRs being disabled in cpuid.

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
V2:
- new patch
---
 arch/x86/include/asm/mtrr.h        |  2 ++
 arch/x86/kernel/cpu/mtrr/generic.c | 38 ++++++++++++++++++++++++++++++
 arch/x86/kernel/cpu/mtrr/mtrr.c    |  9 +++++++
 3 files changed, 49 insertions(+)

diff --git a/arch/x86/include/asm/mtrr.h b/arch/x86/include/asm/mtrr.h
index f0eeaf6e5f5f..0b8f51d683dc 100644
--- a/arch/x86/include/asm/mtrr.h
+++ b/arch/x86/include/asm/mtrr.h
@@ -31,6 +31,8 @@
  */
 # ifdef CONFIG_MTRR
 void mtrr_bp_init(void);
+void mtrr_overwrite_state(struct mtrr_var_range *var, unsigned int num_var,
+			  mtrr_type *fixed, mtrr_type def_type);
 extern u8 mtrr_type_lookup(u64 addr, u64 end, u8 *uniform);
 extern void mtrr_save_fixed_ranges(void *);
 extern void mtrr_save_state(void);
diff --git a/arch/x86/kernel/cpu/mtrr/generic.c b/arch/x86/kernel/cpu/mtrr/generic.c
index ee09d359e08f..788bc16888a5 100644
--- a/arch/x86/kernel/cpu/mtrr/generic.c
+++ b/arch/x86/kernel/cpu/mtrr/generic.c
@@ -240,6 +240,44 @@ static u8 mtrr_type_lookup_variable(u64 start, u64 end, u64 *partial_end,
 	return mtrr_state.def_type;
 }
 
+/**
+ * mtrr_overwrite_state - set fixed MTRR state
+ *
+ * Used to set MTRR state via different means (e.g. with data obtained from
+ * a hypervisor).
+ */
+void mtrr_overwrite_state(struct mtrr_var_range *var, unsigned int num_var,
+			  mtrr_type *fixed, mtrr_type def_type)
+{
+	unsigned int i;
+
+	if (boot_cpu_has(X86_FEATURE_MTRR))
+		return;
+
+	if (var) {
+		if (num_var > MTRR_MAX_VAR_RANGES) {
+			pr_warn("Trying to overwrite MTRR state with %u variable entries\n",
+				num_var);
+			num_var = MTRR_MAX_VAR_RANGES;
+		}
+		for (i = 0; i < num_var; i++)
+			mtrr_state.var_ranges[i] = var[i];
+		num_var_ranges = num_var;
+	}
+
+	if (fixed) {
+		for (i = 0; i < MTRR_NUM_FIXED_RANGES; i++)
+			mtrr_state.fixed_ranges[i] = fixed[i];
+		mtrr_state.enabled |= MTRR_STATE_MTRR_FIXED_ENABLED;
+		mtrr_state.have_fixed = 1;
+	}
+
+	mtrr_state.def_type = def_type;
+	mtrr_state.enabled |= MTRR_STATE_MTRR_ENABLED;
+
+	mtrr_state_set = 1;
+}
+
 /**
  * mtrr_type_lookup - look up memory type in MTRR
  *
diff --git a/arch/x86/kernel/cpu/mtrr/mtrr.c b/arch/x86/kernel/cpu/mtrr/mtrr.c
index 542ca5639dfd..b73fe243c7fd 100644
--- a/arch/x86/kernel/cpu/mtrr/mtrr.c
+++ b/arch/x86/kernel/cpu/mtrr/mtrr.c
@@ -668,6 +668,15 @@ void __init mtrr_bp_init(void)
 	const char *why = "(not available)";
 	unsigned int phys_addr;
 
+	if (mtrr_state.enabled) {
+		/* Software overwrite of MTRR state, only for generic case. */
+		mtrr_calc_physbits(true);
+		init_table();
+		pr_info("MTRRs set to read-only\n");
+
+		return;
+	}
+
 	phys_addr = mtrr_calc_physbits(boot_cpu_has(X86_FEATURE_MTRR));
 
 	if (boot_cpu_has(X86_FEATURE_MTRR)) {
-- 
2.35.3