xen/arm: Taint Xen on incompatible DCZID values

Use arm64 cpu feature sanitization to TAINT Xen if different DCZID values
are found (ftr_dczid is using only STRICT method).
In this case actual memory being cleaned by DC ZVA operations would be
different depending on the cores which could make a guest zeroing too
much or too little memory if it is merged between CPUs.

We could, on processors supporting it, trap access to DCZID_EL0 register
using HFGRTR_EL2 register but this would not solve the case where a
process is being migrated during a copy or if it cached the value of the
register.

Signed-off-by: Bertrand Marquis <bertrand.marquis@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

xen/arch/arm/arm64/cpufeature.c

@@ -329,14 +329,11 @@ static const struct arm64_ftr_bits ftr_mvfr2[] = {
 	ARM64_FTR_END,
 };
 
-#if 0
-/* TODO: handle this when sanitizing cache related registers */
 static const struct arm64_ftr_bits ftr_dczid[] = {
 	ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_EXACT, DCZID_DZP_SHIFT, 1, 1),
 	ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, DCZID_BS_SHIFT, 4, 0),
 	ARM64_FTR_END,
 };
-#endif
 
 static const struct arm64_ftr_bits ftr_id_isar0[] = {
 	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_ISAR0_DIVIDE_SHIFT, 4, 0),
@@ -598,6 +595,17 @@ void update_system_features(const struct cpuinfo_arm *new)
 
 	SANITIZE_ID_REG(zfr64, 0, aa64zfr0);
 
+	/*
+	 * Comment from Linux:
+	 * Userspace may perform DC ZVA instructions. Mismatched block sizes
+	 * could result in too much or too little memory being zeroed if a
+	 * process is preempted and migrated between CPUs.
+	 *
+	 * ftr_dczid is using STRICT comparison so we will taint Xen if different
+	 * values are found.
+	 */
+	SANITIZE_REG(dczid, 0, dczid);
+
 	if ( cpu_feature64_has_el0_32(&system_cpuinfo) )
 	{
 		SANITIZE_ID_REG(pfr32, 0, pfr0);

xen/arch/arm/cpufeature.c

@@ -125,6 +125,8 @@ void identify_cpu(struct cpuinfo_arm *c)
 
     c->zfr64.bits[0] = READ_SYSREG(ID_AA64ZFR0_EL1);
 
+    c->dczid.bits[0] = READ_SYSREG(DCZID_EL0);
+
     aarch32_el0 = cpu_feature64_has_el0_32(c);
 #endif
 

xen/include/asm-arm/cpufeature.h

@@ -259,6 +259,14 @@ struct cpuinfo_arm {
         register_t bits[1];
     } zfr64;
 
+    /*
+     * DCZID is only used to check for incoherent values between cores
+     * and taint Xen in this case
+     */
+    struct {
+        register_t bits[1];
+    } dczid;
+
 #endif
 
     /*