[PATCH] arm64: kvm: fix IDMAP overlap with HYP VA

[Russell King <rmk+kernel@armlinux.org.uk>]

Booting 5.4 on LX2160A reveals that KVM is non-functional:

kvm: Limiting the IPA size due to kernel Virtual Address limit
kvm [1]: IPA Size Limit: 43bits
kvm [1]: IDMAP intersecting with HYP VA, unable to continue
kvm [1]: error initializing Hyp mode: -22

Debugging shows:

kvm [1]: IDMAP page: 81a26000
kvm [1]: HYP VA range: 0:22ffffffff

as RAM is located at:

80000000-fbdfffff : System RAM
2080000000-237fffffff : System RAM

Comparing this with the same kernel on Armada 8040 shows:

kvm: Limiting the IPA size due to kernel Virtual Address limit
kvm [1]: IPA Size Limit: 43bits
kvm [1]: IDMAP page: 2a26000
kvm [1]: HYP VA range: 4800000000:493fffffff
...
kvm [1]: Hyp mode initialized successfully

which indicates that hyp_va_msb is set, and is always set to the
opposite value of the idmap page to avoid the overlap. This does not
happen with the LX2160A.

Further debugging shows vabits_actual = 39, kva_msb = 38 on LX2160A and
kva_msb = 33 on Armada 8040. Looking at the bit layout of the HYP VA,
there is still one bit available for hyp_va_msb. Set this bit
appropriately. This allows kvm to be functional on the LX2160A, but
without any HYP VA randomisation:

kvm: Limiting the IPA size due to kernel Virtual Address limit
kvm [1]: IPA Size Limit: 43bits
kvm [1]: IDMAP page: 81a24000
kvm [1]: HYP VA range: 4000000000:62ffffffff
...
kvm [1]: Hyp mode initialized successfully

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
---
 arch/arm64/kvm/va_layout.c | 22 +++++++++++++++-------
 1 file changed, 15 insertions(+), 7 deletions(-)

diff --git a/arch/arm64/kvm/va_layout.c b/arch/arm64/kvm/va_layout.c
index 2cf7d4b606c3..83f8b3f51cf4 100644
--- a/arch/arm64/kvm/va_layout.c
+++ b/arch/arm64/kvm/va_layout.c
@@ -22,6 +22,17 @@ static u8 tag_lsb;
 static u64 tag_val;
 static u64 va_mask;
 
+/*
+ * We want to generate a hyp VA with the following format:
+ *
+ *  63 ... V |     V-1    | V-2 .. tag_lsb | tag_lsb - 1 .. 0
+ *  ---------------------------------------------------------
+ * | 0000000 | hyp_va_msb |    random tag  |  kern linear VA |
+ *
+ * which does not conflict with the idmap regions. This means that hyp_va_msb
+ * must always be present. Luckily, when kva_msb == (vabits_actual - 1) we
+ * still have one bit for this, but no bits for the random tag.
+ */
 static void compute_layout(void)
 {
 	phys_addr_t idmap_addr = __pa_symbol(__hyp_idmap_text_start);
@@ -39,19 +50,16 @@ static void compute_layout(void)
 		/*
 		 * No space in the address, let's compute the mask so
 		 * that it covers (vabits_actual - 1) bits, and the region
-		 * bit. The tag stays set to zero.
+		 * bit.
 		 */
-		va_mask  = BIT(vabits_actual - 1) - 1;
-		va_mask |= hyp_va_msb;
+		tag_lsb = kva_msb;
+		va_mask = BIT(vabits_actual - 1) - 1;
+		tag_val = hyp_va_msb >> tag_lsb;
 	} else {
 		/*
 		 * We do have some free bits to insert a random tag.
 		 * Hyp VAs are now created from kernel linear map VAs
 		 * using the following formula (with V == vabits_actual):
-		 *
-		 *  63 ... V |     V-1    | V-2 .. tag_lsb | tag_lsb - 1 .. 0
-		 *  ---------------------------------------------------------
-		 * | 0000000 | hyp_va_msb |    random tag  |  kern linear VA |
 		 */
 		tag_lsb = kva_msb;
 		va_mask = GENMASK_ULL(tag_lsb - 1, 0);
-- 
2.20.1


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