arm64: vgic-v2: Fix proxying of cpuif access

Proxying the cpuif accesses at EL2 makes use of vcpu_data_guest_to_host
and co, which check the endianness, which call into vcpu_read_sys_reg...
which isn't mapped at EL2 (it was inlined before, and got moved OoL
with the VHE optimizations).

The result is of course a nice panic. Let's add some specialized
cruft to keep the broken platforms that require this hack alive.

But, this code used vcpu_data_guest_to_host(), which expected us to
write the value to host memory, instead we have trapped the guest's
read or write to an mmio-device, and are about to replay it using the
host's readl()/writel() which also perform swabbing based on the host
endianness. This goes wrong when both host and guest are big-endian,
as readl()/writel() will undo the guest's swabbing, causing the
big-endian value to be written to device-memory.

What needs doing?
A big-endian guest will have pre-swabbed data before storing, undo this.
If its necessary for the host, writel() will re-swab it.

For a read a big-endian guest expects to swab the data after the load.
The hosts's readl() will correct for host endianness, giving us the
device-memory's value in the register. For a big-endian guest, swab it
as if we'd only done the load.

For a little-endian guest, nothing needs doing as readl()/writel() leave
the correct device-memory value in registers.

Tested on Juno with that rarest of things: a big-endian 64K host.
Based on a patch from Marc Zyngier.
Reported-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Fixes: bf8feb39 ("arm64: KVM: vgic-v2: Add GICV access from HYP")
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

arch/arm64/kvm/hyp/vgic-v2-cpuif-proxy.c

@@ -18,11 +18,20 @@
 #include <linux/compiler.h>
 #include <linux/irqchip/arm-gic.h>
 #include <linux/kvm_host.h>
+#include <linux/swab.h>
 
 #include <asm/kvm_emulate.h>
 #include <asm/kvm_hyp.h>
 #include <asm/kvm_mmu.h>
 
+static bool __hyp_text __is_be(struct kvm_vcpu *vcpu)
+{
+	if (vcpu_mode_is_32bit(vcpu))
+		return !!(read_sysreg_el2(spsr) & COMPAT_PSR_E_BIT);
+
+	return !!(read_sysreg(SCTLR_EL1) & SCTLR_ELx_EE);
+}
+
 /*
  * __vgic_v2_perform_cpuif_access -- perform a GICV access on behalf of the
  *				     guest.
@@ -64,14 +73,19 @@ int __hyp_text __vgic_v2_perform_cpuif_access(struct kvm_vcpu *vcpu)
 	addr += fault_ipa - vgic->vgic_cpu_base;
 
 	if (kvm_vcpu_dabt_iswrite(vcpu)) {
-		u32 data = vcpu_data_guest_to_host(vcpu,
-						   vcpu_get_reg(vcpu, rd),
-						   sizeof(u32));
+		u32 data = vcpu_get_reg(vcpu, rd);
+		if (__is_be(vcpu)) {
+			/* guest pre-swabbed data, undo this for writel() */
+			data = swab32(data);
+		}
 		writel_relaxed(data, addr);
 	} else {
 		u32 data = readl_relaxed(addr);
-		vcpu_set_reg(vcpu, rd, vcpu_data_host_to_guest(vcpu, data,
-							       sizeof(u32)));
+		if (__is_be(vcpu)) {
+			/* guest expects swabbed data */
+			data = swab32(data);
+		}
+		vcpu_set_reg(vcpu, rd, data);
 	}
 
 	return 1;