hw/arm/boot: Set SCR_EL3.HXEn when booting kernel

When we direct boot a kernel on a CPU which emulates EL3, we need to
set up the EL3 system registers as the Linux kernel documentation
specifies:
     https://www.kernel.org/doc/Documentation/arm64/booting.rst

For CPUs with FEAT_HCX support this includes:
    - SCR_EL3.HXEn (bit 38) must be initialised to 0b1.

but we forgot to do this when implementing FEAT_HCX, which would mean
that a guest trying to access the HCRX_EL2 register would crash.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20221027140207.413084-3-peter.maydell@linaro.org

hw/arm/boot.c

@@ -771,6 +771,9 @@ static void do_cpu_reset(void *opaque)
                         env->cp15.scr_el3 |= SCR_ENTP2;
                         env->vfp.smcr_el[3] = 0xf;
                     }
+                    if (cpu_isar_feature(aa64_hcx, cpu)) {
+                        env->cp15.scr_el3 |= SCR_HXEN;
+                    }
                     /* AArch64 kernels never boot in secure mode */
                     assert(!info->secure_boot);
                     /* This hook is only supported for AArch32 currently: