[PATCH] xen/arm: add forward_smc command line option for debugging

[Stefano Stabellini <sstabellini@kernel.org>]

It has become clear that an option to disable trapping SMC calls to Xen
is very useful for debugging user issues. Instead of having to provide a
patch to users every time, it would be great if we could just tell them
to add forward_smc=true to the Xen command line.

This option is obviously unsafe and unsecure and only meant for
debugging. Make clear in the description that if you pass
forward_smc=true then the system is not security supported.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>

diff --git a/docs/misc/xen-command-line.pandoc b/docs/misc/xen-command-line.pandoc
index 3ece83a427..0833fe80fc 100644
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -2501,6 +2501,16 @@ vwfi to `native` reduces irq latency significantly. It can also lead to
 suboptimal scheduling decisions, but only when the system is
 oversubscribed (i.e., in total there are more vCPUs than pCPUs).
 
+### forward_smc (arm)
+> `= <boolean>`
+
+> Default: `false`
+
+If enabled, instead of trapping firmware SMC calls to Xen, allow SMC
+calls from VMs directly to the firmware. This option is UNSAFE and it is
+only meant for debugging. Systems with forward_smc=true are not security
+supported.
+
 ### watchdog (x86)
 > `= force | <boolean>`
 
diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
index e7384381cc..0580ac5762 100644
--- a/xen/arch/arm/traps.c
+++ b/xen/arch/arm/traps.c
@@ -95,11 +95,15 @@ static int __init parse_vwfi(const char *s)
 }
 custom_param("vwfi", parse_vwfi);
 
+static bool forward_smc = false;
+boolean_param("forward_smc", forward_smc);
+
 register_t get_default_hcr_flags(void)
 {
     return  (HCR_PTW|HCR_BSU_INNER|HCR_AMO|HCR_IMO|HCR_FMO|HCR_VM|
              (vwfi != NATIVE ? (HCR_TWI|HCR_TWE) : 0) |
-             HCR_TID3|HCR_TSC|HCR_TAC|HCR_SWIO|HCR_TIDCP|HCR_FB|HCR_TSW);
+             (forward_smc ? 0 : HCR_TSC) |
+             HCR_TID3|HCR_TAC|HCR_SWIO|HCR_TIDCP|HCR_FB|HCR_TSW);
 }
 
 static enum {