[PATCH] target/arm: adjust program counter for wfi exception in AArch32

[PATCH] target/arm: adjust program counter for wfi exception in AArch32

[Jeff Kubascik]

The wfi instruction can be configured to be trapped by a higher exception
level, such as the EL2 hypervisor. When the instruction is trapped, the
program counter should contain the address of the wfi instruction that
caused the exception. The program counter is adjusted for this in the wfi op
helper function.

However, this correction is done to env->pc, which only applies to AArch64
mode. For AArch32, the program counter is stored in env->regs[15]. This
adds an if-else statement to modify the correct program counter location
based on the the current CPU mode.

Signed-off-by: Jeff Kubascik <jeff.kubascik@dornerworks.com>
---
Hello,

I am using the ARMv8 version of QEMU to run the Xen hypervisor with a guest
virtual machine compiled for AArch32/Thumb code. I have noticed that when
the AArch32 guest VM executes the wfi instruction, the hypervisor trap of
the wfi instruction sees the program counter contain the address of the
instruction following the wfi. This does not occur for an AARch64 guest VM;
in this case, the program counter contains the address of the wfi
instruction. I am confident the correct behavior in both cases is for the
program counter to contain the address of the wfi instruction, as this works
on actual hardware (Xilinx Zynq UltraScale+ MPSoC).

I have tested the above patch and it works for Xen with both an AArch64
guest (Linux) and an AArch32 guest (RTEMS). I'm still getting accustomed to
the QEMU code base, so it may not be correct. Any feedback would be greatly
appreciated.

Sincerely,
Jeff Kubascik
---
 target/arm/op_helper.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/target/arm/op_helper.c b/target/arm/op_helper.c
index e5a346cb87..7295645854 100644
--- a/target/arm/op_helper.c
+++ b/target/arm/op_helper.c
@@ -295,7 +295,11 @@ void HELPER(wfi)(CPUARMState *env, uint32_t insn_len)
     }
 
     if (target_el) {
-        env->pc -= insn_len;
+        if (env->aarch64)
+            env->pc -= insn_len;
+        else
+            env->regs[15] -= insn_len;
+
         raise_exception(env, EXCP_UDEF, syn_wfx(1, 0xe, 0, insn_len == 2),
                         target_el);
     }
-- 
2.17.1

Re: [PATCH] target/arm: adjust program counter for wfi exception in AArch32

[no-reply]

Patchew URL: https://patchew.org/QEMU/20200110180211.29025-1-jeff.kubascik@dornerworks.com/



Hi,

This series seems to have some coding style problems. See output below for
more information:

Subject: [PATCH] target/arm: adjust program counter for wfi exception in AArch32
Type: series
Message-id: 20200110180211.29025-1-jeff.kubascik@dornerworks.com

=== TEST SCRIPT BEGIN ===
#!/bin/bash
git rev-parse base > /dev/null || exit 0
git config --local diff.renamelimit 0
git config --local diff.renames True
git config --local diff.algorithm histogram
./scripts/checkpatch.pl --mailback base..
=== TEST SCRIPT END ===

Switched to a new branch 'test'
800339c target/arm: adjust program counter for wfi exception in AArch32

=== OUTPUT BEGIN ===
ERROR: braces {} are necessary for all arms of this statement
#30: FILE: target/arm/op_helper.c:298:
+        if (env->aarch64)
[...]
+        else
[...]

total: 1 errors, 0 warnings, 12 lines checked

Commit 800339c7aaeb (target/arm: adjust program counter for wfi exception in AArch32) has style problems, please review.  If any of these errors
are false positives report them to the maintainer, see
CHECKPATCH in MAINTAINERS.
=== OUTPUT END ===

Test command exited with code: 1


The full log is available at
http://patchew.org/logs/20200110180211.29025-1-jeff.kubascik@dornerworks.com/testing.checkpatch/?type=message.
---
Email generated automatically by Patchew [https://patchew.org/].
Please send your feedback to patchew-devel@redhat.com

Re: [PATCH] target/arm: adjust program counter for wfi exception in AArch32

[Richard Henderson]

On 1/10/20 8:02 AM, Jeff Kubascik wrote:
> -        env->pc -= insn_len;
> +        if (env->aarch64)
> +            env->pc -= insn_len;
> +        else
> +            env->regs[15] -= insn_len;

QEMU requires all braces.  See CODING_STYLE.

Otherwise,
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>


r~

Re: [PATCH] target/arm: adjust program counter for wfi exception in AArch32

[Jeff Kubascik]

On 1/13/2020 9:41 PM, Richard Henderson wrote:
> On 1/10/20 8:02 AM, Jeff Kubascik wrote:
>> -        env->pc -= insn_len;
>> +        if (env->aarch64)
>> +            env->pc -= insn_len;
>> +        else
>> +            env->regs[15] -= insn_len;
> 
> QEMU requires all braces.  See CODING_STYLE.

I received an automated email from Patchew regarding the coding style issue. I
have submitted a v2 of the patch with this correction yesterday.

> Otherwise,
> Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
> 
> 
> r~
> 

Thanks!
Jeff Kubascik