[Qemu-devel] [PATCH] Correct value of ARM Cortex-A8 MVFR1 register.

[Qemu-devel] [PATCH] Correct value of ARM Cortex-A8 MVFR1 register.

[Julian Brown]

The value of the MVFR1 (Media and VFP Feature Register 1) register for
the Cortex-A8 appears to be incorrect (according to the TRM, DDI0344K),
with the "full denormal arithmetic" and "propagation of NaN" fields
holding both 0 instead of both 1.

I had a go tracing the history of the use of this value, and it seems
it's always just been wrong in QEMU: maybe it was derived from early
documentation, or guessed based on the use of a "VFP Lite" implementation
in the Cortex-A8.

Depending on the startup/early-boot code in use, this can manifest as
failure to perform denormal arithmetic properly: in our case, selecting
a Cortex-A8 CPU when using QEMU as an instruction-set simulator for
bare-metal GCC testing caused tests using denormal arithmetic to
fail. Problems might be masked (or not occur) when using a full OS kernel
with suitable trap handlers (I'm not sure).

Signed-off-by: Julian Brown <julian@codesourcery.com>
---
 target-arm/cpu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/target-arm/cpu.c b/target-arm/cpu.c
index 7946877..a609211 100644
--- a/target-arm/cpu.c
+++ b/target-arm/cpu.c
@@ -1109,7 +1109,7 @@ static void cortex_a8_initfn(Object *obj)
     cpu->midr = 0x410fc080;
     cpu->reset_fpsid = 0x410330c0;
     cpu->mvfr0 = 0x11110222;
-    cpu->mvfr1 = 0x00011100;
+    cpu->mvfr1 = 0x00011111;
     cpu->ctr = 0x82048004;
     cpu->reset_sctlr = 0x00c50078;
     cpu->id_pfr0 = 0x1031;
-- 
1.9.1

Re: [Qemu-devel] [PATCH] Correct value of ARM Cortex-A8 MVFR1 register.

[Peter Maydell]

On 7 December 2016 at 17:14, Julian Brown <julian@codesourcery.com> wrote:
> The value of the MVFR1 (Media and VFP Feature Register 1) register for
> the Cortex-A8 appears to be incorrect (according to the TRM, DDI0344K),
> with the "full denormal arithmetic" and "propagation of NaN" fields
> holding both 0 instead of both 1.
>
> I had a go tracing the history of the use of this value, and it seems
> it's always just been wrong in QEMU: maybe it was derived from early
> documentation, or guessed based on the use of a "VFP Lite" implementation
> in the Cortex-A8.
>
> Depending on the startup/early-boot code in use, this can manifest as
> failure to perform denormal arithmetic properly: in our case, selecting
> a Cortex-A8 CPU when using QEMU as an instruction-set simulator for
> bare-metal GCC testing caused tests using denormal arithmetic to
> fail. Problems might be masked (or not occur) when using a full OS kernel
> with suitable trap handlers (I'm not sure).
>
> Signed-off-by: Julian Brown <julian@codesourcery.com>

Applied to target-arm.next for 2.9, thanks.

-- PMM