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Re: [Minios-devel] [UNIKRAFT PATCHv6 11/37] plat/common: Introduce SYSREG_READ32/64 SYSTEM_WRITE32/64 for different size





On 09/17/2018 08:18 AM, Wei Chen (Arm Technology China) wrote:
Hi Julien,

Hi,


-----Original Message-----
From: Julien Grall <julien.grall@xxxxxxx>
Sent: 2018年9月17日 8:44
To: Wei Chen (Arm Technology China) <Wei.Chen@xxxxxxx>; minios-
devel@xxxxxxxxxxxxxxxxxxxx; simon.kuenzer@xxxxxxxxx
Cc: Kaly Xin (Arm Technology China) <Kaly.Xin@xxxxxxx>; nd <nd@xxxxxxx>
Subject: Re: [Minios-devel] [UNIKRAFT PATCHv6 11/37] plat/common: Introduce
SYSREG_READ32/64 SYSTEM_WRITE32/64 for different size

Hi,

On 09/14/2018 08:56 AM, Wei Chen wrote:
On Arm, the system registers may have different size. Introduce these
four helpers will give us a direct visual about the register's size we
are accessing.

Signed-off-by: Wei Chen <wei.chen@xxxxxxx>
---
   plat/common/include/arm/arm64/cpu.h | 14 ++++++++++++++
   1 file changed, 14 insertions(+)

diff --git a/plat/common/include/arm/arm64/cpu.h
b/plat/common/include/arm/arm64/cpu.h
index 702761c..db8be9c 100644
--- a/plat/common/include/arm/arm64/cpu.h
+++ b/plat/common/include/arm/arm64/cpu.h
@@ -90,3 +90,17 @@ static inline void ioreg_write64(volatile uint64_t *addr,
uint64_t value)
   #define SYSREG_WRITE(reg, val) \
        __asm__ __volatile__("msr " __STRINGIFY(reg) ", %0" \
                        : : "r" ((uint64_t)(val)))
+
+#define SYSREG_READ32(reg) \
+({     uint32_t val; \
+       __asm__ __volatile__("mrs %0, " __STRINGIFY(reg) \
+                       : "=&r" (val)); \

I am not entirely sure why you need the earlycloberr (&) here. The
operand will not be written before the instruction has finished.


Is there any harm that I am using the "&", because I always use it
in such case?
My point here is you add & which does not make sense in the current context. Can you explain why you always put it in such case?

Cheers,

--
Julien Grall

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