[PATCH] target/arm: Fix reads of CNTFRQ_EL0 in linux-user mode

[PATCH] target/arm: Fix reads of CNTFRQ_EL0 in linux-user mode

[Peter Maydell]

In commit bd8e9ddf6f6 ("target/arm: Refactor default generic timer
frequency handling") we changed how we initialized the generic timer
frequency as reported in the CNTFRQ_EL0 register.  As part of that,
we chanegd the linux-user version of the CNTFRQ_EL0 sysreg from
having a constant value set at compile time through the .resetvalue
field to having a reset value which we compute in a .resetfn.

This accidentally broke the reading of CNTFRQ_EL0 in linux-user mode,
because the cpreg is marked as ARM_CP_CONST, which means we translate
it as a read of the compile-time constant value in the .resetvalue
field.  This is now zero, so userspace sees a 0 frequency value.

Fix the bug by dropping the ARM_CP_CONST marking.  This will cause us
to translate the read as a load of the value from the CPU state
struct cp15.c14_cntfrq field, which is where the real frequency value
now lives.

Cc: qemu-stable@nongnu.org
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3159
Fixes: bd8e9ddf6f6 ("target/arm: Refactor default generic timer frequency handling")
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
 target/arm/helper.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/target/arm/helper.c b/target/arm/helper.c
index 167f2909b3f..1e1c0e41415 100644
--- a/target/arm/helper.c
+++ b/target/arm/helper.c
@@ -2306,7 +2306,7 @@ static uint64_t gt_virt_cnt_read(CPUARMState *env, const ARMCPRegInfo *ri)
 static const ARMCPRegInfo generic_timer_cp_reginfo[] = {
     { .name = "CNTFRQ_EL0", .state = ARM_CP_STATE_AA64,
       .opc0 = 3, .opc1 = 3, .crn = 14, .crm = 0, .opc2 = 0,
-      .type = ARM_CP_CONST, .access = PL0_R /* no PL1_RW in linux-user */,
+      .access = PL0_R /* no PL1_RW in linux-user */,
       .fieldoffset = offsetof(CPUARMState, cp15.c14_cntfrq),
       .resetfn = arm_gt_cntfrq_reset,
     },
-- 
2.43.0

Re: [PATCH] target/arm: Fix reads of CNTFRQ_EL0 in linux-user mode

[Philippe Mathieu-Daudé]

On 13/10/25 18:10, Peter Maydell wrote:
> In commit bd8e9ddf6f6 ("target/arm: Refactor default generic timer
> frequency handling") we changed how we initialized the generic timer
> frequency as reported in the CNTFRQ_EL0 register.  As part of that,
> we chanegd the linux-user version of the CNTFRQ_EL0 sysreg from

"changed"

> having a constant value set at compile time through the .resetvalue
> field to having a reset value which we compute in a .resetfn.
> 
> This accidentally broke the reading of CNTFRQ_EL0 in linux-user mode,
> because the cpreg is marked as ARM_CP_CONST, which means we translate
> it as a read of the compile-time constant value in the .resetvalue
> field.  This is now zero, so userspace sees a 0 frequency value.
> 
> Fix the bug by dropping the ARM_CP_CONST marking.  This will cause us
> to translate the read as a load of the value from the CPU state
> struct cp15.c14_cntfrq field, which is where the real frequency value
> now lives.
> 
> Cc: qemu-stable@nongnu.org
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3159
> Fixes: bd8e9ddf6f6 ("target/arm: Refactor default generic timer frequency handling")
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
>   target/arm/helper.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>