Re: [PATCH] RISC-V: restore U-mode counter access (scounteren)

[Manfred Schlaegl]

On 3/26/26 23:27, Andrew Jones wrote:
> On Thu, Mar 26, 2026 at 12:17:13PM +0100, Manfred SCHLAEGL wrote:
>> In commit 5bc409786752 ("RISC-V: KVM: Remove scounteren initialization")
>> the initialization of scounteren was moved from the hypervisor (KVM) to
>> the supervisor -- a well-argued change. However, the same change also
>> restricted access to performance counters without comment: previously,
>> scounteren was set to 0x7 (0b111), allowing U-mode (userspace) access
>> to instret (IR), cycle (CY), and time (TM). After the change,
>> scounteren is set to 0x2 (0b010), which allows access only to time (TM)
>> and prohibits access to instret (IR) and cycle (CY). This breaks user-
>> space programs that directly use these performance counters.
>>
>> This change restores the prior behavior: scounteren is set to 0x7,
>> which re-enables U-mode (userspace) access to instret (IR), cycle (CY),
>> and time (TM).
>>
>> Fixes: 5bc409786752 ("RISC-V: KVM: Remove scounteren initialization")
>> Signed-off-by: Manfred SCHLAEGL <manfred.schlaegl@xxxxxx>
>> ---
>>  arch/riscv/kernel/head.S | 8 ++++----
>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
>> index 9c99c5ad6fe8..5ad5e12a8299 100644
>> --- a/arch/riscv/kernel/head.S
>> +++ b/arch/riscv/kernel/head.S
>> @@ -133,8 +133,8 @@ secondary_start_sbi:
>>  	csrw CSR_IP, zero
>>  
>>  #ifndef CONFIG_RISCV_M_MODE
>> -	/* Enable time CSR */
>> -	li t0, 0x2
>> +	/* Enable CY, TM, and IR counters in U mode */
>> +	li t0, 0x7
>>  	csrw CSR_SCOUNTEREN, t0
>>  #endif
>>  
>> @@ -247,8 +247,8 @@ SYM_CODE_START(_start_kernel)
>>  	 */
>>  	csrr a0, CSR_MHARTID
>>  #else
>> -	/* Enable time CSR */
>> -	li t0, 0x2
>> +	/* Enable CY, TM, and IR counters in U mode */
>> +	li t0, 0x7
>>  	csrw CSR_SCOUNTEREN, t0
>>  #endif /* CONFIG_RISCV_M_MODE */
>>  
>> -- 
>> 2.47.3
>>
> 
> 5bc409786752 should have explained that change by referencing the
> same links as commit cc4c07c89aad ("drivers: perf: Implement perf
> event mmap support in the SBI backend") does.

Ahhh, I see :-)
Thanks for the clarification!

> 
> Thanks,
> drew

Best regards,
Manfred