Re: riscv64: use sstc for clocks when available

On Mon, Jul 13, 2026 at 09:18:20AM -0700, Nick Owens wrote:
> hi,

Hi, sorry for the delay ;)

> this implements support for sstc extension. with sstc, we can write the
> stimecmp csr directly instead of needing an sbi ecall to set the timer
> comparison register.
> 
> both the k1 and k3 support it, and since it's a direct register write it
> takes only ~10ns instead of several hundred ns for the ecall and
> possible hypervisor emulation in the case of linux kvm.
> 
> i've tested on orangepi rv2 (k1) directly, and on the k3 under linux
> kvm. fallback path tested on kvm by disabling sstc in the advertised
> features of qemu kvm as well. without sstc, the k1 takes about 250ns and
> the k3 + kvm takes about 600ns. with this change, both are around 10ns
> per timer write.
>
> this also conveniently works around a bug in upstream kvm where timer
> writes via sbi ecall can get lost if there's a context switch at the
> wrong time, which makes kvm guest timers permanently stop firing. see
> https://lore.kernel.org/all/[email protected]/

Getting rid of this overhead appears desirable, working around a bug
in a hypervisor is just a bonus...

No visible regression on my k1 which exposes Sstc.  Please find a
tweaked diff below: I'd rather have the check for the feature in a
single place.  The "timer_set_timer" name is admittedly a bit
redundant, suggestions welcome.

I'll commit this in a few days unless I hear objections.  Tests & oks
welcome.


Index: clock.c
===================================================================
RCS file: /cvs/src/sys/arch/riscv64/riscv64/clock.c,v
diff -u -p -r1.14 clock.c
--- clock.c	27 Jan 2024 12:05:40 -0000	1.14
+++ clock.c	24 Jul 2026 14:54:28 -0000
@@ -25,6 +25,7 @@
 #include <sys/stdint.h>
 #include <sys/timetc.h>
 
+#include <machine/elf.h>
 #include <machine/cpufunc.h>
 #include <machine/sbi.h>
 
@@ -60,6 +61,15 @@ static struct timecounter tb_timecounter
 void	(*cpu_startclock_fcn)(void) = timer_startclock;
 int	clock_intr(void *);
 
+static inline void
+timer_set_timer(uint64_t nsecs)
+{
+	if (riscv_hwcap & HWCAP_ISA_SSTC)
+		csr_write(stimecmp, nsecs);
+	else
+		sbi_set_timer(nsecs);
+}
+
 void
 timer_rearm(void *unused, uint64_t nsecs)
 {
@@ -68,13 +78,13 @@ timer_rearm(void *unused, uint64_t nsecs
 	if (nsecs > timer_nsec_max)
 		nsecs = timer_nsec_max;
 	cycles = (nsecs * timer_nsec_cycle_ratio) >> 32;
-	sbi_set_timer(rdtime() + cycles);
+	timer_set_timer(rdtime() + cycles);
 }
 
 void
 timer_trigger(void *unused)
 {
-	sbi_set_timer(0);
+	timer_set_timer(0);
 }
 
 u_int
@@ -125,7 +135,7 @@ clock_intr(void *frame)
 	struct cpu_info *ci = curcpu();
 	int s;
 
-	sbi_set_timer(UINT64_MAX);	/* clear timer interrupt */
+	timer_set_timer(UINT64_MAX); /* clear timer interrupt */
 
 	/*
 	 * If the clock interrupt is masked, defer all clock interrupt


-- 
jca