RISC-V: Add Sstc extension support

This series implements Sstc extension support which was ratified
recently.  Before the Sstc extension, an SBI call is necessary to
generate timer interrupts as only M-mode have access to the timecompare
registers. Thus, there is significant latency to generate timer
interrupts at kernel.  For virtualized enviornments, its even worse as
the KVM handles the SBI call and uses a software timer to emulate the
timecomapre register.

Sstc extension solves both these problems by defining a
stimecmp/vstimecmp at supervisor (host/guest) level. It allows kernel to
program a timer and recieve interrupt without supervisor execution
enviornment (M-mode/HS mode) intervention.

* palmer/riscv-sstc:
  RISC-V: Prefer sstc extension if available
  RISC-V: Enable sstc extension parsing from DT
  RISC-V: Add SSTC extension CSR details

Author: palmer-dabbelt

arch/riscv/include/asm/csr.h

@@ -247,6 +247,9 @@
 #define CSR_SIP			0x144
 #define CSR_SATP		0x180
 
+#define CSR_STIMECMP		0x14D
+#define CSR_STIMECMPH		0x15D
+
 #define CSR_VSSTATUS		0x200
 #define CSR_VSIE		0x204
 #define CSR_VSTVEC		0x205
@@ -256,6 +259,8 @@
 #define CSR_VSTVAL		0x243
 #define CSR_VSIP		0x244
 #define CSR_VSATP		0x280
+#define CSR_VSTIMECMP		0x24D
+#define CSR_VSTIMECMPH		0x25D
 
 #define CSR_HSTATUS		0x600
 #define CSR_HEDELEG		0x602

arch/riscv/include/asm/hwcap.h

@@ -57,6 +57,7 @@ enum riscv_isa_ext_id {
 	RISCV_ISA_EXT_SVPBMT,
 	RISCV_ISA_EXT_ZICBOM,
 	RISCV_ISA_EXT_ZIHINTPAUSE,
+	RISCV_ISA_EXT_SSTC,
 	RISCV_ISA_EXT_ID_MAX = RISCV_ISA_EXT_MAX,
 };
 

arch/riscv/kernel/cpu.c

@@ -95,6 +95,7 @@ static struct riscv_isa_ext_data isa_ext_arr[] = {
 	__RISCV_ISA_EXT_DATA(svpbmt, RISCV_ISA_EXT_SVPBMT),
 	__RISCV_ISA_EXT_DATA(zicbom, RISCV_ISA_EXT_ZICBOM),
 	__RISCV_ISA_EXT_DATA(zihintpause, RISCV_ISA_EXT_ZIHINTPAUSE),
+	__RISCV_ISA_EXT_DATA(sstc, RISCV_ISA_EXT_SSTC),
 	__RISCV_ISA_EXT_DATA("", RISCV_ISA_EXT_MAX),
 };
 

arch/riscv/kernel/cpufeature.c

@@ -203,6 +203,7 @@ void __init riscv_fill_hwcap(void)
 				SET_ISA_EXT_MAP("svpbmt", RISCV_ISA_EXT_SVPBMT);
 				SET_ISA_EXT_MAP("zicbom", RISCV_ISA_EXT_ZICBOM);
 				SET_ISA_EXT_MAP("zihintpause", RISCV_ISA_EXT_ZIHINTPAUSE);
+				SET_ISA_EXT_MAP("sstc", RISCV_ISA_EXT_SSTC);
 			}
 #undef SET_ISA_EXT_MAP
 		}

drivers/clocksource/timer-riscv.c

@@ -7,6 +7,9 @@
  * either be read from the "time" and "timeh" CSRs, and can use the SBI to
  * setup events, or directly accessed using MMIO registers.
  */
+
+#define pr_fmt(fmt) "riscv-timer: " fmt
+
 #include <linux/clocksource.h>
 #include <linux/clockchips.h>
 #include <linux/cpu.h>
@@ -20,14 +23,28 @@
 #include <linux/of_irq.h>
 #include <clocksource/timer-riscv.h>
 #include <asm/smp.h>
+#include <asm/hwcap.h>
 #include <asm/sbi.h>
 #include <asm/timex.h>
 
+static DEFINE_STATIC_KEY_FALSE(riscv_sstc_available);
+
 static int riscv_clock_next_event(unsigned long delta,
 		struct clock_event_device *ce)
 {
+	u64 next_tval = get_cycles64() + delta;
+
 	csr_set(CSR_IE, IE_TIE);
-	sbi_set_timer(get_cycles64() + delta);
+	if (static_branch_likely(&riscv_sstc_available)) {
+#if defined(CONFIG_32BIT)
+		csr_write(CSR_STIMECMP, next_tval & 0xFFFFFFFF);
+		csr_write(CSR_STIMECMPH, next_tval >> 32);
+#else
+		csr_write(CSR_STIMECMP, next_tval);
+#endif
+	} else
+		sbi_set_timer(next_tval);
+
 	return 0;
 }
 
@@ -166,6 +183,12 @@ static int __init riscv_timer_init_dt(struct device_node *n)
 	if (error)
 		pr_err("cpu hp setup state failed for RISCV timer [%d]\n",
 		       error);
+
+	if (riscv_isa_extension_available(NULL, SSTC)) {
+		pr_info("Timer interrupt in S-mode is available via sstc extension\n");
+		static_branch_enable(&riscv_sstc_available);
+	}
+
 	return error;
 }