Add RISC-V base support

kernel/comps/time/src/rtc/goldfish.rs

@@ -0,0 +1,46 @@
+// SPDX-License-Identifier: MPL-2.0
+
+use ostd::{arch::riscv::timer::GOLDFISH_IO_MEM, mm::VmIoOnce};
+use chrono::{DateTime, Datelike, Timelike};
+
+use crate::{SystemTime, rtc::Driver};
+
+pub struct RtcGoldfish;
+
+impl Driver for RtcGoldfish {
+    fn try_new() -> Option<RtcGoldfish> {
+        GOLDFISH_IO_MEM.get()?;
+        Some(RtcGoldfish)
+    }
+
+    fn read_rtc(&self) -> SystemTime {
+        const TIME_LOW: usize = 0;
+        const TIME_HIGH: usize = 4;
+
+        let io_mem = GOLDFISH_IO_MEM.get().unwrap();
+
+        let mut last_time_high = io_mem.read_once(TIME_HIGH).unwrap();
+        let timestamp = loop {
+            let time_low: u32 = io_mem.read_once(TIME_LOW).unwrap();
+            let time_high: u32 = io_mem.read_once(TIME_HIGH).unwrap();
+            if last_time_high == time_high {
+                break ((time_high as u64) << 32) | time_low as u64;
+            }
+            last_time_high = time_high;
+        };
+
+        let time = DateTime::from_timestamp_nanos(timestamp as i64).naive_utc();
+        let (is_ad, year) = time.year_ce();
+        debug_assert!(is_ad, "non-negative timestamp should always be AD");
+
+        SystemTime {
+            year: year as u16,
+            month: time.month() as u8,
+            day: time.day() as u8,
+            hour: time.hour() as u8,
+            minute: time.minute() as u8,
+            second: time.second() as u8,
+            nanos: time.nanosecond() as u64,
+        }
+    }
+}

kernel/comps/time/src/rtc/mod.rs

@@ -39,4 +39,5 @@ macro_rules! declare_rtc_drivers {
 
 declare_rtc_drivers! {
     #[cfg(target_arch = "x86_64")] cmos::RtcCmos,
+    #[cfg(target_arch = "riscv64")] goldfish::RtcGoldfish,
 }