This commit adds support for legacy SBI extensions (version 0.1) in Xen
for guest domains.

The changes include:
1. Define all legacy SBI extension IDs (0x0 to 0x8) for better clarity and
   completeness.
2. Implement handling of legacy SBI extensions, starting with support for
   SBI_EXT_0_1_CONSOLE_{PUT,GET}CHAR.

The implementation uses the existing virtual SBI framework to handle legacy
SBI ecalls, ensuring compatibility with older SBI specifications in
RISC-V guests.

Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
---
Changes in v2:
 - s/vsbi-legacy-extension.*/legacy-extension.*
 - Correct padding for SBI_EXT_0_1_* macros.
 - Set ret = SBI_ERR_NOT_SUPPORTED; in the case of vsbi_legacy_ecall_handler()
   instead of regs->a0 = ... as regs->a0 will be overwritten in
   vsbi_handle_ecall().
 - Use domain_crash() instead of panic() in vsbi_legacy_ecall_handler() and
   add TODO.
 - Use newly introduced VSBI_EXT macros instead of VSBI_EXT_{START,END}.
 - Introduce vsbi_print_line() and use it instead of plain printk() in the
   handler of SBI_EXT_0_1_CONSOLE_PUTCHAR.
---
 xen/arch/riscv/include/asm/sbi.h       | 11 ++++-
 xen/arch/riscv/vsbi/Makefile           |  1 +
 xen/arch/riscv/vsbi/legacy-extension.c | 65 ++++++++++++++++++++++++++
 3 files changed, 75 insertions(+), 2 deletions(-)
 create mode 100644 xen/arch/riscv/vsbi/legacy-extension.c

diff --git a/xen/arch/riscv/include/asm/sbi.h b/xen/arch/riscv/include/asm/sbi.h
index ade24a572d..751bae6d66 100644
--- a/xen/arch/riscv/include/asm/sbi.h
+++ b/xen/arch/riscv/include/asm/sbi.h
@@ -14,8 +14,15 @@
 
 #include <xen/cpumask.h>
 
-#define SBI_EXT_0_1_CONSOLE_PUTCHAR		0x1
-#define SBI_EXT_0_1_SHUTDOWN			0x8
+#define SBI_EXT_0_1_SET_TIMER               0x0
+#define SBI_EXT_0_1_CONSOLE_PUTCHAR         0x1
+#define SBI_EXT_0_1_CONSOLE_GETCHAR         0x2
+#define SBI_EXT_0_1_CLEAR_IPI               0x3
+#define SBI_EXT_0_1_SEND_IPI                0x4
+#define SBI_EXT_0_1_REMOTE_FENCE_I          0x5
+#define SBI_EXT_0_1_REMOTE_SFENCE_VMA       0x6
+#define SBI_EXT_0_1_REMOTE_SFENCE_VMA_ASID  0x7
+#define SBI_EXT_0_1_SHUTDOWN                0x8
 
 #define SBI_EXT_BASE                    0x10
 #define SBI_EXT_RFENCE                  0x52464E43
diff --git a/xen/arch/riscv/vsbi/Makefile b/xen/arch/riscv/vsbi/Makefile
index 820eb10ac2..bc5755cb13 100644
--- a/xen/arch/riscv/vsbi/Makefile
+++ b/xen/arch/riscv/vsbi/Makefile
@@ -1 +1,2 @@
 obj-y += core.o
+obj-y += legacy-extension.o
diff --git a/xen/arch/riscv/vsbi/legacy-extension.c b/xen/arch/riscv/vsbi/legacy-extension.c
new file mode 100644
index 0000000000..75280de3bc
--- /dev/null
+++ b/xen/arch/riscv/vsbi/legacy-extension.c
@@ -0,0 +1,65 @@
+
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#include <xen/console.h>
+#include <xen/lib.h>
+#include <xen/sched.h>
+
+#include <asm/processor.h>
+#include <asm/vsbi.h>
+
+static void vsbi_print_line(char c)
+{
+    struct domain *cd = current->domain;
+    struct domain_console *cons = cd->console;
+
+    if ( !is_console_printable(c) )
+        return;
+
+    spin_lock(&cons->lock);
+    ASSERT(cons->idx < ARRAY_SIZE(cons->buf));
+    if ( c != '\n' )
+        cons->buf[cons->idx++] = c;
+    if ( (cons->idx == (ARRAY_SIZE(cons->buf) - 1)) || (c == '\n') )
+    {
+        cons->buf[cons->idx] = '\0';
+        guest_printk(cd, XENLOG_G_DEBUG "%s\n", cons->buf);
+        cons->idx = 0;
+    }
+    spin_unlock(&cons->lock);
+}
+
+static int vsbi_legacy_ecall_handler(struct vcpu *vcpu, unsigned long eid,
+                                     unsigned long fid,
+                                     struct cpu_user_regs *regs)
+{
+    int ret = 0;
+
+    switch ( eid )
+    {
+    case SBI_EXT_0_1_CONSOLE_PUTCHAR:
+        vsbi_print_line((char)regs->a0);
+        break;
+
+    case SBI_EXT_0_1_CONSOLE_GETCHAR:
+        ret = SBI_ERR_NOT_SUPPORTED;
+        break;
+
+    default:
+        /*
+         * TODO: domain_crash() is acceptable here while things are still under
+         * development.
+         * It shouldn't stay like this in the end though: guests should not
+         * be punished like this for something Xen hasn't implemented.
+         */
+        domain_crash(vcpu->domain,
+                     "%s: Unsupported ecall: FID: #%lx, EID: #%lx\n",
+                     __func__, fid, eid);
+        break;
+    }
+
+    return ret;
+}
+
+VSBI_EXT(legacy, SBI_EXT_0_1_SET_TIMER, SBI_EXT_0_1_SHUTDOWN,
+         vsbi_legacy_ecall_handler);
-- 
2.52.0