xen/riscv: add RISC-V legacy SBI extension support for guests

Add 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_PUTCHAR. SBI_EXT_0_1_CONSOLE_GETCHAR is marked as
   not supported as legacy SBI console related stuff is expected to be used
   only for early debugging of guest.

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>
Acked-by: Jan Beulich <jbeulich@suse.com>

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

xen/arch/riscv/vsbi/Makefile

@@ -1 +1,2 @@
 obj-y += core.o
+obj-y += legacy-extension.o

xen/arch/riscv/vsbi/legacy-extension.c

@@ -0,0 +1,64 @@
+
+/* 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_char(char c)
+{
+    struct domain *currd = current->domain;
+    struct domain_console *cons = currd->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(currd, XENLOG_G_DEBUG "%s\n", cons->buf);
+        cons->idx = 0;
+    }
+    spin_unlock(&cons->lock);
+}
+
+static int vsbi_legacy_ecall_handler(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_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(current->domain,
+                     "%s: Unsupported legacy ecall: EID: #%#lx\n",
+                     __func__, eid);
+        break;
+    }
+
+    return ret;
+}
+
+VSBI_EXT(legacy, SBI_EXT_0_1_SET_TIMER, SBI_EXT_0_1_SHUTDOWN,
+         vsbi_legacy_ecall_handler);