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

Add support of virtual SBI base extension calls for RISC-V guests, delegating
hardware-specific queries to the underlying SBI and handling version and
firmware ID queries directly.

The changes include:
1. Define new SBI base extension function IDs (SBI_EXT_BASE_GET_MVENDORID,
   SBI_EXT_BASE_GET_MARCHID, SBI_EXT_BASE_GET_MIMPID).
2. Introduce XEN_SBI_VER_MAJOR, XEN_SBI_VER_MINOR for imeplenataion of
   SBI_EXT_BASE_GET_SPEC_VERSION.
4. Introduce SBI_XEN_IMPID to implement SBI_EXT_BASE_GET_IMP_ID.
5. Implement handling of SBI base extension functions, including version,
   firmware ID, and machine-specific queries.

Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/arch/riscv/include/asm/sbi.h

@@ -14,6 +14,9 @@
 
 #include <xen/cpumask.h>
 
+/* SBI-defined implementation ID */
+#define SBI_XEN_IMPID 7
+
 #define SBI_EXT_0_1_SET_TIMER               0x0
 #define SBI_EXT_0_1_CONSOLE_PUTCHAR         0x1
 #define SBI_EXT_0_1_CONSOLE_GETCHAR         0x2
@@ -32,6 +35,9 @@
 #define SBI_EXT_BASE_GET_IMP_ID         0x1
 #define SBI_EXT_BASE_GET_IMP_VERSION    0x2
 #define SBI_EXT_BASE_PROBE_EXT          0x3
+#define SBI_EXT_BASE_GET_MVENDORID      0x4
+#define SBI_EXT_BASE_GET_MARCHID        0x5
+#define SBI_EXT_BASE_GET_MIMPID         0x6
 
 /* SBI function IDs for RFENCE extension */
 #define SBI_EXT_RFENCE_REMOTE_FENCE_I           0x0

xen/arch/riscv/vsbi/Makefile

@@ -1,2 +1,3 @@
+obj-y += base-extension.o
 obj-y += core.o
 obj-y += legacy-extension.o

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

@@ -0,0 +1,82 @@
+
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#include <xen/lib.h>
+#include <xen/sched.h>
+#include <xen/version.h>
+
+#include <asm/processor.h>
+#include <asm/sbi.h>
+#include <asm/vsbi.h>
+
+/* Xen-controlled SBI version reported to guests */
+#define XEN_SBI_VER_MAJOR 0
+#define XEN_SBI_VER_MINOR 2
+
+static int vsbi_base_ecall_handler(unsigned long eid, unsigned long fid,
+                                   struct cpu_user_regs *regs)
+{
+    int ret = 0;
+    struct sbiret sbi_ret;
+
+    ASSERT(eid == SBI_EXT_BASE);
+
+    switch ( fid )
+    {
+    case SBI_EXT_BASE_GET_SPEC_VERSION:
+        regs->a1 = MASK_INSR(XEN_SBI_VER_MAJOR, SBI_SPEC_VERSION_MAJOR_MASK) |
+                   XEN_SBI_VER_MINOR;
+        break;
+
+    case SBI_EXT_BASE_GET_IMP_ID:
+        regs->a1 = SBI_XEN_IMPID;
+        break;
+
+    case SBI_EXT_BASE_GET_IMP_VERSION:
+        regs->a1 = (xen_major_version() << 16) | xen_minor_version();
+        break;
+
+    case SBI_EXT_BASE_GET_MVENDORID:
+    case SBI_EXT_BASE_GET_MARCHID:
+    case SBI_EXT_BASE_GET_MIMPID:
+        if ( is_hardware_domain(current->domain) )
+        {
+            sbi_ret = sbi_ecall(SBI_EXT_BASE, fid, 0, 0, 0, 0, 0, 0);
+            ret = sbi_ret.error;
+            regs->a1 = sbi_ret.value;
+        }
+        else
+            /*
+             * vSBI should present a consistent, virtualized view to guests.
+             * In particular, DomU-visible data must remain stable across
+             * migration and must not expose hardware-specific details.
+             *
+             * These register(s) must be readable in any implementation,
+             * but a value of 0 can be returned to indicate the field
+             * is not implemented.
+             */
+            regs->a1 = 0;
+
+        break;
+
+    case SBI_EXT_BASE_PROBE_EXT:
+        regs->a1 = vsbi_find_extension(regs->a0) ? 1 : 0;
+        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 ecall: EID: #%#lx FID: #%lu\n",
+                     __func__, eid, fid);
+        break;
+    }
+
+    return ret;
+}
+
+VSBI_EXT(base, SBI_EXT_BASE, SBI_EXT_BASE, vsbi_base_ecall_handler)