target/arm/arm-semi: Implement support for semihosting feature detection

Version 2.0 of the semihosting specification added support for
allowing a guest to detect whether the implementation supported
particular features. This works by the guest opening a magic
file ":semihosting-features", which contains a fixed set of
data with some magic numbers followed by a sequence of bytes
with feature flags. The file is expected to behave sensibly
for the various semihosting calls which operate on files
(SYS_FLEN, SYS_SEEK, etc).

Implement this as another kind of guest FD using our function
table dispatch mechanism. Initially we report no extended
features, so we have just one feature flag byte which is zero.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20190916141544.17540-14-peter.maydell@linaro.org

target/arm/arm-semi.c

@@ -110,6 +110,7 @@ typedef enum GuestFDType {
     GuestFDUnused = 0,
     GuestFDHost = 1,
     GuestFDGDB = 2,
+    GuestFDFeatureFile = 3,
 } GuestFDType;
 
 /*
@@ -118,7 +119,10 @@ typedef enum GuestFDType {
  */
 typedef struct GuestFD {
     GuestFDType type;
+    union {
         int hostfd;
+        target_ulong featurefile_offset;
+    };
 } GuestFD;
 
 static GArray *guestfd_array;
@@ -504,6 +508,89 @@ static uint32_t gdb_flenfn(ARMCPU *cpu, GuestFD *gf)
                            gf->hostfd, arm_flen_buf(cpu));
 }
 
+#define SHFB_MAGIC_0 0x53
+#define SHFB_MAGIC_1 0x48
+#define SHFB_MAGIC_2 0x46
+#define SHFB_MAGIC_3 0x42
+
+static const uint8_t featurefile_data[] = {
+    SHFB_MAGIC_0,
+    SHFB_MAGIC_1,
+    SHFB_MAGIC_2,
+    SHFB_MAGIC_3,
+    0, /* Feature byte 0 */
+};
+
+static void init_featurefile_guestfd(int guestfd)
+{
+    GuestFD *gf = do_get_guestfd(guestfd);
+
+    assert(gf);
+    gf->type = GuestFDFeatureFile;
+    gf->featurefile_offset = 0;
+}
+
+static uint32_t featurefile_closefn(ARMCPU *cpu, GuestFD *gf)
+{
+    /* Nothing to do */
+    return 0;
+}
+
+static uint32_t featurefile_writefn(ARMCPU *cpu, GuestFD *gf,
+                                    target_ulong buf, uint32_t len)
+{
+    /* This fd can never be open for writing */
+    CPUARMState *env = &cpu->env;
+
+    errno = EBADF;
+    return set_swi_errno(env, -1);
+}
+
+static uint32_t featurefile_readfn(ARMCPU *cpu, GuestFD *gf,
+                                   target_ulong buf, uint32_t len)
+{
+    uint32_t i;
+#ifndef CONFIG_USER_ONLY
+    CPUARMState *env = &cpu->env;
+#endif
+    char *s;
+
+    s = lock_user(VERIFY_WRITE, buf, len, 0);
+    if (!s) {
+        return len;
+    }
+
+    for (i = 0; i < len; i++) {
+        if (gf->featurefile_offset >= sizeof(featurefile_data)) {
+            break;
+        }
+        s[i] = featurefile_data[gf->featurefile_offset];
+        gf->featurefile_offset++;
+    }
+
+    unlock_user(s, buf, len);
+
+    /* Return number of bytes not read */
+    return len - i;
+}
+
+static uint32_t featurefile_isattyfn(ARMCPU *cpu, GuestFD *gf)
+{
+    return 0;
+}
+
+static uint32_t featurefile_seekfn(ARMCPU *cpu, GuestFD *gf,
+                                   target_ulong offset)
+{
+    gf->featurefile_offset = offset;
+    return 0;
+}
+
+static uint32_t featurefile_flenfn(ARMCPU *cpu, GuestFD *gf)
+{
+    return sizeof(featurefile_data);
+}
+
 typedef struct GuestFDFunctions {
     sys_closefn *closefn;
     sys_writefn *writefn;
@@ -530,6 +617,14 @@ static const GuestFDFunctions guestfd_fns[] = {
         .seekfn = gdb_seekfn,
         .flenfn = gdb_flenfn,
     },
+    [GuestFDFeatureFile] = {
+        .closefn = featurefile_closefn,
+        .writefn = featurefile_writefn,
+        .readfn = featurefile_readfn,
+        .isattyfn = featurefile_isattyfn,
+        .seekfn = featurefile_seekfn,
+        .flenfn = featurefile_flenfn,
+    },
 };
 
 /* Read the input value from the argument block; fail the semihosting
@@ -616,6 +711,18 @@ target_ulong do_arm_semihosting(CPUARMState *env)
             unlock_user(s, arg0, 0);
             return guestfd;
         }
+        if (strcmp(s, ":semihosting-features") == 0) {
+            unlock_user(s, arg0, 0);
+            /* We must fail opens for modes other than 0 ('r') or 1 ('rb') */
+            if (arg1 != 0 && arg1 != 1) {
+                dealloc_guestfd(guestfd);
+                errno = EACCES;
+                return set_swi_errno(env, -1);
+            }
+            init_featurefile_guestfd(guestfd);
+            return guestfd;
+        }
+
         if (use_gdb_syscalls()) {
             arm_semi_open_guestfd = guestfd;
             ret = arm_gdb_syscall(cpu, arm_semi_open_cb, "open,%s,%x,1a4", arg0,