Re: [Qemu-devel] [Qemu-arm] [PATCH 11/13] target/arm/arm-semi: Implement support for semihosting feature detection

["Alex Bennée" <alex.bennee@linaro.org>]

Peter Maydell <peter.maydell@linaro.org> writes:

> 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>

with your EACCESS suggestion:

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

> ---
>  target/arm/arm-semi.c | 107 +++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 106 insertions(+), 1 deletion(-)
>
> diff --git a/target/arm/arm-semi.c b/target/arm/arm-semi.c
> index f9019b00b8d..531084b7799 100644
> --- a/target/arm/arm-semi.c
> +++ b/target/arm/arm-semi.c
> @@ -117,6 +117,7 @@ typedef enum GuestFDType {
>      GuestFDUnused = 0,
>      GuestFDHost = 1,
>      GuestFDGDB = 2,
> +    GuestFDFeatureFile = 3,
>  } GuestFDType;
>
>  /*
> @@ -125,7 +126,10 @@ typedef enum GuestFDType {
>   */
>  typedef struct GuestFD {
>      GuestFDType type;
> -    int hostfd;
> +    union {
> +        int hostfd;
> +        target_ulong featurefile_offset;
> +    };
>  } GuestFD;
>
>  static GArray *guestfd_array;
> @@ -467,6 +471,87 @@ static uint32_t gdb_flenfn(TaskState *ts, 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(TaskState *ts, ARMCPU *cpu, GuestFD *gf)
> +{
> +    /* Nothing to do */
> +    return 0;
> +}
> +
> +static uint32_t featurefile_writefn(TaskState *ts, ARMCPU *cpu, GuestFD *gf,
> +                                    target_ulong buf, uint32_t len)
> +{
> +    /* This fd can never be open for writing */
> +    errno = EBADF;
> +    return set_swi_errno(ts, -1);
> +}
> +
> +static uint32_t featurefile_readfn(TaskState *ts, 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(TaskState *ts, ARMCPU *cpu, GuestFD *gf)
> +{
> +    return 0;
> +}
> +
> +static uint32_t featurefile_seekfn(TaskState *ts, ARMCPU *cpu, GuestFD *gf,
> +                                   target_ulong offset)
> +{
> +    gf->featurefile_offset = offset;
> +    return 0;
> +}
> +
> +static uint32_t featurefile_flenfn(TaskState *ts, ARMCPU *cpu, GuestFD *gf)
> +{
> +    return sizeof(featurefile_data);
> +}
> +
>  typedef struct GuestFDFunctions {
>      sys_closefn *closefn;
>      sys_writefn *writefn;
> @@ -493,6 +578,14 @@ 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
> @@ -586,6 +679,18 @@ 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 = EINVAL;
> +                return set_swi_errno(ts, -1);
> +            }
> +            init_featurefile_guestfd(guestfd);
> +            return guestfd;
> +        }
> +
>          if (use_gdb_syscalls()) {
>              ret = arm_gdb_syscall(cpu, arm_semi_cb, "open,%s,%x,1a4", arg0,
>                                    (int)arg2+1, gdb_open_modeflags[arg1]);


--
Alex Bennée