Re: [Qemu-devel] [PATCH v5 12/23] RISC-V HTIF Console

[Michael Clark]

On Fri, Feb 9, 2018 at 5:35 AM, Richard Henderson <
address@hidden> wrote:

> On 02/07/2018 05:28 PM, Michael Clark wrote:
> > +++ b/hw/riscv/riscv_elf.c
> > @@ -0,0 +1,244 @@
> > +/*
> > + * elf.c - A simple package for manipulating symbol tables in elf
> binaries.
> > + *
> > + * Taken from
> > + * https://www.cs.cmu.edu/afs/cs.cmu.edu/academic/class/15213-f03/www/
> > + * ftrace/elf.c
> > + *
> > + */
> > +#include <stdio.h>
> > +#include <stdlib.h>
> > +#include <unistd.h>
> > +#include <string.h>
> > +#include <sys/stat.h>
> > +#include <sys/types.h>
> > +#include <sys/mman.h>
> > +#include <errno.h>
> > +#include <fcntl.h>
> > +#include <glib.h>
> > +
> > +#include "hw/riscv/riscv_elf.h"
> > +
> > +/*
> > + * elf_open - Map a binary into the address space and extract the
> > + * locations of the static and dynamic symbol tables and their string
> > + * tables. Return this information in a Elf object file handle that will
> > + * be passed to all of the other elf functions.
> > + */
> > +Elf_obj64 *elf_open64(const char *filename)
> > +{
> > +    int i, fd;
> > +    struct stat sbuf;
> > +    Elf_obj64 *ep;
> > +    Elf64_Shdr *shdr;
> > +
> > +    ep = g_new(Elf_obj64, 1);
> > +
> > +    /* Do some consistency checks on the binary */
> > +    fd = open(filename, O_RDONLY);
> > +    if (fd == -1) {
> > +        fprintf(stderr, "Can't open \"%s\": %s\n", filename,
> strerror(errno));
> > +        exit(1);
> > +    }
> > +    if (fstat(fd, &sbuf) == -1) {
> > +        fprintf(stderr, "Can't stat \"%s\": %s\n", filename,
> strerror(errno));
> > +        exit(1);
> > +    }
> > +    if (sbuf.st_size < sizeof(Elf64_Ehdr)) {
> > +        fprintf(stderr, "\"%s\" is not an ELF binary object\n",
> filename);
> > +        exit(1);
> > +    }
> > +
> > +    /* It looks OK, so map the Elf binary into our address space */
> > +    ep->mlen = sbuf.st_size;
> > +    ep->maddr = mmap(NULL, ep->mlen, PROT_READ, MAP_SHARED, fd, 0);
> > +    if (ep->maddr == (void *)-1) {
> > +        fprintf(stderr, "Can't mmap \"%s\": %s\n", filename,
> strerror(errno));
> > +        exit(1);
> > +    }
> > +    close(fd);
> > +
> > +    /* The Elf binary begins with the Elf header */
> > +    ep->ehdr = ep->maddr;
> > +
> > +    /* check we have a 64-bit little-endian RISC-V ELF object */
> > +    if (ep->ehdr->e_ident[EI_MAG0] != ELFMAG0 ||
> > +        ep->ehdr->e_ident[EI_MAG1] != ELFMAG1 ||
> > +        ep->ehdr->e_ident[EI_MAG2] != ELFMAG2 ||
> > +        ep->ehdr->e_ident[EI_MAG3] != ELFMAG3 ||
> > +        ep->ehdr->e_ident[EI_CLASS] != ELFCLASS64 ||
> > +        ep->ehdr->e_ident[EI_DATA] != ELFDATA2LSB ||
> > +        ep->ehdr->e_machine != EM_RISCV)
> > +    {
> > +        fprintf(stderr, "\"%s\" is not a 64-bit RISC-V ELF object\n",
> filename);
> > +        exit(1);
> > +    }
> > +
> > +    /*
> > +     * Find the static and dynamic symbol tables and their string
> > +     * tables in the the mapped binary. The sh_link field in symbol
> > +     * table section headers gives the section index of the string
> > +     * table for that symbol table.
> > +     */
> > +    shdr = (Elf64_Shdr *)(ep->maddr + ep->ehdr->e_shoff);
>
> This duplicates, badly, existing code within include/hw/elf_ops.h.
>
> In particular, this fails to bswap these fields.  As such, this code will
> only
> work on little-endian hosts.
>

There is enough information in the structures held in the syminfo table
used by the disassembler however elf_ops.h currently throws away all
symbols that are not STT_FUNC, and we are after STT_OBJECT symbols. We
would need to add an option to load_elf to load the entire symbol table.
Also, it runs qsort on the table by address, so we'd need to do a linear
scan for the HTIF symbols by name. There are only two symbols we need:
'fromhost' and 'tohost'.

        /* We are only interested in function symbols.
           Throw everything else away.  */
        if (syms[i].st_shndx == SHN_UNDEF ||
                syms[i].st_shndx >= SHN_LORESERVE ||
                ELF_ST_TYPE(syms[i].st_info) != STT_FUNC) {
            nsyms--;
            if (i < nsyms) {
                syms[i] = syms[nsyms];
            }
            continue;
        }

It would be really nice if HTIF just specified registers in device-tree;
alas it doesn't; and there is still quite a bit of code that depend on
using these magic HTIF 'fromhost' and 'tohost' symbols. riscv-tests for one
uses the HTIF mechanism and spike the official ISA simulator uses HTIF for
IO.