Re: [Xen-devel] [PATCH v2 08/16] SVM: use generic instruction decoding

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 28/09/16 09:13, Jan Beulich wrote:
> ... instead of custom handling. To facilitate this break out init code
> from _hvm_emulate_one() into the new hvm_emulate_init(), and make
> hvmemul_insn_fetch( globally available.

)

>  int __get_instruction_length_from_list(struct vcpu *v,
>          const enum instruction_index *list, unsigned int list_count)
>  {
>      struct vmcb_struct *vmcb = v->arch.hvm_svm.vmcb;
> -    unsigned int i, j, inst_len = 0;
> -    enum instruction_index instr = 0;
> -    u8 buf[MAX_INST_LEN];
> -    const u8 *opcode = NULL;
> -    unsigned long fetch_addr, fetch_limit;
> -    unsigned int fetch_len, max_len;
> +    struct hvm_emulate_ctxt ctxt;
> +    struct x86_emulate_state *state;
> +    unsigned int inst_len, j, modrm_rm, modrm_reg;
> +    int modrm_mod;
>  

Despite knowing how this works, it is still confusing to read.  Do you
mind putting in a comment such as:

/* In a debug build, always use x86_decode_insn() and compare with
hardware. */

> +#ifdef NDEBUG
>      if ( (inst_len = svm_nextrip_insn_length(v)) != 0 )
>          return inst_len;
>  
>      if ( vmcb->exitcode == VMEXIT_IOIO )
>          return vmcb->exitinfo2 - vmcb->rip;
> +#endif
>  
> -    /* Fetch up to the next page break; we'll fetch from the next page
> -     * later if we have to. */
> -    fetch_addr = svm_rip2pointer(v, &fetch_limit);
> -    if ( vmcb->rip > fetch_limit )
> -        return 0;
> -    max_len = min(fetch_limit - vmcb->rip + 1, MAX_INST_LEN + 0UL);
> -    fetch_len = min_t(unsigned int, max_len,
> -                      PAGE_SIZE - (fetch_addr & ~PAGE_MASK));
> -    if ( !fetch(vmcb, buf, fetch_addr, fetch_len) )
> +    ASSERT(v == current);
> +    hvm_emulate_prepare(&ctxt, guest_cpu_user_regs());
> +    hvm_emulate_init(&ctxt, NULL, 0);
> +    state = x86_decode_insn(&ctxt.ctxt, hvmemul_insn_fetch);
> +    if ( IS_ERR_OR_NULL(state) )
>          return 0;
>  
> -    while ( (inst_len < max_len) && is_prefix(buf[inst_len]) )
> -    {
> -        inst_len++;
> -        if ( inst_len >= fetch_len )
> -        {
> -            if ( !fetch(vmcb, buf + fetch_len, fetch_addr + fetch_len,
> -                        max_len - fetch_len) )
> -                return 0;
> -            fetch_len = max_len;
> -        }
> +    inst_len = x86_insn_length(state, &ctxt.ctxt);
> +    modrm_mod = x86_insn_modrm(state, &modrm_rm, &modrm_reg);
> +    x86_emulate_free_state(state);
> +#ifndef NDEBUG
> +    if ( vmcb->exitcode == VMEXIT_IOIO )
> +        j = vmcb->exitinfo2 - vmcb->rip;
> +    else
> +        j = svm_nextrip_insn_length(v);
> +    if ( j && j != inst_len )
> +    {
> +        gprintk(XENLOG_WARNING, "insn-len[%02x]=%u (exp %u)\n",
> +                ctxt.ctxt.opcode, inst_len, j);
> +        return j;
>      }
> +#endif
>  
>      for ( j = 0; j < list_count; j++ )
>      {
> -        instr = list[j];
> -        opcode = opc_bytes[instr];
> +        enum instruction_index instr = list[j];
>  
> -        for ( i = 0; (i < opcode[0]) && ((inst_len + i) < max_len); i++ )
> +        ASSERT(instr >= 0 && instr < ARRAY_SIZE(opc_tab));

This is another ASSERT() used as a bounds check, and will suffer a build
failure on clang.

You need to use s/enum instruction_index/unsigned int/ to fix the build
issue.  Can I also request the use of

if ( instr >= ARRAY_SIZE(opc_tab) )
{
    ASSERT_UNREACHABLE();
    return 0;
}

instead?

> --- a/xen/arch/x86/x86_emulate/x86_emulate.c
> +++ b/xen/arch/x86/x86_emulate/x86_emulate.c
> @@ -5200,3 +5214,89 @@ x86_emulate(
>  #undef vex
>  #undef override_seg
>  #undef ea
> +
> +#ifdef __XEN__
> +
> +#include <xen/err.h>
> +
> +struct x86_emulate_state *
> +x86_decode_insn(
> +    struct x86_emulate_ctxt *ctxt,
> +    int (*insn_fetch)(
> +        enum x86_segment seg, unsigned long offset,
> +        void *p_data, unsigned int bytes,
> +        struct x86_emulate_ctxt *ctxt))
> +{
> +    static DEFINE_PER_CPU(struct x86_emulate_state, state);
> +    struct x86_emulate_state *state = &this_cpu(state);
> +    const struct x86_emulate_ops ops = {

This can be static, to avoid having it reconstructed on the stack each
function call.

Otherwise, Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>


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