Re: [Xen-devel] [PATCH] x86/ioemul: Account for ioemul_handle_quirk() in stub length check

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 10.01.18 at 10:56, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 10/01/2018 09:52, Jan Beulich wrote:
>>>>> On 09.01.18 at 17:47, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> --- a/xen/arch/x86/pv/emul-priv-op.c
>>> +++ b/xen/arch/x86/pv/emul-priv-op.c
>>> @@ -89,19 +89,24 @@ static io_emul_stub_t *io_emul_stub_setup(struct 
>>> priv_op_ctxt *ctxt, u8 opcode,
>>>      /* callq *%rcx */
>>>      ctxt->io_emul_stub[10] = 0xff;
>>>      ctxt->io_emul_stub[11] = 0xd1;
>>> -    /* data16 or nop */
>>> -    ctxt->io_emul_stub[12] = (bytes != 2) ? 0x90 : 0x66;
>>> -    /* <io-access opcode> */
>>> -    ctxt->io_emul_stub[13] = opcode;
>>> -    /* imm8 or nop */
>>> -    ctxt->io_emul_stub[14] = !(opcode & 8) ? port : 0x90;
>>> -    /* ret (jumps to guest_to_host_gpr_switch) */
>>> -    ctxt->io_emul_stub[15] = 0xc3;
>>> -    BUILD_BUG_ON(STUB_BUF_SIZE / 2 < 16);
>>> -
>>> -    if ( ioemul_handle_quirk )
>>> +
>>> +    if ( likely(!ioemul_handle_quirk) )
>>> +    {
>>> +        /* data16 or nop */
>>> +        ctxt->io_emul_stub[12] = (bytes != 2) ? 0x90 : 0x66;
>>> +        /* <io-access opcode> */
>>> +        ctxt->io_emul_stub[13] = opcode;
>>> +        /* imm8 or nop */
>>> +        ctxt->io_emul_stub[14] = !(opcode & 8) ? port : 0x90;
>>> +        /* ret (jumps to guest_to_host_gpr_switch) */
>>> +        ctxt->io_emul_stub[15] = 0xc3;
>>> +    }
>>> +    else
>>>          ioemul_handle_quirk(opcode, &ctxt->io_emul_stub[12], 
>>> ctxt->ctxt.regs);
>>>  
>>> +    BUILD_BUG_ON(STUB_BUF_SIZE / 2 < MAX(16, /* Regular stubs */
>> Now that we have it available globally, would you mind using
>> MAX_INST_LEN + 1 here instead of the literal 16?
> 
> This raw 16 is because we write 16 bytes into ctxt->io_emul_stub[], and
> this jumps to 19 with the INDIRECT_THUNK work, not because it is related
> to MAX_INST_LEN.

I don't follow - as the comment says, the 16 refers to regular
stubs, which are there to allow for maximum insn length plus RET.

Jan


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