On Thu, 2017-01-26 at 09:05 -0800, Andy Lutomirski wrote:
On Wed, Jan 25, 2017 at 9:50 PM, Ricardo Neri ricardo.neri-calderon@linux.intel.com wrote:
On Wed, 2017-01-25 at 13:58 -0800, Andy Lutomirski wrote:
On Wed, Jan 25, 2017 at 12:23 PM, Ricardo Neri ricardo.neri-calderon@linux.intel.com wrote:
Tasks running in virtual-8086 mode will use 16-bit addressing form encodings as described in the Intel 64 and IA-32 Architecture Software Developer's Manual Volume 2A Section 2.1.5. 16-bit addressing encodings differ in several ways from the 32-bit/64-bit addressing form encodings: the r/m part of the ModRM byte points to different registers and, in some cases, addresses can be indicated by the addition of the value of two registers. Also, there is no support for SiB bytes. Thus, a separate function is needed to parse this form of addressing.
Furthermore, virtual-8086 mode tasks will use real-mode addressing. This implies that the segment selectors do not point to a segment descriptor but are used to compute logical addresses. Hence, there is a need to add support to compute addresses using the segment selectors. If segment- override prefixes are present in the instructions, they take precedence.
Lastly, it is important to note that when a tasks is running in virtual- 8086 mode and an interrupt/exception occurs, the CPU pushes to the stack the segment selectors for ds, es, fs and gs. These are accesible via the struct kernel_vm86_regs rather than pt_regs.
Code for 16-bit addressing encodings is likely to be used only by virtual- 8086 mode tasks. Thus, this code is wrapped to be built only if the option CONFIG_VM86 is selected.
That's not true. It's used in 16-bit protected mode, too. And there are (ugh!) six possibilities:
Thanks for the clarification. I will enable the decoding of addresses for 16-bit as well... and test the emulation code.
- Normal 32-bit protected mode. This should already work.
- Normal 64-bit protected mode. This should also already work. (I
forget whether a 16-bit SS is either illegal or has no effect in this case.)
For these two cases I am just taking the effective address that the user space application provides, given that the segment selectors were set beforehand (and with a base of 0).
What do you mean by the base being zero? User code can set a nonzero DS base if it wants. In 64-bit mode (user_64bit_mode(regs)), the base is ignored unless there's an fs or gs prefix, and in 32-bit mode the base is never ignored.
Yes, I take this back. At the time of writing I was thinking about the __USER_CS and _USER_DS descriptors. You ar right, the base is not ignored.
- Virtual 8086 mode
In this case I calculate the linear address as: (segment_select << 4) + effective address.
- Normal 16-bit protected mode, used by DOSEMU and Wine. (16-bit CS,
16-bit address segment)
- 16-bit CS, 32-bit address segment. IIRC this might be used by some
32-bit DOS programs to call BIOS.
- 32-bit CS, 16-bit address segment. I don't know whether anything uses this.
In all these protected modes, are you referring to the size in bits of the base address of in the descriptor selected in the CS register? In such a case I would need to get the base address and add it to the effective address given in the operands of the instructions, right?
No, I'm referring to the D/B bit. I'm a bit fuzzy on exactly how the instruction encoding works, but I think that 16-bit x86 code is encoded just like real mode code except that the selectors are used for real.
I see. I have the logic to differentiate between 16-bit and 32-bit addresses. What I am missing is code to look at the value of this bit and any potential operand overrides. I will work on adding this.
size, but I suspect you'll need to handle 16-bit CS.
Unless I am missing what is special with the 16-bit base address, I only would need to add that base address to whatever effective address (aka, offset) is encoded in the ModRM and displacement bytes.
Exactly. (And make sure the instruction decoder can decode 16-bit instructions correctly.)
Will do. I have tested my 16-bit decoding extensively. I think it will work.
Thanks and BR, Ricardo