Re: PATCH: 0/6 [2nd try]: Add AVX support

H.J. Lu
2010-03-07T16:31:43Z

On Sun, Mar 7, 2010 at 6:37 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Sun, Mar 7, 2010 at 6:16 AM, Mark Kettenis <mark.kettenis@xs4all.nl> wrote:
>>> Date: Sat, 6 Mar 2010 14:16:34 -0800
>>> From: "H.J. Lu" <hongjiu.lu@intel.com>
>>>
>>> AVX registers are saved and restored via the XSAVE extended state. The
>>> extended control register 0 (the XFEATURE_ENABLED_MASK register), XCR0,
>>> is used to determine which states, x87, SSE, AVX, ... are supported
>>> in the XSAVE extended state.  XCR0 can be read with the new "xgetbv"
>>> instruction.  The xstate_bv field at byte offset 512 in the XSAVE
>>> extended state indicates what states the current process is in. If
>>> the feature bit is cleared, the corresponding registers should be read as
>>> 0. If we update a register, we should set the corresponding feature
>>> bit in the xstate_bv field.
>>>
>>> We added PTRACE_GETREGSET and PTRACE_SETREGSET to Linux kernel to
>>> fetch and store AVX registers with ptrace. Linux kernel also stores
>>> XCR0 at the first 8 bytes of the software usable bytes, starting at
>>> byte offset 464.
>>>
>>> There are total 6 patches to add AVX support for Linux.  They support:
>>>
>>> 1. The upper 128bit YMM registers are added for AVX support. The upper
>>> 128bit YMM registers are hidden from users. Gdb combines XMM register,
>>> %xmmX, with 128bit YMM register, %ymmXh, and present the whole 256bit
>>> YMM register, %ymmX, as pseudo register to users.
>>> 2. Backward compatible. If AVX isn't supported, SSE will be used.
>>> 3. Forward compatible. If new state beyond AVX is supported in
>>> the XSAVE extended state, only AVX state will be used.
>>> 4. Remote gdb protocol extension. GDB will send "x86=xml" in qSupported
>>> request packet to indicate that GDB supports x86 XML target desciption.
>>> The gdb stub will send x86 XML target desciption if it sees "x86=xml"
>>> in qSupported request packet.
>>>
>>> One advantage of this approach is YMM registers are actually stored as
>>> XMM registers and upper YMM registers in the XSAVE extended state.  It
>>> is easy and natural to access them as %xmmX and %ymmXh internally.  We
>>> just need to hide %ymmXh from users.
>>>
>>> To support AVX on other OSes, the following changes are needed:
>>>
>>> 1. Kernel support to get/set the XSAVE extended state.
>>> 2. Handle 8/16 upper YMM registers.
>>> 3. Provide target to_read_description to return SSE or AVX target
>>> description.
>>> 4. Update gdbarch_core_read_description to return SSE or AVX target
>>> description based on contents of core dump.
>>
>> Wait; there is something important missing here.  How are the new %ymm
>> registers referred to in debug info?  The AMD64 SysV psABI defines the
>> DWARF register Number Mapping, but the 0.99.4 draft copy I have
>> doesn't define any mappings for the %ymm registers.  What mapping does
>> GCC use?
>>
>
> In gcc, XMM and YMM registers have the same register number. They map
> to be the same DWARF register with different sizes.  Since XMM and YMM
> registers are caller-saved, they don't appear in unwind info. So, the same
> DWARF register with different sizes for XMM/YMM registers isn't a problem.
>
>

Yes, there is a problem. amd64_dwarf_reg_to_regnum needs to map 256bit
register to YMM. How do other arches solve this?

--
H.J.