Re: ABI change for device drivers using future AVX instruction set

On Wed, 25 Jun 2008 17:32:36 +0200
Agner Fog <agner@xxxxxxxxx> wrote:


One problem that has not been resolved yet, AFAIK, is how to handle
the possible use of YMM registers in device drivers. Should these
registers be saved before calling a device driver from an interrupt
or should it be the responsibility of the device driver?

the answer is: you don't use AVX in drivers just as you don't use SSE
in drivers.

Let me repeat this loud and clear:

It is not allowed to use floating point, SSE of AVX in device drivers.

(there are few places that very carefully do this anyway, the raid5
code being the most obvious one, but we should not add more)


2. There is a performance cost to using XSAVE / XRESTOR.

a patch introducing xsave/xrestor is being discussed already here...
it's not really more expensive.


3. When compiling a device driver, the compiler may insert implicit
calls to library functions such as memcpy and memset. These functions
typically have a CPU dispatcher that chooses the largest register
size available. The device driver may therefore use YMM registers
without the knowledge of the programmer and without compiling with
the AVX switch on.

this is not correct; the library functions are provided by the kernel
and do not use AVX etc.


--
If you want to reach me at my work email, use arjan@xxxxxxxxxxxxxxx
For development, discussion and tips for power savings,
visit http://www.lesswatts.org
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

Relevant Pages

  • ABI change for device drivers using future AVX instruction set
    ... The announced future Intel AVX instruction set extends the 128-bit XMM registers to 256-bit YMM registers. ... Intel has proposed relevant ABI extensions, which I assume will be adopted in the System V ABI. ... One problem that has not been resolved yet, AFAIK, is how to handle the possible use of YMM registers in device drivers. ... Should these registers be saved before calling a device driver from an interrupt or should it be the responsibility of the device driver? ...
    (Linux-Kernel)
  • Re: ABI change for device drivers using future AVX instruction set
    ... calls to library functions such as memcpy and memset. ... typically have a CPU dispatcher that chooses the largest register size ... The device driver may therefore use YMM registers without the ... generally avoid kernel usage in order to avoid the overheads. ...
    (Linux-Kernel)