Re: git: b4efc6277634 - main - LinuxKPI: Do not use explicit context in FPU sections on powerpc64

On 12/30/23 7:14 PM, Vladimir Kondratyev wrote:
> On 27.12.2023 20:05, John Baldwin wrote:
>> On 12/24/23 5:20 AM, Vladimir Kondratyev wrote:
>>> The branch main has been updated by wulf:
>>>
>>> URL:
>>> https://cgit.FreeBSD.org/src/commit/?id=b4efc62776344a9aaada5a0866e453e528a0e977
>>>
>>> commit b4efc62776344a9aaada5a0866e453e528a0e977
>>> Author:     Vladimir Kondratyev <wulf@FreeBSD.org>
>>> AuthorDate: 2023-12-24 12:48:06 +0000
>>> Commit:     Vladimir Kondratyev <wulf@FreeBSD.org>
>>> CommitDate: 2023-12-24 12:48:06 +0000
>>>
>>>       LinuxKPI: Do not use explicit context in FPU sections on
>>> powerpc64
>>>       It is not supported yet.
>>>       Sponsored by:   Serenity Cyber Security, LLC
>>>       Fixes:  5a3bd281672b ("LinuxKPI: Add explicit software context to
>>> FPU sections")
>>>       MFC after:      1 week
>>> ---
>>>    sys/compat/linuxkpi/common/include/linux/compat.h | 5 -----
>>>    sys/compat/linuxkpi/common/src/linux_current.c    | 9 ++++++---
>>>    sys/compat/linuxkpi/common/src/linux_fpu.c        | 3 ++-
>>>    3 files changed, 8 insertions(+), 9 deletions(-)
>>
>> Do you need explicit contexts at all?
> 
> Original version of https://reviews.freebsd.org/D42822 did not use
> explicit contexts.
> 
>    That is, can you not just
>> use FPU_KERN_NOCTX all the time?  Most code in the tree now uses
>> FPU_KERN_NOCTX now (all the crypto drivers for example), and I've
>> been thinking about removing support for the !FPU_KERN_NOCTX case.
>> Is there a reason drm-kmod can't use FPU_KERN_NOCTX?  Do you really
>> need to save FPU registers in one block of code wrapped by fpu_kern_*
>> and then use those register values in a future section wrapped by
>> fpu_kern_*?
>>
> 
> I can revert current code end use previous version. Just give me some
> time.
> I will be AFK till the end of january.

Hmm, well the review for that answers my question which is that
there is GPU driver code that calls malloc() while in an FPU
section.  So the answer to my original question is, no, you can't
just use FPU_KERN_NOCTX.  Or if you did you would need to patch
the driver code instead to narrow the FPU context usage so it
wasn't enabled when malloc() is called.  I don't know how feasible
it would be to fix the drivers to narrow the FPU sections, but I
think you would have to do that if you wanted to go back to always
using FPU_KERN_NOCTX.

-- 
John Baldwin