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== Background ==
The XSTATE init code initializes all enabled and supported components.
Then, the init states are saved in the init_fpstate buffer that is
statically allocated in about one page.
The AMX TILE_DATA state is large (8KB) but its init state is zero. And the
feature comes only with the compacted format with these established
dependencies: AMX->XFD->XSAVES. So this state is excludable from
init_fpstate.
== Problem ==
But the buffer is formatted to include that large state. Then, this can be
the cause of a noisy splat like the below.
This came from XRSTORS for the task with init_fpstate in its XSAVE buffer.
It is reproducible on AMX systems when the running kernel is built with
CONFIG_DEBUG_PAGEALLOC=y and CONFIG_DEBUG_PAGEALLOC_ENABLE_DEFAULT=y:
Bad FPU state detected at restore_fpregs_from_fpstate+0x57/0xd0, reinitializing FPU registers.
...
RIP: 0010:restore_fpregs_from_fpstate+0x57/0xd0
? restore_fpregs_from_fpstate+0x45/0xd0
switch_fpu_return+0x4e/0xe0
exit_to_user_mode_prepare+0x17b/0x1b0
syscall_exit_to_user_mode+0x29/0x40
do_syscall_64+0x67/0x80
? do_syscall_64+0x67/0x80
? exc_page_fault+0x86/0x180
entry_SYSCALL_64_after_hwframe+0x63/0xcd
== Solution ==
Adjust init_fpstate to exclude dynamic states. XRSTORS from init_fpstate
still initializes those states when their bits are set in the
requested-feature bitmap.
Fixes: 2308ee57d9 ("x86/fpu/amx: Enable the AMX feature in 64-bit mode")
Reported-by: Lin X Wang <lin.x.wang@intel.com>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Lin X Wang <lin.x.wang@intel.com>
Link: https://lore.kernel.org/r/20220824191223.1248-4-chang.seok.bae@intel.com
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.