linux-kernel - [PATCH 0/3] x86/fpu: Improve the init

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Message-Id: <20220824191223.1248-1-chang.seok.bae@intel.com>
Date:   Wed, 24 Aug 2022 12:12:20 -0700
From:   "Chang S. Bae" <chang.seok.bae@...el.com>
To:     linux-kernel@...r.kernel.org
Cc:     x86@...nel.org, chang.seok.bae@...el.com
Subject: [PATCH 0/3] x86/fpu: Improve the init_fpstate setup code

Hi all,

This set of patches fixes the init_fpstate code. So this first version is
sent to the maintainers hoping the fix is reviewable.

Thanks,
Chang

== Background ==

The init_fpstate is an XSAVE image that records init states during the boot
time. It is presumed to cover all the supported and enabled features. The
setup code has been recently optimized to capture legacy states only as all
of the other init states are all zeros.

== Problem with AMX state ==

When AMX is enabled, this buffer is too small to include AMX TILE_DATA
(8KB) as it is statically allocated with about a page. But, the buffer is
formatted to have them all although using the compacted format.

This also leads to a noisy splat with XRSTORS as it expects all the buffer
memory accessible. This is mentioned in Intel SDM Vol.1 13.13 Memory Access
By The XSAVE Feature Set:
    "An execution of an instruction in the XSAVE feature set may access any
     byte of any state component on which that execution operates."

== Other minor issues ==

The existing sanity check could help finding this issue as it checks
whether the allocated init_fpstate is enough for the expected size or not.
But what is currently measured is not matched -- the size without the AMX
state.

Also, these size and features are better to be configured first before
setting up the init image.

== Patchset ==

As AMX requires the compacted format, init_fpstate may exclude dynamic
states. The series also includes other improvments:
* Set up the init_fpstate buffer after its scope is clarified.
* Fix the size that is validated against the static allocation.

Chang S. Bae (3):
  x86/fpu: Configure init_fpstate attributes orderly
  x86/fpu: Fix the init_fpstate size check with the actual size
  x86/fpu: Exclude dynamic states from init_fpstate

 arch/x86/kernel/fpu/init.c   |  8 --------
 arch/x86/kernel/fpu/xstate.c | 33 ++++++++++++++-------------------
 2 files changed, 14 insertions(+), 27 deletions(-)

base-commit: cf90f46223eef9d5f389b4b88ee2fc7914458b06
-- 
2.17.1