linux-kernel - [patch 0/4] x86/fpu/kvm: Sanitize the FPU guest/user handling

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Message-ID: <20211017151447.829495362@linutronix.de>
Date:   Sun, 17 Oct 2021 19:03:11 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     x86@...nel.org, "Liu, Jing2" <jing2.liu@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "Bae, Chang Seok" <chang.seok.bae@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        kvm@...r.kernel.org, "Nakajima, Jun" <jun.nakajima@...el.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: [patch 0/4] x86/fpu/kvm: Sanitize the FPU guest/user handling

Currently KVM allocates two FPU structs which are used for saving the user
state of the vCPU thread and restoring the guest state when entering
vcpu_run() and doing the reverse operation before leaving vcpu_run().

With the new fpstate mechanism this can be reduced to one extra buffer by
swapping the fpstate pointer in current::thread::fpu. This makes also the
upcoming support for AMX and XFD simpler because then fpstate information
(features, sizes, xfd) are always consistent and it does not require any
nasty workarounds.

The following series cleans that up and replaces the current scheme with a
single guest state which is switched in when entering vcpu_run() and
switched out before leaving it.

The rework is valuable even without AMX/XFD because it consumes less memory
and when swapping the fpstates there is no memory copy required when
TIF_NEED_LOAD_FPU is set on the going out fpstate.

The series is based on:

  git://git.kernel.org/pub/scm/linux/kernel/git/tglx/devel.git x86/fpu-3

and is now part of the full AMX series:

  git://git.kernel.org/pub/scm/linux/kernel/git/tglx/devel.git x86/fpu

On top of that I've integrated the KVM reallocation mechanism into:

  git://git.kernel.org/pub/scm/linux/kernel/git/tglx/devel.git x86/fpu-kvm

The latter builds, boots and runs KVM guests, but that reallocation
functionality is obviously completely untested. I want to share this with
KVM folks so they can start to look how to integrate their XFD/XCR0 and
reallocation scheme as discussed and outlined here:

   https://lore.kernel.org/r/87mtn93u58.ffs@tglx

and the related thread. It's a tiny incremental update on top of x86/fpu (6
files changed, 183 insertions(+), 36 deletions(-)) which reuses the host
side mechanisms.

Thanks,

	tglx
---
 include/asm/fpu/api.h   |   19 ++++++--
 include/asm/fpu/types.h |   44 ++++++++++++++++++-
 include/asm/kvm_host.h  |    7 ---
 kernel/fpu/core.c       |  110 +++++++++++++++++++++++++++++++++---------------
 kvm/svm/svm.c           |    7 +--
 kvm/x86.c               |   88 ++++++++++----------------------------
 6 files changed, 165 insertions(+), 110 deletions(-)