linux-kernel - [PATCH 0/3] x86/kvm: avoid expensive rdmsrs for FS/GS base MSRs

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Date:   Mon, 12 Mar 2018 15:02:57 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     kvm@...r.kernel.org, x86@...nel.org
Cc:     linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>
Subject: [PATCH 0/3] x86/kvm: avoid expensive rdmsrs for FS/GS base MSRs

Changes since RFC [Andy Lutomirski]:
- Export new save_current_fsgs() API and call it before reading
  current->thread.fs/gsbase
- New cpu_kernelmode_gs_base() API.
- Some comments added.

Some time ago Paolo suggested to take a look at probably unneeded expensive
rdmsrs for FS/GS base MSR in vmx_save_host_state(). This is called on every
vcpu run when we need to handle vmexit in userspace.

vmx_save_host_state() is always called in a very well defined context
(ioctl from userspace) so we may try to get the required values for FS/GS
bases from in-kernel variables and avoid expensive rdmsrs.

My debug shows we're shaving off 240 cpu cycles (E5-2603 v3).

Vitaly Kuznetsov (3):
  x86/kvm/vmx: read MSR_FS_BASE from current->thread
  x86/kvm/vmx: read MSR_KERNEL_GS_BASE from current->thread
  x86/kvm/vmx: avoid expensive rdmsr for MSR_GS_BASE

 arch/x86/include/asm/processor.h |  8 ++++++++
 arch/x86/kernel/cpu/common.c     |  3 ++-
 arch/x86/kernel/process_64.c     | 20 ++++++++++++++++++++
 arch/x86/kvm/vmx.c               |  9 ++++++---
 4 files changed, 36 insertions(+), 4 deletions(-)

-- 
2.14.3