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

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Date:   Fri,  2 Mar 2018 11:55:00 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     kvm@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, x86@...nel.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 RFC 0/3] x86/kvm: avoid expensive rdmsrs for FS/GS base MSRs

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.

I have to admit I got a bit lost in our kernel FS/GS magic. I managed to 
convince myself that in the well defined context (ioctl from userspace)
we can always get the required values from in-kernel variables and avoid
rdmsrs. But I may have missed something really important, thus RFC.

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

In case these patches turn out to be worthwile AMD SVM can probably be
optimized the ame way.

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/kernel/cpu/common.c | 1 +
 arch/x86/kvm/vmx.c           | 7 ++++---
 2 files changed, 5 insertions(+), 3 deletions(-)

-- 
2.14.3