linux-kernel - Re: [PATCH v2 0/2] x86/kvm: avoid expensive rdmsrs for FS/GS base MSRs

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Message-ID: <426fa4fd-7c4d-b6dc-cc19-b0034b7226aa@redhat.com>
Date:   Wed, 14 Mar 2018 17:12:13 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org,
        x86@...nel.org
Cc:     linux-kernel@...r.kernel.org,
        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: Re: [PATCH v2 0/2] x86/kvm: avoid expensive rdmsrs for FS/GS base
 MSRs

On 13/03/2018 18:48, Vitaly Kuznetsov wrote:
> Changes since v1:
> - Merged patches 1 and 2, skip savesegment() for FS and FS for x86_64
>   as save_fsgs() already does that [Paolo Bonzini]
> - Rename save_current_fsgs() -> save_fsgs_for_kvm() [Andy Lutomirski]
> - Trimmed comments [Andy Lutomirski]
> - Add Andy's A-b to what is now PATCH2
> 
> 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 (2):
>   x86/kvm/vmx: read MSR_{FS,KERNEL_GS}_BASE from current->thread
>   x86/kvm/vmx: avoid expensive rdmsr for MSR_GS_BASE
> 
>  arch/x86/include/asm/processor.h | 10 ++++++++++
>  arch/x86/kernel/cpu/common.c     |  3 ++-
>  arch/x86/kernel/process_64.c     | 14 ++++++++++++++
>  arch/x86/kvm/vmx.c               | 16 ++++++++++++----
>  4 files changed, 38 insertions(+), 5 deletions(-)
> 

Queued, thanks.

Paolo