From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andi Kleen <ak@linux.intel.com>
Subject: Re: [PATCH] KVM: VMX: Cache IA32_DEBUGCTL in memory
Date: Wed, 29 Nov 2017 10:20:51 -0800
Message-ID: <87fu8xj69o.fsf@linux.intel.com>
References: <1511935673-7371-1-git-send-email-wanpeng.li@hotmail.com>
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Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        Paolo Bonzini <pbonzini@redhat.com>,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Wanpeng Li <wanpeng.li@hotmail.com>,
        Jim Mattson <jmattson@google.com>
To: Wanpeng Li <kernellwp@gmail.com>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <1511935673-7371-1-git-send-email-wanpeng.li@hotmail.com>
        (Wanpeng Li's message of "Tue, 28 Nov 2017 22:07:53 -0800")
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List-Id: kvm.vger.kernel.org

Wanpeng Li <kernellwp@gmail.com> writes:

> From: Wanpeng Li <wanpeng.li@hotmail.com>
>
> MSR_IA32_DEBUGCTLMSR is zeroed on VMEXIT, so it is saved/restored 
> each time during world switch. Jim from Google pointed out that 
> when running schbench in L2, vmx_vcpu_run will occupy 4% cpu time, 
> and the 25% of vmx_vcpu_run cpu time is occupied by get_debugctlmsr(). 
> This patch caches the host IA32_DEBUGCTL MSR and saves/restores 
> the host IA32_DEBUGCTL msr when guest/host switches to avoid to 
> save/restore each time during world switch.

FWIW i've seen this too on L2 profiles.

But I haven't looked too closely, but I suspect you'll clobber global
kernel debugger state this way.

You would at least need some interface for KDB etc. to invalidate
your cache.

-Andi