From mboxrd@z Thu Jan 1 00:00:00 1970 From: Alex Williamson Subject: Re: [PATCH 0/4] KVM: x86: Let the guest write to multiple debug registers with one vmexit Date: Wed, 26 Feb 2014 10:00:40 -0700 Message-ID: <1393434040.18087.2.camel@ul30vt.home> References: <1393429753-20857-1-git-send-email-pbonzini@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, mtosatti@redhat.com, gleb@kernel.org, jan.kiszka@siemens.com To: Paolo Bonzini Return-path: In-Reply-To: <1393429753-20857-1-git-send-email-pbonzini@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On Wed, 2014-02-26 at 16:49 +0100, Paolo Bonzini wrote: > Alex Williamson reported that a Windows game does something weird that > makes the guest save and restore debug registers on each context switch. > This cause several hundred thousands vmexits per second, and basically > cuts performance in half when running under KVM. > > However, when not running in guest-debug mode, the guest controls the > debug registers and having to take an exit for each DR access is a waste > of time. We just need one vmexit to load any stale values of DR0-DR6, > and then we can let the guest run freely. On the next vmexit (whatever > the reason) we will read out whatever changes the guest made to the > debug registers. > > On top of this, we can implement SVM support and let nested guests run > with dirty debug registers too. > > Paolo Bonzini (4): > KVM: vmx: we do rely on loading DR7 on entry > KVM: x86: change vcpu->arch.switch_db_regs to a bit mask > KVM: x86: Allow the guest to run with dirty debug registers > KVM: vmx: Allow the guest to run with dirty debug registers > > arch/x86/include/asm/kvm_host.h | 8 ++++++- > arch/x86/kvm/vmx.c | 48 +++++++++++++++++++++++++++++++++++++---- > arch/x86/kvm/x86.c | 29 +++++++++++++++++++++++-- > 3 files changed, 78 insertions(+), 7 deletions(-) > I see a slightly better than 100% improvement in the frame rate in Borderlands2 running in a VM with assigned Nvidia GPU with this series. Tested-by: Alex Williamson