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Date:	Sun, 09 Mar 2014 09:11:36 +0100
From:	Jan Kiszka <>
To:	Paolo Bonzini <>,
Subject: Re: [PATCH 0/7] KVM: x86: Let the guest write to multiple debug registers
 with one vmexit

On 2014-03-07 12:42, 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.
> Tested with x86/debug.flat on both Intel and AMD, both direct and
> nested virtualization.
> Changes from RFC: changed get_dr7 callback to sync_dirty_debug_regs,
> 	new patches 5-7.

This looks good now to me from KVM perspective. I was just wondering how
the case is handled that the host used debug registers on the thread the
runs a VCPU? What if I set a hw breakpoint on its userspace path e.g.?
What if I debug the kernel side with kgdb?


> Paolo Bonzini (7):
>   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
>   KVM: nVMX: Allow nested guests to run with dirty debug registers
>   KVM: svm: set/clear all DR intercepts in one swoop
>   KVM: svm: Allow the guest to run with dirty debug registers
>  arch/x86/include/asm/kvm_host.h |  8 ++++-
>  arch/x86/kvm/svm.c              | 68 ++++++++++++++++++++++++++++-------------
>  arch/x86/kvm/vmx.c              | 43 ++++++++++++++++++++++++--
>  arch/x86/kvm/x86.c              | 20 +++++++++++-
>  4 files changed, 114 insertions(+), 25 deletions(-)

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