Re: [PATCH 0/7] KVM: x86: Let the guest write to multiple debug registers with one vmexit

Jan Kiszka
2014-03-09T08:11:36Z

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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?

Jan

>
> 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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