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

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Date:	Sun, 09 Mar 2014 09:15:41 +0100
From:	Jan Kiszka <jan.kiszka@....de>
To:	Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org
CC:	kvm@...r.kernel.org, alex.williamson@...hat.com,
	mtosatti@...hat.com, gleb@...nel.org
Subject: Re: [PATCH 0/7] KVM: x86: Let the guest write to multiple debug registers
 with one vmexit

On 2014-03-09 09:11, Jan Kiszka wrote:
> 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?

Ah, this part didn't change, we still switch between host and guest
state on vmentries/exits. All fine!

Jan



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