linux-kernel - Re: [PATCH 1/3] KVM: trace mmio read event properly

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Message-ID: <4F14DEB5.6020902@linux.vnet.ibm.com>
Date:	Tue, 17 Jan 2012 10:36:37 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH 1/3] KVM: trace mmio read event properly

On 01/16/2012 06:18 PM, Avi Kivity wrote:

> On 01/16/2012 11:31 AM, Xiao Guangrong wrote:
>> In current code, we use KVM_TRACE_MMIO_READ to trace mmio read event which
>> only can be completed immediately, instead of it, we trace the time when
>> read event occur, then cooperate with then later patch, we can know the time
>> of mmio read emulation
>>
>> @@ -3744,6 +3740,8 @@ mmio:
>>  	/*
>>  	 * Is this MMIO handled locally?
>>  	 */
>> +	trace_kvm_mmio(write ? KVM_TRACE_MMIO_WRITE : KVM_TRACE_MMIO_READ,
> 
> It's better to push the conditional to the trace event itself, so it's
> only evaluated if tracing is enabled.
> 


Yes.

>> +			bytes, gpa, *(u64 *)val);
> 
> We get the wrong value for reads here, no?
> 


Yes, the value is meaningless in this point.

> Can't we leave the code as is, and infer the start of the event from the
> last kvm_exit trace?


Yes, we can do it, but the same problems is we need trace vcpu id to get its
vmexit, i will do it in the next version if you like this way :)

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