linux-kernel - Re: [PATCH v4 3/5] KVM: x86: clean up reexecute

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Date:	Sat, 05 Jan 2013 15:20:24 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
CC:	Gleb Natapov <gleb@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH v4 3/5] KVM: x86: clean up reexecute_instruction

On 01/05/2013 06:21 AM, Marcelo Tosatti wrote:
> On Fri, Jan 04, 2013 at 09:55:40PM +0800, Xiao Guangrong wrote:
>> Little cleanup for reexecute_instruction, also use gpa_to_gfn in
>> retry_instruction
>>
>> Signed-off-by: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
>> ---
>>  arch/x86/kvm/x86.c |   13 ++++++-------
>>  1 files changed, 6 insertions(+), 7 deletions(-)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 1c9c834..ad39018 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -4761,19 +4761,18 @@ static bool reexecute_instruction(struct kvm_vcpu *vcpu, gva_t gva)
>>  	if (tdp_enabled)
>>  		return false;
>>
>> +	gpa = kvm_mmu_gva_to_gpa_read(vcpu, gva, NULL);
>> +	if (gpa == UNMAPPED_GVA)
>> +		return true; /* let cpu generate fault */
>> +
> 
> Why change from _system to _read here? Purely cleanup patch should
> have no logical changes.

Ouch, my mistake, will drop this change.

> 
> BTW, there is not much logic in using reexecute_instruction() at
> for x86_decode_insn (checks in reexecute_instruction() assume 
> write to the cr2, for instance).
> Fault propagation for x86_decode_insn seems completly broken
> (which is perhaps why reexecute_instruction() there survived).

Currently, reexecute_instruction can work only if it is called on page
fault path where cr2 is valid. On other paths, cr2 is 0 which is always
not be mapped on guest since it is NULL pointer, so reexecute_instruction
always retry the instruction.

Yes, as you point it out, it is better if the fault address can be got
from x86_decode_insn. I will consider it later.


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