Re: [PATCH 3/3] KVM: x86: improve reexecute_instruction

[Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>]

On 11/27/2012 06:41 AM, Marcelo Tosatti wrote:

>>
>> -	return false;
>> +again:
>> +	page_fault_count = ACCESS_ONCE(vcpu->kvm->arch.page_fault_count);
>> +
>> +	/*
>> +	 * if emulation was due to access to shadowed page table
>> +	 * and it failed try to unshadow page and re-enter the
>> +	 * guest to let CPU execute the instruction.
>> +	 */
>> +	kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(gpa));
>> +	emulate = vcpu->arch.mmu.page_fault(vcpu, cr3, PFERR_WRITE_MASK, false);
> 
> Can you explain what is the objective here?
> 

Sure. :)

The instruction emulation is caused by fault access on cr3. After unprotect
the target page, we call vcpu->arch.mmu.page_fault to fix the mapping of cr3.
if it return 1, mmu can not fix the mapping, we should report the error,
otherwise it is good to return to guest and let it re-execute the instruction
again.

page_fault_count is used to avoid the race on other vcpus, since after we
unprotect the target page, other cpu can enter page fault path and let the
page be write-protected again.

This way can help us to detect all the case that mmu can not be fixed.