Re: [PATCH 0/3] x86: clear vmcss on all cpus when doing kdump if necessary

[Zhang Yanfei <zhangyanfei@cn.fujitsu.com>]

于 2012年10月18日 18:55, Avi Kivity 写道:
> On 10/18/2012 03:12 AM, Zhang Yanfei wrote:
>> 于 2012年10月17日 18:16, Avi Kivity 写道:
>>> On 10/17/2012 04:28 AM, Zhang Yanfei wrote:
>>>> 于 2012年10月15日 23:43, Avi Kivity 写道:
>>>>> On 10/12/2012 08:40 AM, Zhang Yanfei wrote:
>>>>>> Currently, kdump just makes all the logical processors leave VMX operation by
>>>>>> executing VMXOFF instruction, so any VMCSs active on the logical processors may
>>>>>> be corrupted. But, sometimes, we need the VMCSs to debug guest images contained
>>>>>> in the host vmcore. To prevent the corruption, we should VMCLEAR the VMCSs before
>>>>>> executing the VMXOFF instruction.
>>>>>
>>>>> How have you verified that VMXOFF doesn't flush cached VMCSs already?
>>>>>
>>>>
>>>> I tried some tests, for example, I made copies for every vmcs, and in the kdump
>>>> path, I backed up all the loaded vmcs into the copies before vmxoff.
>>>> After generating the vmcore, I retrieve the vmcss and their copies, and compare them,
>>>> no differences.
>>>>
>>>> Another test is using VMCLEAR to clear all the loaded vmcs before VMXOFF,
>>>> and compare the vmcss and their copies, there are indeed differences between the
>>>> vmcs and its copy.
>>>>
>>>> I know the tests may be not so convincing, for example, I used memcpy to back up
>>>> the vmcss and it is an ordinary memory operation. But to ensure the non-corruption
>>>> of the vmcss in the vmcore, I think we should VMCLEAR the vmcss before VMXOFF just
>>>> as the Intel spec says.
>>>
>>> Sorry, I was unclear -- I was referring to the spec, I wasn't sure
>>> whether VMXOFF is defined to flush VMCSes or whether it just invalidates
>>> on-chip caches so that it won't flush them out in the future, corrupting
>>> memory.  We don't want to depend on actual behaviour as it may change
>>> with future version.
>>>
>>> Copying some Intel folk, maybe they can clarify it.
>>>
>>
>> Yes, the Intel spec says "may be" about the VMCS-corruption thing. From
>> chapter 24.10.1 in Intel® 64 and IA-32 Architectures Software Developer’s
>> Manual Volume 3C:System Programming Guide, Part 3, there is the description:
>>
>> "If a logical processor leaves VMX operation, any VMCSs active on that logical
>> processor may be corrupted (see below). To prevent such corruption of a VMCS that
>> may be used either after a return to VMX operation or on another logical processor,
>> software should VMCLEAR that VMCS before executing the VMXOFF instruction or
>> removing power from the processor (e.g., as part of a transition to the S3 and S4
>> power states)."
>>
>> Our purpose is to make sure the VMCSs in the vmcore are updated and non-corrupted. So
>> according to the description above, no matter whether VMXOFF is defined to flush
>> VMCSs or whether it just invalidates on-chip caches, we'd better VMCLEAR the
>> VMCSs before executing the VMXOFF.
> 
> Ok, that's clear then.  So all we need is to remove the sysctl and clear
> VMCSs unconditionally.
> 

OK, I'll make the new patch and resend it again.

Thanks
Zhang Yanfei


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