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

[Avi Kivity <avi@redhat.com>]

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.



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