On 10/18/2012 03:12 AM, Zhang Yanfei wrote: > ? 2012?10?17? 18:16, Avi Kivity ??: >> On 10/17/2012 04:28 AM, Zhang Yanfei wrote: >>> ? 2012?10?15? 23:43, Avi Kivity ??: >>>> 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. -- error compiling committee.c: too many arguments to function
