? 2012?10?18? 18:55, Avi Kivity ??: > 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. > OK, I'll make the new patch and resend it again. Thanks Zhang Yanfei
