? 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. Thanks Zhang Yanfei
