Messages in this thread | | | From | (Eric W. Biederman) | Date | Thu, 08 Nov 2018 19:16:39 -0600 | Subject | Re: x86_64 INIT/SIPI Bug |
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Rian Quinn <rianquinn@gmail.com> writes:
> I apologize upfront if this is the wrong place to post this, pretty new to this. > > We are working on the Bareflank Hypervisor (www.bareflank.org), and we > are passing through the INIT/SIPI process (similar to how a VMX > rootkit from EFI might boot the OS) and we noticed that on Arch Linux, > the INIT/SIPI process stalls, something we are not seeing on Ubuntu. > > Turns out, to fix the issue, we had to turn on cpu_init_udelay=10000. > The problem is, once a hypervisor is turned on, even one that is doing > nothing but passing through the instructions, the "quick" that is > detailed below fails as the kernel is not giving the CPU enough time > to perform a VMExit/VMEntry (even through the VMExit is not doing > anything). > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/smpboot.c?h=v4.20-rc1#n650 > > You can see our INIT/SIPI code here if you are interested: > https://github.com/rianquinn/extended_apis/blob/hyperkernel_1/bfvmm/src/hve/arch/intel_x64/vmexit/init_signal.cpp > > The reason I suggest this is a bug is the manual clearly states that a > wait is required and the "quirk" that turns off this delay prevents > code like this from working. Would it be possible to either: > - Turn this off by default, but still allow someone to turn it on if > they are confident the delay is not needed? > - Provide a generic way to turn this off (maybe if a hypervisor is > detected, it defaults to off)? > > I'd be more than happy to provide a patch and test, but I'm not sure > if there is any interest in changing this code.
I would suggest testing either for your hypervisor or simply for being inside some hypervisor. As I read the code it is only turning off the 10ms delay on processors where it is know that it is safe to do so. This is a case where it is not safe to disable the 10ms delay, so it makes sense not not turn off the delay.
Eric
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