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VMCALL_USERMODE causes CPU freeze

 
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lcineyes
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PostPosted: Sun Feb 01, 2026 4:59 am    Post subject: VMCALL_USERMODE causes CPU freeze Reply with quote

When using VMCALL_KERNELMODEto enter kernel mode, if the CPU core executing VMCALL_USERMODE is not the same core that originally entered kernel mode due to thread scheduling, it causes a CPU freeze, followed by a Windows watchdog blue screen.
My current solution is to set thread affinity, which works fine on higher-performance computers. However, on lower-performance machines, blue screens still occur because Windows does not guarantee that affinity will remain effective. This is one scenario.
Another scenario involves a specific driver on a high-performance machine that consistently triggers the watchdog blue screen. I suspect that certain drivers may cause thread affinity to become ineffective, but I have not found any relevant documentation on this issue.
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Dark Byte
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PostPosted: Sun Feb 01, 2026 3:01 pm    Post subject: Reply with quote

that's what you get for re-enabling interrupts. They where turned of for this reason

anyhow, for every single cpu do a switch to kernel and then immediately back again (do not enable interrupts)
This will setup the return state for CS, SS CR4 and RFLAGS for every CPU. (Which should be pretty much the same for each running cpu anyhow)\

that way when vmcall_usermode is called and it returns in a different cpu it doesn't restore CS, SS and CR4 to 0 (which effectively crashes the cpu) but to the previously stored state

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