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The pcb is saved at the top of the kernel stack on x86 platforms. The initial kenrel stack pointer is set in the TSS so that the trapframe from user -> kernel transitions begins directly below the pcb and grows down. The XSAVE changes moved the FPU save area out of the pcb and into a variable-sized area after the pcb. This required updating the expressions to calculate the initial stack pointer from 'stacktop - sizeof(pcb)' to 'stacktop - sizeof(pcb) + FPU save area size'. The i386_set_ioperm() system call allows user applications to access individual I/O ports via the I/O port permission bitmap in the TSS. On FreeBSD this requires allocating a custom per-process TSS instead of using the shared per-CPU TSS. The expression to initialize the initial kernel stack pointer in the per-process TSS created for i386_set_ioperm() was not properly updated after the XSAVE changes. Processes that used i386_set_ioperm() would trash the trapframe during subsequent context switches resulting in panics from memory corruption. This changes fixes the kernel stack pointer calculation for the per-process TSS. Reviewed by: kib, n_hibma Reported by: n_hibma MFC after: 1 week
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