On Mon, Mar 5, 2018 at 1:25 PM, Joerg Roedel <joro@xxxxxxxxxx> wrote: > Hi Brian, > > thanks for your review and helpful input. > > On Mon, Mar 05, 2018 at 11:41:01AM -0500, Brian Gerst wrote: >> On Mon, Mar 5, 2018 at 5:25 AM, Joerg Roedel <joro@xxxxxxxxxx> wrote: >> > +.Lentry_from_kernel_\@: >> > + >> > + /* >> > + * This handles the case when we enter the kernel from >> > + * kernel-mode and %esp points to the entry-stack. When this >> > + * happens we need to switch to the task-stack to run C code, >> > + * but switch back to the entry-stack again when we approach >> > + * iret and return to the interrupted code-path. This usually >> > + * happens when we hit an exception while restoring user-space >> > + * segment registers on the way back to user-space. >> > + * >> > + * When we switch to the task-stack here, we can't trust the >> > + * contents of the entry-stack anymore, as the exception handler >> > + * might be scheduled out or moved to another CPU. Therefore we >> > + * copy the complete entry-stack to the task-stack and set a >> > + * marker in the iret-frame (bit 31 of the CS dword) to detect >> > + * what we've done on the iret path. >> >> We don't need to worry about preemption changing the entry stack. The >> faults that IRET or segment loads can generate just run the exception >> fixup handler and return. Interrupts were disabled when the fault >> occurred, so the kernel cannot be preempted. The other case to watch >> is #DB on SYSENTER, but that simply returns and doesn't sleep either. >> >> We can keep the same process as the existing debug/NMI handlers - >> leave the current exception pt_regs on the entry stack and just switch >> to the task stack for the call to the handler. Then switch back to >> the entry stack and continue. No copying needed. > > Okay, I'll look into that. Will it even be true for fully preemptible > and RT kernels that there can't be any preemption of these handlers? See resume_kernel in the 32-bit entry for how preemption is handled on return to kernel mode. Looking at the RT patches, they still respect disabling interrupts also disabling preemption. -- Brian Gerst -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
