On Tue, 2024-11-12 at 11:14 +0100, Peter Zijlstra wrote: > On Tue, Nov 12, 2024 at 08:44:33AM +0000, David Woodhouse wrote: > > On Fri, 2024-11-08 at 12:26 +0100, H. Peter Anvin wrote: > > > > > > > --- a/arch/x86/kernel/vmlinux.lds.S > > > > +++ b/arch/x86/kernel/vmlinux.lds.S > > > > @@ -100,7 +100,7 @@ const_pcpu_hot = pcpu_hot; > > > > . = ALIGN(PAGE_SIZE); \ > > > > __relocate_kernel_start = .; \ > > > > *(.text.relocate_kernel); \ > > > > - *(.rodata.relocate_kernel); \ > > > > + *(.data.relocate_kernel); \ > > Why are we having data in the middle of the text section? This is the relocate_kernel() page. It's the last thing the kernel calls on kexec. The kernel first takes a *copy* of it and places it into the identity mapping page tables which are set up for kexec. The relocate_kernel() function is then called at its *original* location in the kernel text. Before reloading %cr3, it stores some information which is going to become unavailable into its own page (currently using a bit of a nasty hack based on a hard-coded KEXEC_CONTROL_CODE_MAX_SIZE because we can't just use data symbol references). Then it reloads %cr3 and jumps to the identity-mapped copy of itself. Could we put .text.relocate_kernel and .data.relocate_kernel somewhere *other* than the main kernel text segment? Probably... it use use alternative instructions, but we could deal with that. And if we call from machine_kexec() directly into the *copy* (having marked it executable), maybe...?
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