[PATCH 08/10] arm64/kexec: Add core kexec support

[Geoff Levand]

Hi Mark,

On Fri, 2014-10-24 at 11:28 +0100, Mark Rutland wrote:
> > +++ b/arch/arm64/Kconfig
> > @@ -313,6 +313,15 @@ config ARCH_HAS_CACHE_LINE_SIZE
> >
> >  source "mm/Kconfig"
> >
> > +config KEXEC
> > +       depends on (!SMP || PM_SLEEP_SMP)
> 
> In its current state this also depends on !KVM && !EFI (technically you
> could detect those cases at runtime, but I don't see that in this
> series).

A kernel built with CONFIG_EFI is OK if run on a non-EFI system or
without using a system's EFI support.  I added a patch that adds
runtime checks in the kexec_load syscall path to print a message
and return failure for situations where KVM or EFI won't work.

> > +/**
> > + * machine_kexec_prepare - Prepare for a kexec reboot.
> > + *
> > + * Called from the core kexec code when a kernel image is loaded.
> > + */
> > +
> > +int machine_kexec_prepare(struct kimage *image)
> > +{
> > +       const struct kexec_segment *dtb_seg = kexec_find_dtb_seg(image);
> > +
> > +       if (!dtb_seg)
> > +               pr_warn("%s: No device tree segment found.\n", __func__);
> > +
> > +       arm64_kexec_dtb_addr = dtb_seg ? dtb_seg->mem : 0;
> > +       arm64_kexec_kimage_start = image->start;
> > +
> > +       return 0;
> > +}
> 
> I thought all of the DTB handling was moving to purgatory?

Non-purgatory booting is needed for kexec-lite.  We can do
this simple check here which optionally sets x0 to the dtb
address to support that.  The other solution is to have a
trampoline in kexec-lite that sets x0 (basically an absolute
minimal purgatory), but I think to do it here is nicer, and
is also the same way that the arm arch code does it.

Maybe removing this pr_warn message and just relying on the
kexec_image_info() output would be better.

> > +/**
> > + * kexec_list_flush - Helper to flush the kimage list to PoC.
> > + */
> > +
> > +static void kexec_list_flush(unsigned long kimage_head)
> > +{
> > +       void *dest;
> > +       unsigned long *entry;
> > +
> > +       for (entry = &kimage_head, dest = NULL; ; entry++) {
> > +               unsigned int flag = *entry &
> > +                       (IND_DESTINATION | IND_INDIRECTION | IND_DONE |
> > +                       IND_SOURCE);
> > +               void *addr = phys_to_virt(*entry & PAGE_MASK);
> > +
> > +               switch (flag) {
> > +               case IND_INDIRECTION:
> > +                       entry = (unsigned long *)addr - 1;
> > +                       __flush_dcache_area(addr, PAGE_SIZE);
> > +                       break;
> > +               case IND_DESTINATION:
> > +                       dest = addr;
> > +                       break;
> > +               case IND_SOURCE:
> > +                       __flush_dcache_area(addr, PAGE_SIZE);
> > +                       dest += PAGE_SIZE;
> > +                       break;
> > +               case IND_DONE:
> > +                       return;
> > +               default:
> > +                       break;
> 
> Can an image ever have no flags? Given the presence of IND_NONE I'd
> assume not, so this looks like a candidate for a BUG().

Sure, I guess things will blow up before it ever gets here though.

> 
> > +               }
> > +       }
> > +}
> > +
> > +/**
> > + * machine_kexec - Do the kexec reboot.
> > + *
> > + * Called from the core kexec code for a sys_reboot with LINUX_REBOOT_CMD_KEXEC.
> > + */
> > +
> > +void machine_kexec(struct kimage *image)
> > +{
> > +       phys_addr_t reboot_code_buffer_phys;
> > +       void *reboot_code_buffer;
> > +
> > +       BUG_ON(num_online_cpus() > 1);
> > +
> > +       arm64_kexec_kimage_head = image->head;
> > +
> > +       reboot_code_buffer_phys = page_to_phys(image->control_code_page);
> > +       reboot_code_buffer = phys_to_virt(reboot_code_buffer_phys);
> > +
> > +       /*
> > +        * Copy relocate_new_kernel to the reboot_code_buffer for use
> > +        * after the kernel is shut down.
> > +        */
> > +
> > +       memcpy(reboot_code_buffer, relocate_new_kernel,
> > +               relocate_new_kernel_size);
> 
> Can we get rid of the line gaps between comments and the single function
> calls they apply to, please? I realise it's a minor thing, but this
> looks rather inconsistent with the rest of arch/arm64/.

checkpatch doesn't seem to mind, but sure, I can do that.

> > +
> > +       /* Flush the reboot_code_buffer in preparation for its execution. */
> > +
> > +       __flush_dcache_area(reboot_code_buffer, relocate_new_kernel_size);
> 
> That code should already be at the PoC per the boot protocol (the entire
> kernel image should have been clean to the PoC at boot, so the
> instructions forming relocate_new_kernel are globally visible).
> 
> From the looks of it you only need to flush the variables at the very
> end.

We copy the relocate_new_kernel routine to reboot_code_buffer, which is
a buffer allocated by the kexec core with alloc_pages().  That copy of
relocate_new_kernel is what we are flushing here.

I believe we need to flush that buffer out to PoC so we can execute
the code it contains after cpu_soft_restart().

> > --- /dev/null
> > +++ b/arch/arm64/kernel/relocate_kernel.S
> > @@ -0,0 +1,184 @@
> > +/*
> > + * kexec for arm64
> > + *
> > + * Copyright (C) Linaro.
> > + *
> > + * This program is free software; you can redistribute it and/or modify
> > + * it under the terms of the GNU General Public License version 2 as
> > + * published by the Free Software Foundation.
> > + */
> > +
> > +#include <asm/assembler.h>
> > +#include <asm/kexec.h>
> > +#include <asm/memory.h>
> > +#include <asm/page.h>
> > +#include <asm/proc-macros.S>
> > +
> > +/* The list entry flags. */
> > +
> > +#define IND_DESTINATION_BIT 0
> > +#define IND_INDIRECTION_BIT 1
> > +#define IND_DONE_BIT        2
> > +#define IND_SOURCE_BIT      3
> 
> As previously, I think these need to be moved into a common header, and
> defined in terms of the existing IND_* macros (or vice-versa). I believe
> you had a patch doing so; what's the status of that?

Still working to get it merged:

  https://lkml.org/lkml/2014/11/12/675

> > +/*
> > + * relocate_new_kernel - Put a 2nd stage kernel image in place and boot it.
> > + *
> > + * The memory that the old kernel occupies may be overwritten when coping the
> > + * new image to its final location.  To assure that the relocate_new_kernel
> > + * routine which does that copy is not overwritten all code and data needed
> > + * by relocate_new_kernel must be between the symbols relocate_new_kernel and
> > + * relocate_new_kernel_end.  The machine_kexec() routine will copy
> > + * relocate_new_kernel to the kexec control_code_page, a special page which
> > + * has been set up to be preserved during the copy operation.
> > + */
> > +
> > +.globl relocate_new_kernel
> > +relocate_new_kernel:

...

> > +
> > +       /* start_new_image */
> > +
> > +       ldr     x4, arm64_kexec_kimage_start
> > +       ldr     x0, arm64_kexec_dtb_addr
> > +       mov     x1, xzr
> > +       mov     x2, xzr
> > +       mov     x3, xzr
> > +       br      x4
> 
> This last part should be in userspace-provided purgatory. If you have
> purgatory code which does this then we should be able to rely on that,
> and we don't have to try to maintain this DTB handling in kernelspace
> (which I suspect may become painful as the boot protocol evolves).

I think the putting the dtb address in x0 is already fixed.  There are
users with firmware that does this and any change to the boot protocol
will have to work with it.

As I mentioned above, we need a solution for non-purgatory re-boot and I
think this is the best way.

Thanks for taking the time to review.  I'll post an updated patch set
soon.

-Geoff