linux-kernel - Re: [Xen-devel] [PATCH v3 00/11] xen: Initial kexec/kdump implementation

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Message-ID: <20130111173452.GE5150@host-192-168-1-59.local.net-space.pl>
Date:	Fri, 11 Jan 2013 18:34:52 +0100
From:	Daniel Kiper <daniel.kiper@...cle.com>
To:	David Vrabel <david.vrabel@...rix.com>
Cc:	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
	Andrew Cooper <Andrew.Cooper3@...rix.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"virtualization@...ts.linux-foundation.org" 
	<virtualization@...ts.linux-foundation.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"jbeulich@...e.com" <jbeulich@...e.com>,
	"maxim.uvarov@...cle.com" <maxim.uvarov@...cle.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"vgoyal@...hat.com" <vgoyal@...hat.com>
Subject: Re: [Xen-devel] [PATCH v3 00/11] xen: Initial kexec/kdump
 implementation

On Fri, Jan 11, 2013 at 03:22:35PM +0000, David Vrabel wrote:
> On 11/01/13 13:22, Daniel Kiper wrote:
> > On Thu, Jan 10, 2013 at 02:19:55PM +0000, David Vrabel wrote:
> >> On 04/01/13 17:01, Daniel Kiper wrote:
> >>> My .5 cents:
> >>>   - We should focus on KEXEC_CMD_kexec_load and KEXEC_CMD_kexec_unload;
> >>>     probably we should introduce KEXEC_CMD_kexec_load2 and KEXEC_CMD_kexec_unload2;
> >>>     load should __LOAD__ kernel image and other things into hypervisor memory;
> >>
> >> Yes, but I don't see how we can easily support both ABIs easily.  I'd be
> >> in favour of replacing the existing hypercalls and requiring updated
> >> kexec tools in dom0 (this isn't that different to requiring the correct
> >> libxc in dom0).
> >
> > Why? Just define new strutures for new functions of kexec hypercall.
> > That should suffice.
>
> The current hypervisor ABI depends on an internal kernel ABI (i.e., the
> ABI provided by relocate_kernel).  We do not want hypervisor internals
> to be constrained by having to be compatible with kernel internals.

I agree. I did not sugest to stay with current interface. Old KEXEC_CMD_kexec_load
and KEXEC_CMD_kexec_unload should stay as is for backward compatibility (maybe
someday they should be removed). However, I do not see any problem in adding
new KEXEC_CMD_kexec_load2 and KEXEC_CMD_kexec_unload2 functions with completely
new arguments to existing kexec hypercall. Let's say something like that:

struct kexec_segment {
  void *buf;
  size_t bufsz;
  unsigned long mem;
  size_t memsz;
};

struct xen_kexec_load2 {
  unsigned long entry;
  unsigned long nr_segments;
  struct kexec_segment *segments;
  unsigned long flags;
};

struct xen_kexec_load2 xkl2;

...

rc = HYPERVISOR_kexec_op(KEXEC_CMD_kexec_load2, &xkl2);

Regarding relocate_kernel(), it should be Xen hypervisor specific but
probably most of the code will be similar to its Linux Kernel version.
It should only at the end leave machine in state identical with state
left by Linux Kernel version of relocate_kernel(). Just to be compatible
with existing kexec/kdump implementations.

> >>>     probably we should introduce KEXEC_CMD_kexec_load2 and KEXEC_CMD_k
>
> >>>   - Hmmm... Now I think that we should still use kexec syscall to load image
> >>>     into Xen memory (with new KEXEC_CMD_kexec_load2) because it establishes
> >>>     all things which are needed to call kdump if dom0 crashes; however,
> >>>     I could be wrong...
> >>
> >> I don't think we need the kexec syscall.  The kernel can unconditionally
> >> do the crash hypercall, which will return if the kdump kernel isn't
> >> loaded and the kernel can fall back to the regular non-kexec panic.
> >
> > No, please do not do that. When you call HYPERVISOR_kexec_op(KEXEC_CMD_kexec)
> > system is completly shutdown. Return form HYPERVISOR_kexec_op(KEXEC_CMD_kexec)
> > would require to restore some kernel functionalities. It maybe impossible
> > in some cases. Additionally, it means that some changes should be made
> > in generic kexec code path. As I know kexec maintainers are very reluctant
> > to make such things.
>
> Huh?  There only needs to be a call to a new hypervisor_crash_kexec()
> function (which would then call the Xen specific crash hypercall) at the
> very beginning of crash_kexec().  If this returns the normal
> crash/shutdown path is done (which could even include a guest kexec!).

I am still not convinced. Howerver, go ahead with your vision in this case.
Later we will see it makes sense.

Daniel
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