Re: [PATCH v14 3/6] crash: add a new kexec flag for FDT update

From: Baoquan He <bhe@redhat.com>
To: Sourabh Jain <sourabhjain@linux.ibm.com>
Cc: linuxppc-dev@ozlabs.org, Akhil Raj <lf32.dev@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	"Aneesh Kumar K . V" <aneesh.kumar@kernel.org>,
	Borislav Petkov <bp@alien8.de>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Christophe Leroy <christophe.leroy@csgroup.eu>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Dave Young <dyoung@redhat.com>,
	David Hildenbrand <david@redhat.com>,
	Eric DeVolder <eric.devolder@oracle.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Hari Bathini <hbathini@linux.ibm.com>,
	Laurent Dufour <laurent.dufour@fr.ibm.com>,
	Mahesh Salgaonkar <mahesh@linux.ibm.com>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Mimi Zohar <zohar@linux.ibm.com>,
	Naveen N Rao <naveen@kernel.org>,
	Oscar Salvador <osalvador@suse.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	Valentin Schneider <vschneid@redhat.com>,
	Vivek Goyal <vgoyal@redhat.com>,
	kexec@lists.infradead.org, x86@kernel.org
Subject: Re: [PATCH v14 3/6] crash: add a new kexec flag for FDT update
Date: Fri, 22 Dec 2023 08:28:23 +0800	[thread overview]
Message-ID: <ZYTYJ6sjdzZwjGpS@MiWiFi-R3L-srv> (raw)
In-Reply-To: <cdde877b-e620-45db-ae28-4b691c1ae5b6@linux.ibm.com>

On 12/21/23 at 11:36am, Sourabh Jain wrote:
......
> > > > diff --git a/include/uapi/linux/kexec.h b/include/uapi/linux/kexec.h
> > > > index 3d5b3d757bed..df6a6505e267 100644
> > > > --- a/include/uapi/linux/kexec.h
> > > > +++ b/include/uapi/linux/kexec.h
> > > > @@ -13,7 +13,7 @@
> > > >    #define KEXEC_ON_CRASH         0x00000001
> > > >    #define KEXEC_PRESERVE_CONTEXT 0x00000002
> > > > -#define KEXEC_UPDATE_FDT       0x00000008
> > > > +#define KEXEC_CRASH_HOTPLUG_UPDATE 0x00000004
> > > >    #define KEXEC_UPDATE_ELFCOREHDR        0x00000004
> > > >    #define KEXEC_ARCH_MASK                0xffff0000
> > > >    /*
> > > > 
> > > > With my understanding, the kexec flag should be indicating the action,
> > > > the mem/cpu hotplug, but not relating to any detail. Imagine later
> > > > another segment need be skipped on one ARCH again, then another flag
> > > > need be added, this sounds not reasonable.
> > > I strongly agree with you. The KEXEC_CRASH_HOTPLUG_UPDATE kexec flag
> > > should be sufficient to inform the kernel that the kexec tool has been
> > > updated
> > > to support CPU/Memory hotplug for the kexec_load system call. Unfortunately,
> > > we cannot use the 0x00000004 kexec flags bit for KEXEC_CRASH_HOTPLUG_UPDATE
> > > at the moment.
> > I am fine with 0x00000008 and a new flag, it has the same effect as
> > #define KEXEC_CRASH_HOTPLUG_UPDATE 0x00000004
> > 
> > I am worried about the header file incompatiblity.
> 
> If we are OK to have KEXEC_CRASH_HOTPLUG_UPDATE 0x00000008 as new bit
> to introduce CPU/Memory hotplug feature for kexec_load syscall, we will not
> have
> compatibility issue.
> 
> Let me write next version for this patch with KEXEC_CRASH_HOTPLUG_UPDATE
> 0x00000008
> as new flag bit and show how it will be handled. I will also share kexec
> code for clarity.

It's great we are in the same page about segments excluding done in arch
function. While It's a little unclear to me why we can't reuse 0x00000004
flag value. Then KEXEC_UPDATE_ELFCOREHDR will only exist in v6.6 kernel,
and that bit won't be used in v6.7 and future version.

Except of the existence in kexec-tools utility for XEN, do you see other
barrier? I would like to know so that one day I can explain
KEXEC_UPDATE_ELFCOREHDR to someone else if asked.

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