[PATCH v3 0/3] Add restrictions for kexec/kdump jumping between 5-level and 4-level kernel

[PATCH v3 0/3] Add restrictions for kexec/kdump jumping between 5-level and 4-level kernel

[Baoquan He]

This is v3 post.

The original v1 post can be found here:
http://lkml.kernel.org/r/20180829141624.13985-1-bhe@redhat.com

Later a v1 RESEND version:
http://lkml.kernel.org/r/20190125022817.29506-1-bhe@redhat.com

V2 post is here:
http://lkml.kernel.org/r/20190312005004.19182-1-bhe@redhat.com

This patchset is trying to fix several issues for kexec/kdump when
dynamic switching of paging mode is enabled in x86_64. The current
kernel supports 5-level paging mode, and supports dynamically choosing
paging mode during bootup according to kernel image, hardware and
kernel parameter setting. This flexibility brings several issues for
kexec/kdump:

Issues:
1)
Dynamic switching between paging mode requires code change in target
kernel. So we can't kexec jump from 5-level kernel to old 4-level
kernel which lacks the code change.

2)
Switching from 5-level paging to 4-level paging kernel would fail, if
kexec() put kernel image above 64TiB of memory.

3)
Kdump jumping has similar issue as 2). This require us to only
reserve crashkernel below 64TB, otherwise jumping from 5-level to
4-level kernel will fail.

Note:
Since we have two interfaces kexec_load() and kexec_file_load() to load
kexec/kdump kernel, handling for them is a little different. For
kexec_load(), most of the loading job is done in user space utility
kexec_tools. However, for kexec_file_load(), most of the loading codes
have moved into kernel because of kernel image verification.

Fixes:
a) For issue 1), we need check if XLF_5LEVEL is set, otherwise error out
   a message. 
  -This need be done in both kernel and kexec_tools utility.
  -Patch 2/3 is the handling of kernel part.
  -Will post user space patch to kexec mailing list later.

b) For issue 2), we need check if both XLF_5LEVEL and XLF_5LEVEL_ENABLED
   are set, otherwise error out a message.
  -This only need be done in kexec_tools utility. Because for
   kexec_file_load(), the current code searches area to put kernel from
   bottom to up in system RAM, we usually can always find an area below
   4 GB, no need to worry about 5-level kernel jumping to 4-level
   kernel. While for kexec_load(), it's top down seraching area for kernel
   loading, and implemented in user space. We need make sure that
   5-level kernel find an area under 64 TB for a kexec-ed kernel of
   4-level.
  -Will post user space patch to kexec mailing list later.

c) For issues 3), just limit kernel to reserve crashkernel below 64 TB.
  -This only need be done in kernel.
  -It doesn't need to check bit XLF_5LEVEL or XLF_5LEVEL_ENABLED, we
   just simply limit it below 64 TB which should be enough. Because
   crashernel is reserved during the 1st kernel's bootup, we don't know
   what kernel will be loaded for kdump usage.
  -Patch 3/3 handles this.

Concerns from reviewing comments:
1)
In v1, hpa raised concern that why the paging mode checking is not done
before kexec jumping, the discussion can be found here:

http://lkml.kernel.org/r/alpine.DEB.2.21.1809051002020.1416@nanos.tec.linutronix.de

As tglx said, it might be not doable for kdump since kdump kernel's
reserved crashkernel region only owns a portion of memory, may
be above 4G; and might be not safer to do paging mode checking and
switching thing after crash.

2)
In v1 RESEND post, tglx asked why only bit XLF_5LEVEL is checked, even
though two bits XLF_5LEVEL or XLF_5LEVEL_ENABLED added. So add more
words to explain it in *Fixes* b).

Changelog:
v2->v3:
  Change the constant to match the notation for the rest of defines as
  Kirill suggested;
v1->v2:
  Correct the subject of patch 1 according to tglx's comment;
  Add more information to cover-letter to address reviewers' concerns;

Baoquan He (3):
  x86/boot: Add xloadflags bits for 5-level kernel checking
  x86/kexec/64: Error out if try to jump to old 4-level kernel from
    5-level kernel
  x86/kdump/64: Change the upper limit of crashkernel reservation

 arch/x86/boot/header.S                | 12 +++++++++++-
 arch/x86/include/uapi/asm/bootparam.h |  2 ++
 arch/x86/kernel/kexec-bzimage64.c     |  5 +++++
 arch/x86/kernel/setup.c               | 18 ++++++++++++++----
 4 files changed, 32 insertions(+), 5 deletions(-)

-- 
2.17.2

[PATCH v3 1/3] x86/boot: Add xloadflags bits for 5-level kernel checking

[Baoquan He]

Add two bits XLF_5LEVEL and XLF_5LEVEL_ENABLED for 5-level kernel.
Bit XLF_5LEVEL indicates if 5-level related code is contained
in this kernel.
Bit XLF_5LEVEL_ENABLED indicates if CONFIG_X86_5LEVEL=y is set.

They are being used in later patch to check if kexec/kdump kernel
is loaded in right place.

Signed-off-by: Baoquan He <bhe@redhat.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 arch/x86/boot/header.S                | 12 +++++++++++-
 arch/x86/include/uapi/asm/bootparam.h |  2 ++
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
index 850b8762e889..be19f4199727 100644
--- a/arch/x86/boot/header.S
+++ b/arch/x86/boot/header.S
@@ -419,7 +419,17 @@ xloadflags:
 # define XLF4 0
 #endif
 
-			.word XLF0 | XLF1 | XLF23 | XLF4
+#ifdef CONFIG_X86_64
+#ifdef CONFIG_X86_5LEVEL
+#define XLF56 (XLF_5LEVEL|XLF_5LEVEL_ENABLED)
+#else
+#define XLF56 XLF_5LEVEL
+#endif
+#else
+#define XLF56 0
+#endif
+
+			.word XLF0 | XLF1 | XLF23 | XLF4 | XLF56
 
 cmdline_size:   .long   COMMAND_LINE_SIZE-1     #length of the command line,
                                                 #added with boot protocol
diff --git a/arch/x86/include/uapi/asm/bootparam.h b/arch/x86/include/uapi/asm/bootparam.h
index 60733f137e9a..c895df5482c5 100644
--- a/arch/x86/include/uapi/asm/bootparam.h
+++ b/arch/x86/include/uapi/asm/bootparam.h
@@ -29,6 +29,8 @@
 #define XLF_EFI_HANDOVER_32		(1<<2)
 #define XLF_EFI_HANDOVER_64		(1<<3)
 #define XLF_EFI_KEXEC			(1<<4)
+#define XLF_5LEVEL			(1<<5)
+#define XLF_5LEVEL_ENABLED		(1<<6)
 
 #ifndef __ASSEMBLY__
 
-- 
2.17.2

[PATCH v3 2/3] x86/kexec/64: Error out if try to jump to old 4-level kernel from 5-level kernel

[Baoquan He]

In relocate_kernel() CR4.LA57 flag is set before kexec jumping if
the kernel has 5-level paging enabled. Then in boot/compressed/head_64.S,
it will check if the booting kernel is in 4-level or 5-level paging
mode, and handle accordingly. However, the old kernel which doesn't
contain the 5-level codes doesn't know how to cope with it, then #GP
triggered.

Instead of triggering #GP during kexec kernel boot, error out during
kexec loading if find out we are trying to jump to old 4-level kernel
from 5-level kernel.

Signed-off-by: Baoquan He <bhe@redhat.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 arch/x86/kernel/kexec-bzimage64.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/x86/kernel/kexec-bzimage64.c b/arch/x86/kernel/kexec-bzimage64.c
index 1f3b77367948..4c9c079b5673 100644
--- a/arch/x86/kernel/kexec-bzimage64.c
+++ b/arch/x86/kernel/kexec-bzimage64.c
@@ -321,6 +321,11 @@ static int bzImage64_probe(const char *buf, unsigned long len)
 		return ret;
 	}
 
+	if (!(header->xloadflags & XLF_5LEVEL) && pgtable_l5_enabled()) {
+		pr_err("Can not jump to old 4-level kernel from 5-level kernel.\n");
+		return ret;
+	}
+
 	/* I've got a bzImage */
 	pr_debug("It's a relocatable bzImage64\n");
 	ret = 0;
-- 
2.17.2

[PATCH v3 3/3] x86/kdump/64: Change the upper limit of crashkernel reservation

[Baoquan He]

Restrict kdump to only reserve crashkernel below 64TB. Since the kdump
jumping may be from 5-level to 4-level, and the kdump kernel is put
above 64TB in 5-level kernel, then the jumping will fail. And the
crashkernel reservation is done during the 1st kernel bootup, there's
no way to detect the paging mode of kdump kernel at that time.

Hence change the upper limit of crashkernel reservation to 64TB
on x86_64.

Signed-off-by: Baoquan He <bhe@redhat.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 arch/x86/kernel/setup.c | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
index 3d872a527cd9..5508b981f33d 100644
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -451,16 +451,26 @@ static void __init memblock_x86_reserve_range_setup_data(void)
 #define CRASH_ALIGN		(16 << 20)
 
 /*
- * Keep the crash kernel below this limit.  On 32 bits earlier kernels
- * would limit the kernel to the low 512 MiB due to mapping restrictions.
- * On 64bit, old kexec-tools need to under 896MiB.
+ * Keep the crash kernel below this limit.
+ *
+ * On 32 bits earlier kernels would limit the kernel to the low
+ * 512 MiB due to mapping restrictions.
+ *
+ * On 64bit, old kexec-tools need to be under 896MiB. The later
+ * supports to put kernel above 4G, up to system RAM top. Here
+ * kdump kernel need be restricted to be under 64TB, which is
+ * the upper limit of system RAM in 4-level paing mode. Since
+ * the kdump jumping could be from 5-level to 4-level, the jumping
+ * will fail if kernel is put above 64TB, and there's no way to
+ * detect the paging mode of the kernel which will be loaded for
+ * dumping during the 1st kernel bootup.
  */
 #ifdef CONFIG_X86_32
 # define CRASH_ADDR_LOW_MAX	(512 << 20)
 # define CRASH_ADDR_HIGH_MAX	(512 << 20)
 #else
 # define CRASH_ADDR_LOW_MAX	(896UL << 20)
-# define CRASH_ADDR_HIGH_MAX	MAXMEM
+# define CRASH_ADDR_HIGH_MAX	(64UL << 40)
 #endif
 
 static int __init reserve_crashkernel_low(void)
-- 
2.17.2

Re: [PATCH v3 0/3] Add restrictions for kexec/kdump jumping between 5-level and 4-level kernel

[Baoquan He]

PING

Any comment for this patch, Or consider to merge?

On 03/12/19 at 06:30pm, Baoquan He wrote:
> This is v3 post.
> 
> The original v1 post can be found here:
> http://lkml.kernel.org/r/20180829141624.13985-1-bhe@redhat.com
> 
> Later a v1 RESEND version:
> http://lkml.kernel.org/r/20190125022817.29506-1-bhe@redhat.com
> 
> V2 post is here:
> http://lkml.kernel.org/r/20190312005004.19182-1-bhe@redhat.com
> 
> This patchset is trying to fix several issues for kexec/kdump when
> dynamic switching of paging mode is enabled in x86_64. The current
> kernel supports 5-level paging mode, and supports dynamically choosing
> paging mode during bootup according to kernel image, hardware and
> kernel parameter setting. This flexibility brings several issues for
> kexec/kdump:
> 
> Issues:
> 1)
> Dynamic switching between paging mode requires code change in target
> kernel. So we can't kexec jump from 5-level kernel to old 4-level
> kernel which lacks the code change.
> 
> 2)
> Switching from 5-level paging to 4-level paging kernel would fail, if
> kexec() put kernel image above 64TiB of memory.
> 
> 3)
> Kdump jumping has similar issue as 2). This require us to only
> reserve crashkernel below 64TB, otherwise jumping from 5-level to
> 4-level kernel will fail.
> 
> Note:
> Since we have two interfaces kexec_load() and kexec_file_load() to load
> kexec/kdump kernel, handling for them is a little different. For
> kexec_load(), most of the loading job is done in user space utility
> kexec_tools. However, for kexec_file_load(), most of the loading codes
> have moved into kernel because of kernel image verification.
> 
> Fixes:
> a) For issue 1), we need check if XLF_5LEVEL is set, otherwise error out
>    a message. 
>   -This need be done in both kernel and kexec_tools utility.
>   -Patch 2/3 is the handling of kernel part.
>   -Will post user space patch to kexec mailing list later.
> 
> b) For issue 2), we need check if both XLF_5LEVEL and XLF_5LEVEL_ENABLED
>    are set, otherwise error out a message.
>   -This only need be done in kexec_tools utility. Because for
>    kexec_file_load(), the current code searches area to put kernel from
>    bottom to up in system RAM, we usually can always find an area below
>    4 GB, no need to worry about 5-level kernel jumping to 4-level
>    kernel. While for kexec_load(), it's top down seraching area for kernel
>    loading, and implemented in user space. We need make sure that
>    5-level kernel find an area under 64 TB for a kexec-ed kernel of
>    4-level.
>   -Will post user space patch to kexec mailing list later.
> 
> c) For issues 3), just limit kernel to reserve crashkernel below 64 TB.
>   -This only need be done in kernel.
>   -It doesn't need to check bit XLF_5LEVEL or XLF_5LEVEL_ENABLED, we
>    just simply limit it below 64 TB which should be enough. Because
>    crashernel is reserved during the 1st kernel's bootup, we don't know
>    what kernel will be loaded for kdump usage.
>   -Patch 3/3 handles this.
> 
> Concerns from reviewing comments:
> 1)
> In v1, hpa raised concern that why the paging mode checking is not done
> before kexec jumping, the discussion can be found here:
> 
> http://lkml.kernel.org/r/alpine.DEB.2.21.1809051002020.1416@nanos.tec.linutronix.de
> 
> As tglx said, it might be not doable for kdump since kdump kernel's
> reserved crashkernel region only owns a portion of memory, may
> be above 4G; and might be not safer to do paging mode checking and
> switching thing after crash.
> 
> 2)
> In v1 RESEND post, tglx asked why only bit XLF_5LEVEL is checked, even
> though two bits XLF_5LEVEL or XLF_5LEVEL_ENABLED added. So add more
> words to explain it in *Fixes* b).
> 
> Changelog:
> v2->v3:
>   Change the constant to match the notation for the rest of defines as
>   Kirill suggested;
> v1->v2:
>   Correct the subject of patch 1 according to tglx's comment;
>   Add more information to cover-letter to address reviewers' concerns;
> 
> Baoquan He (3):
>   x86/boot: Add xloadflags bits for 5-level kernel checking
>   x86/kexec/64: Error out if try to jump to old 4-level kernel from
>     5-level kernel
>   x86/kdump/64: Change the upper limit of crashkernel reservation
> 
>  arch/x86/boot/header.S                | 12 +++++++++++-
>  arch/x86/include/uapi/asm/bootparam.h |  2 ++
>  arch/x86/kernel/kexec-bzimage64.c     |  5 +++++
>  arch/x86/kernel/setup.c               | 18 ++++++++++++++----
>  4 files changed, 32 insertions(+), 5 deletions(-)
> 
> -- 
> 2.17.2
> 

Re: [PATCH v3 1/3] x86/boot: Add xloadflags bits for 5-level kernel checking

[Thomas Gleixner]

On Tue, 12 Mar 2019, Baoquan He wrote:

> Add two bits XLF_5LEVEL and XLF_5LEVEL_ENABLED for 5-level kernel.
> Bit XLF_5LEVEL indicates if 5-level related code is contained
> in this kernel.
> Bit XLF_5LEVEL_ENABLED indicates if CONFIG_X86_5LEVEL=y is set.
> 
> They are being used in later patch to check if kexec/kdump kernel
> is loaded in right place.

Only XLF_5LEVEL is checked. So what's the second flag for?

Thanks,

	tglx

Re: [PATCH v3 2/3] x86/kexec/64: Error out if try to jump to old 4-level kernel from 5-level kernel

[Thomas Gleixner]

On Tue, 12 Mar 2019, Baoquan He wrote:

> In relocate_kernel() CR4.LA57 flag is set before kexec jumping if
> the kernel has 5-level paging enabled. Then in boot/compressed/head_64.S,
> it will check if the booting kernel is in 4-level or 5-level paging
> mode, and handle accordingly. However, the old kernel which doesn't
> contain the 5-level codes doesn't know how to cope with it, then #GP
> triggered.

The above is more than confusing. I assume you want to say:

  If the running kernel has 5-level paging activated, the 5-level paging
  mode is preserved across kexec. If the kexec'ed kernel does not contain
  support for handling active 5-level paging mode in the decompressor, the
  decompressor will crash with #GP.

> Instead of triggering #GP during kexec kernel boot, error out during
> kexec loading if find out we are trying to jump to old 4-level kernel
> from 5-level kernel.

Prevent this situation at load time. If 5-level paging is active, check the
xloadflags whether the kexec kernel can handle 5-level paging at least in
the decompressor. If not, reject the load attempt.
 
> Signed-off-by: Baoquan He <bhe@redhat.com>
> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
>  arch/x86/kernel/kexec-bzimage64.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/arch/x86/kernel/kexec-bzimage64.c b/arch/x86/kernel/kexec-bzimage64.c
> index 1f3b77367948..4c9c079b5673 100644
> --- a/arch/x86/kernel/kexec-bzimage64.c
> +++ b/arch/x86/kernel/kexec-bzimage64.c
> @@ -321,6 +321,11 @@ static int bzImage64_probe(const char *buf, unsigned long len)
>  		return ret;
>  	}
>  
> +	if (!(header->xloadflags & XLF_5LEVEL) && pgtable_l5_enabled()) {
> +		pr_err("Can not jump to old 4-level kernel from 5-level kernel.\n");

This is confusing at best.

     	"bzImage cannot handle 5-level paging mode\n"

or something like this.

> +		return ret;

Thanks,

	tglx

Re: [PATCH v3 2/3] x86/kexec/64: Error out if try to jump to old 4-level kernel from 5-level kernel

[Baoquan He]

On 04/05/19 at 10:38pm, Thomas Gleixner wrote:
> On Tue, 12 Mar 2019, Baoquan He wrote:
> 
> > In relocate_kernel() CR4.LA57 flag is set before kexec jumping if
> > the kernel has 5-level paging enabled. Then in boot/compressed/head_64.S,
> > it will check if the booting kernel is in 4-level or 5-level paging
> > mode, and handle accordingly. However, the old kernel which doesn't
> > contain the 5-level codes doesn't know how to cope with it, then #GP
> > triggered.
> 
> The above is more than confusing. I assume you want to say:
> 
>   If the running kernel has 5-level paging activated, the 5-level paging
>   mode is preserved across kexec. If the kexec'ed kernel does not contain
>   support for handling active 5-level paging mode in the decompressor, the
>   decompressor will crash with #GP.
> 
> > Instead of triggering #GP during kexec kernel boot, error out during
> > kexec loading if find out we are trying to jump to old 4-level kernel
> > from 5-level kernel.
> 
> Prevent this situation at load time. If 5-level paging is active, check the
> xloadflags whether the kexec kernel can handle 5-level paging at least in
> the decompressor. If not, reject the load attempt.

Yes, exactly. I will rewrite patch log with simpler sentences as you
have demonstrated. Thanks.
>  
> > Signed-off-by: Baoquan He <bhe@redhat.com>
> > Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > ---
> >  arch/x86/kernel/kexec-bzimage64.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/arch/x86/kernel/kexec-bzimage64.c b/arch/x86/kernel/kexec-bzimage64.c
> > index 1f3b77367948..4c9c079b5673 100644
> > --- a/arch/x86/kernel/kexec-bzimage64.c
> > +++ b/arch/x86/kernel/kexec-bzimage64.c
> > @@ -321,6 +321,11 @@ static int bzImage64_probe(const char *buf, unsigned long len)
> >  		return ret;
> >  	}
> >  
> > +	if (!(header->xloadflags & XLF_5LEVEL) && pgtable_l5_enabled()) {
> > +		pr_err("Can not jump to old 4-level kernel from 5-level kernel.\n");
> 
> This is confusing at best.
> 
>      	"bzImage cannot handle 5-level paging mode\n"
> 
> or something like this.
> 
> > +		return ret;

Will change too as you suggested.

Re: [PATCH v3 1/3] x86/boot: Add xloadflags bits for 5-level kernel checking

[Baoquan He]

On 04/05/19 at 10:23pm, Thomas Gleixner wrote:
> On Tue, 12 Mar 2019, Baoquan He wrote:
> 
> > Add two bits XLF_5LEVEL and XLF_5LEVEL_ENABLED for 5-level kernel.
> > Bit XLF_5LEVEL indicates if 5-level related code is contained
> > in this kernel.
> > Bit XLF_5LEVEL_ENABLED indicates if CONFIG_X86_5LEVEL=y is set.
> > 
> > They are being used in later patch to check if kexec/kdump kernel
> > is loaded in right place.
> 
> Only XLF_5LEVEL is checked. So what's the second flag for?

I put the explanation why only XLF_5LEVEL is checked in cover letter.
Will add them in this log.

Re: [PATCH v3 0/3] Add restrictions for kexec/kdump jumping between 5-level and 4-level kernel

[Baoquan He]

On 03/12/19 at 06:30pm, Baoquan He wrote:
> This is v3 post.
> 
> The original v1 post can be found here:
> http://lkml.kernel.org/r/20180829141624.13985-1-bhe@redhat.com
> 
> Later a v1 RESEND version:
> http://lkml.kernel.org/r/20190125022817.29506-1-bhe@redhat.com
> 
> V2 post is here:
> http://lkml.kernel.org/r/20190312005004.19182-1-bhe@redhat.com
> 
> This patchset is trying to fix several issues for kexec/kdump when
> dynamic switching of paging mode is enabled in x86_64. The current
> kernel supports 5-level paging mode, and supports dynamically choosing
> paging mode during bootup according to kernel image, hardware and
> kernel parameter setting. This flexibility brings several issues for
> kexec/kdump:
> 
> Issues:
> 1)
> Dynamic switching between paging mode requires code change in target
> kernel. So we can't kexec jump from 5-level kernel to old 4-level
> kernel which lacks the code change.
> 
> 2)
> Switching from 5-level paging to 4-level paging kernel would fail, if
> kexec() put kernel image above 64TiB of memory.
> 
> 3)
> Kdump jumping has similar issue as 2). This require us to only
> reserve crashkernel below 64TB, otherwise jumping from 5-level to
> 4-level kernel will fail.
> 
> Note:
> Since we have two interfaces kexec_load() and kexec_file_load() to load
> kexec/kdump kernel, handling for them is a little different. For
> kexec_load(), most of the loading job is done in user space utility
> kexec_tools. However, for kexec_file_load(), most of the loading codes
> have moved into kernel because of kernel image verification.
> 
> Fixes:
> a) For issue 1), we need check if XLF_5LEVEL is set, otherwise error out
>    a message. 
>   -This need be done in both kernel and kexec_tools utility.
>   -Patch 2/3 is the handling of kernel part.
>   -Will post user space patch to kexec mailing list later.
> 
> b) For issue 2), we need check if both XLF_5LEVEL and XLF_5LEVEL_ENABLED
>    are set, otherwise error out a message.
>   -This only need be done in kexec_tools utility. Because for
>    kexec_file_load(), the current code searches area to put kernel from
>    bottom to up in system RAM, we usually can always find an area below
>    4 GB, no need to worry about 5-level kernel jumping to 4-level
>    kernel. While for kexec_load(), it's top down seraching area for kernel
>    loading, and implemented in user space. We need make sure that
>    5-level kernel find an area under 64 TB for a kexec-ed kernel of
>    4-level.

This paragraph is used to explain why XLF_5LEVEL_ENABLED is not checked
in kernel. I will add these information to log of patch 1/3.

>   -Will post user space patch to kexec mailing list later.
> 
> c) For issues 3), just limit kernel to reserve crashkernel below 64 TB.
>   -This only need be done in kernel.
>   -It doesn't need to check bit XLF_5LEVEL or XLF_5LEVEL_ENABLED, we
>    just simply limit it below 64 TB which should be enough. Because
>    crashernel is reserved during the 1st kernel's bootup, we don't know
>    what kernel will be loaded for kdump usage.
>   -Patch 3/3 handles this.
> 
> Concerns from reviewing comments:
> 1)
> In v1, hpa raised concern that why the paging mode checking is not done
> before kexec jumping, the discussion can be found here:
> 
> http://lkml.kernel.org/r/alpine.DEB.2.21.1809051002020.1416@nanos.tec.linutronix.de
> 
> As tglx said, it might be not doable for kdump since kdump kernel's
> reserved crashkernel region only owns a portion of memory, may
> be above 4G; and might be not safer to do paging mode checking and
> switching thing after crash.
> 
> 2)
> In v1 RESEND post, tglx asked why only bit XLF_5LEVEL is checked, even
> though two bits XLF_5LEVEL or XLF_5LEVEL_ENABLED added. So add more
> words to explain it in *Fixes* b).
> 
> Changelog:
> v2->v3:
>   Change the constant to match the notation for the rest of defines as
>   Kirill suggested;
> v1->v2:
>   Correct the subject of patch 1 according to tglx's comment;
>   Add more information to cover-letter to address reviewers' concerns;
> 
> Baoquan He (3):
>   x86/boot: Add xloadflags bits for 5-level kernel checking
>   x86/kexec/64: Error out if try to jump to old 4-level kernel from
>     5-level kernel
>   x86/kdump/64: Change the upper limit of crashkernel reservation
> 
>  arch/x86/boot/header.S                | 12 +++++++++++-
>  arch/x86/include/uapi/asm/bootparam.h |  2 ++
>  arch/x86/kernel/kexec-bzimage64.c     |  5 +++++
>  arch/x86/kernel/setup.c               | 18 ++++++++++++++----
>  4 files changed, 32 insertions(+), 5 deletions(-)
> 
> -- 
> 2.17.2
>