Loading a 32 bit kernel from 64 bit grub-xen

Loading a 32 bit kernel from 64 bit grub-xen

[Phillip Susi]

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Hash: SHA1

I have been trying to fix grub to load a 32 bit kernel from the 64 bit
xen build.  After fixing up one or two minor issues with the elf
loader, I believe I now have it to the point where it jumps correctly
to the 32 bit kernel and it crashes there, since it is 32 bit code
still executing on a cpu in 64 bit mode.  The question is how to
return the cpu to 32 bit mode *under xen*?

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Re: Loading a 32 bit kernel from 64 bit grub-xen

[Vladimir 'phcoder' Serbinenko]

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Not possible without xen cooperation and xen has no hypercalls for this
(main problem being page table layout)
On 1 Jul 2014 18:06, "Phillip Susi" <psusi@ubuntu.com> wrote:

> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> I have been trying to fix grub to load a 32 bit kernel from the 64 bit
> xen build.  After fixing up one or two minor issues with the elf
> loader, I believe I now have it to the point where it jumps correctly
> to the 32 bit kernel and it crashes there, since it is 32 bit code
> still executing on a cpu in 64 bit mode.  The question is how to
> return the cpu to 32 bit mode *under xen*?
>
> -----BEGIN PGP SIGNATURE-----
> Version: GnuPG v2.0.17 (MingW32)
> Comment: Using GnuPG with Thunderbird - http://www.enigmail.net/
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>
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>

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Re: Loading a 32 bit kernel from 64 bit grub-xen

[Andrey Borzenkov]

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Hash: SHA1

В Tue, 01 Jul 2014 12:06:08 -0400
Phillip Susi <psusi@ubuntu.com> пишет:

> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
> 
> I have been trying to fix grub to load a 32 bit kernel from the 64 bit
> xen build.  After fixing up one or two minor issues with the elf
> loader, I believe I now have it to the point where it jumps correctly
> to the 32 bit kernel and it crashes there, since it is 32 bit code
> still executing on a cpu in 64 bit mode.  The question is how to
> return the cpu to 32 bit mode *under xen*?
> 

IIRC it was already discussed not long ago and it seems to be Xen
limitation. You probably need to ask on xen-devel to be sure.

Is it not more simple to use 32 bit grub with 32 bit kernel to start
with? 
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Re: Loading a 32 bit kernel from 64 bit grub-xen

[Phillip Susi]

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Hash: SHA1

On 7/1/2014 12:18 PM, Andrey Borzenkov wrote:
> В Tue, 01 Jul 2014 12:06:08 -0400 Phillip Susi <psusi@ubuntu.com>
> пишет:
> 
>> -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1
> 
>> I have been trying to fix grub to load a 32 bit kernel from the
>> 64 bit xen build.  After fixing up one or two minor issues with
>> the elf loader, I believe I now have it to the point where it
>> jumps correctly to the 32 bit kernel and it crashes there, since
>> it is 32 bit code still executing on a cpu in 64 bit mode.  The
>> question is how to return the cpu to 32 bit mode *under xen*?
> 
> 
> IIRC it was already discussed not long ago and it seems to be Xen 
> limitation. You probably need to ask on xen-devel to be sure.
> 
> Is it not more simple to use 32 bit grub with 32 bit kernel to
> start with?

The problem with that is that you have to know in advance which kernel
you are going to boot.  That makes configuring virtual hosts harder;
they just want one grub image they can use to chainload whatever the
guest wants to install in their domain.

Also there must be a way to do this otherwise a 64 bit kernel running
under xen wouldn't be able to execute a 32 bit binary.  I suppose I'll
Cc xen-devel.

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Re: Loading a 32 bit kernel from 64 bit grub-xen

[Phillip Susi]

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Hash: SHA1

On 7/1/2014 12:18 PM, Andrey Borzenkov wrote:
> В Tue, 01 Jul 2014 12:06:08 -0400 Phillip Susi <psusi@ubuntu.com>
> пишет:
>
>> -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1
>
>> I have been trying to fix grub to load a 32 bit kernel from the
>> 64 bit xen build.  After fixing up one or two minor issues with
>> the elf loader, I believe I now have it to the point where it
>> jumps correctly to the 32 bit kernel and it crashes there, since
>> it is 32 bit code still executing on a cpu in 64 bit mode.  The
>> question is how to return the cpu to 32 bit mode *under xen*?
>
>
> IIRC it was already discussed not long ago and it seems to be Xen
> limitation. You probably need to ask on xen-devel to be sure.
>
> Is it not more simple to use 32 bit grub with 32 bit kernel to
> start with?

The problem with that is that you have to know in advance which kernel
you are going to boot.  That makes configuring virtual hosts harder;
they just want one grub image they can use to chainload whatever the
guest wants to install in their domain.

Also there must be a way to do this otherwise a 64 bit kernel running
under xen wouldn't be able to execute a 32 bit binary.  I suppose I'll
Cc xen-devel.

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Re: Loading a 32 bit kernel from 64 bit grub-xen

[Vladimir 'phcoder' Serbinenko]

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On 1 Jul 2014 20:28, "Phillip Susi" <psusi@ubuntu.com> wrote:
>
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> On 7/1/2014 12:18 PM, Andrey Borzenkov wrote:
> > В Tue, 01 Jul 2014 12:06:08 -0400 Phillip Susi <psusi@ubuntu.com>
> > пишет:
> >
> >> -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1
> >
> >> I have been trying to fix grub to load a 32 bit kernel from the
> >> 64 bit xen build.  After fixing up one or two minor issues with
> >> the elf loader, I believe I now have it to the point where it
> >> jumps correctly to the 32 bit kernel and it crashes there, since
> >> it is 32 bit code still executing on a cpu in 64 bit mode.  The
> >> question is how to return the cpu to 32 bit mode *under xen*?
> >
> >
> > IIRC it was already discussed not long ago and it seems to be Xen
> > limitation. You probably need to ask on xen-devel to be sure.
> >
> > Is it not more simple to use 32 bit grub with 32 bit kernel to
> > start with?
>
> The problem with that is that you have to know in advance which kernel
> you are going to boot.  That makes configuring virtual hosts harder;
> they just want one grub image they can use to chainload whatever the
> guest wants to install in their domain.
>
> Also there must be a way to do this otherwise a 64 bit kernel running
> under xen wouldn't be able to execute a 32 bit binary.
Binaries don't have to use e.g. paging table or hypercall mechanism. You
need to either make xen switch all those structures or port kernels to
fake-32-xen mode. When either is done GRUB part will be trivial but either
of those tasks is outside of grub scope.
> I suppose I'll
> Cc xen-devel.
>
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>
> _______________________________________________
> Grub-devel mailing list
> Grub-devel@gnu.org
> https://lists.gnu.org/mailman/listinfo/grub-devel

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Re: Loading a 32 bit kernel from 64 bit grub-xen

[Vladimir 'phcoder' Serbinenko]

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On 1 Jul 2014 20:28, "Phillip Susi" <psusi@ubuntu.com> wrote:
>
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> On 7/1/2014 12:18 PM, Andrey Borzenkov wrote:
> > В Tue, 01 Jul 2014 12:06:08 -0400 Phillip Susi <psusi@ubuntu.com>
> > пишет:
> >
> >> -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1
> >
> >> I have been trying to fix grub to load a 32 bit kernel from the
> >> 64 bit xen build.  After fixing up one or two minor issues with
> >> the elf loader, I believe I now have it to the point where it
> >> jumps correctly to the 32 bit kernel and it crashes there, since
> >> it is 32 bit code still executing on a cpu in 64 bit mode.  The
> >> question is how to return the cpu to 32 bit mode *under xen*?
> >
> >
> > IIRC it was already discussed not long ago and it seems to be Xen
> > limitation. You probably need to ask on xen-devel to be sure.
> >
> > Is it not more simple to use 32 bit grub with 32 bit kernel to
> > start with?
>
> The problem with that is that you have to know in advance which kernel
> you are going to boot.  That makes configuring virtual hosts harder;
> they just want one grub image they can use to chainload whatever the
> guest wants to install in their domain.
>
> Also there must be a way to do this otherwise a 64 bit kernel running
> under xen wouldn't be able to execute a 32 bit binary.
Binaries don't have to use e.g. paging table or hypercall mechanism. You
need to either make xen switch all those structures or port kernels to
fake-32-xen mode. When either is done GRUB part will be trivial but either
of those tasks is outside of grub scope.
> I suppose I'll
> Cc xen-devel.
>
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> =Vadi
> -----END PGP SIGNATURE-----
>
> _______________________________________________
> Grub-devel mailing list
> Grub-devel@gnu.org
> https://lists.gnu.org/mailman/listinfo/grub-devel

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Re: Loading a 32 bit kernel from 64 bit grub-xen

[Jan Beulich]

>>> On 01.07.14 at 20:27, <psusi@ubuntu.com> wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
> 
> On 7/1/2014 12:18 PM, Andrey Borzenkov wrote:
>> В Tue, 01 Jul 2014 12:06:08 -0400 Phillip Susi <psusi@ubuntu.com>
>> пишет:
>> 
>>> -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1
>> 
>>> I have been trying to fix grub to load a 32 bit kernel from the
>>> 64 bit xen build.  After fixing up one or two minor issues with
>>> the elf loader, I believe I now have it to the point where it
>>> jumps correctly to the 32 bit kernel and it crashes there, since
>>> it is 32 bit code still executing on a cpu in 64 bit mode.  The
>>> question is how to return the cpu to 32 bit mode *under xen*?
>> 
>> 
>> IIRC it was already discussed not long ago and it seems to be Xen 
>> limitation. You probably need to ask on xen-devel to be sure.
>> 
>> Is it not more simple to use 32 bit grub with 32 bit kernel to
>> start with?
> 
> The problem with that is that you have to know in advance which kernel
> you are going to boot.  That makes configuring virtual hosts harder;
> they just want one grub image they can use to chainload whatever the
> guest wants to install in their domain.
> 
> Also there must be a way to do this otherwise a 64 bit kernel running
> under xen wouldn't be able to execute a 32 bit binary.  I suppose I'll
> Cc xen-devel.

Perhaps there's some confusion about what "kernel" here means:
The thing that's the kernel from GrUB's perspective is the hypervisor,
i.e. xen.gz or xen.efi. While the former expects to be entered in 32-bit
mode, on the Xen side we all agree that this isn't the way things
should work under UEFI; Daniel already proposed how to handle this
case (Xen should be entered in 64-bit mode, with ExitBootServices()
not having got called yet).

The thing that you'd traditionally call kernel (e.g. Linux) can be 32-
or 64-bit irrespective of GrUB's bitness: Xen knows how to load either
(both for Dom0 and, likely irrelevant here, DomU).

Jan

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Re: [Xen-devel] Loading a 32 bit kernel from 64 bit grub-xen

[Jan Beulich]

>>> On 01.07.14 at 20:27, <psusi@ubuntu.com> wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
> 
> On 7/1/2014 12:18 PM, Andrey Borzenkov wrote:
>> В Tue, 01 Jul 2014 12:06:08 -0400 Phillip Susi <psusi@ubuntu.com>
>> пишет:
>> 
>>> -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1
>> 
>>> I have been trying to fix grub to load a 32 bit kernel from the
>>> 64 bit xen build.  After fixing up one or two minor issues with
>>> the elf loader, I believe I now have it to the point where it
>>> jumps correctly to the 32 bit kernel and it crashes there, since
>>> it is 32 bit code still executing on a cpu in 64 bit mode.  The
>>> question is how to return the cpu to 32 bit mode *under xen*?
>> 
>> 
>> IIRC it was already discussed not long ago and it seems to be Xen 
>> limitation. You probably need to ask on xen-devel to be sure.
>> 
>> Is it not more simple to use 32 bit grub with 32 bit kernel to
>> start with?
> 
> The problem with that is that you have to know in advance which kernel
> you are going to boot.  That makes configuring virtual hosts harder;
> they just want one grub image they can use to chainload whatever the
> guest wants to install in their domain.
> 
> Also there must be a way to do this otherwise a 64 bit kernel running
> under xen wouldn't be able to execute a 32 bit binary.  I suppose I'll
> Cc xen-devel.

Perhaps there's some confusion about what "kernel" here means:
The thing that's the kernel from GrUB's perspective is the hypervisor,
i.e. xen.gz or xen.efi. While the former expects to be entered in 32-bit
mode, on the Xen side we all agree that this isn't the way things
should work under UEFI; Daniel already proposed how to handle this
case (Xen should be entered in 64-bit mode, with ExitBootServices()
not having got called yet).

The thing that you'd traditionally call kernel (e.g. Linux) can be 32-
or 64-bit irrespective of GrUB's bitness: Xen knows how to load either
(both for Dom0 and, likely irrelevant here, DomU).

Jan

Re: [Xen-devel] Loading a 32 bit kernel from 64 bit grub-xen

[Vladimir 'phcoder' Serbinenko]

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On 2 Jul 2014 10:36, "Jan Beulich" <JBeulich@suse.com> wrote:
>
> >>> On 01.07.14 at 20:27, <psusi@ubuntu.com> wrote:
> > -----BEGIN PGP SIGNED MESSAGE-----
> > Hash: SHA1
> >
> > On 7/1/2014 12:18 PM, Andrey Borzenkov wrote:
> >> В Tue, 01 Jul 2014 12:06:08 -0400 Phillip Susi <psusi@ubuntu.com>
> >> пишет:
> >>
> >>> -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1
> >>
> >>> I have been trying to fix grub to load a 32 bit kernel from the
> >>> 64 bit xen build.  After fixing up one or two minor issues with
> >>> the elf loader, I believe I now have it to the point where it
> >>> jumps correctly to the 32 bit kernel and it crashes there, since
> >>> it is 32 bit code still executing on a cpu in 64 bit mode.  The
> >>> question is how to return the cpu to 32 bit mode *under xen*?
> >>
> >>
> >> IIRC it was already discussed not long ago and it seems to be Xen
> >> limitation. You probably need to ask on xen-devel to be sure.
> >>
> >> Is it not more simple to use 32 bit grub with 32 bit kernel to
> >> start with?
> >
> > The problem with that is that you have to know in advance which kernel
> > you are going to boot.  That makes configuring virtual hosts harder;
> > they just want one grub image they can use to chainload whatever the
> > guest wants to install in their domain.
> >
> > Also there must be a way to do this otherwise a 64 bit kernel running
> > under xen wouldn't be able to execute a 32 bit binary.  I suppose I'll
> > Cc xen-devel.
>
> Perhaps there's some confusion about what "kernel" here means:
> The thing that's the kernel from GrUB's perspective is the hypervisor,
> i.e. xen.gz or xen.efi. While the former expects to be entered in 32-bit
> mode, on the Xen side we all agree that this isn't the way things
> should work under UEFI; Daniel already proposed how to handle this
> case (Xen should be entered in 64-bit mode, with ExitBootServices()
> not having got called yet).
>
> The thing that you'd traditionally call kernel (e.g. Linux) can be 32-
> or 64-bit irrespective of GrUB's bitness: Xen knows how to load either
> (both for Dom0 and, likely irrelevant here, DomU).
>
Actually you're the first one to speak about dom0 here. Whole discussion
was about domU.
> Jan
>
> _______________________________________________
> Grub-devel mailing list
> Grub-devel@gnu.org
> https://lists.gnu.org/mailman/listinfo/grub-devel

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Re: Loading a 32 bit kernel from 64 bit grub-xen

[Ian Campbell]

On Tue, 2014-07-01 at 14:27 -0400, Phillip Susi wrote:
> On 7/1/2014 12:18 PM, Andrey Borzenkov wrote:
> > В Tue, 01 Jul 2014 12:06:08 -0400 Phillip Susi <psusi@ubuntu.com>
> > пишет:
> > 
> >> -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1
> > 
> >> I have been trying to fix grub to load a 32 bit kernel from the
> >> 64 bit xen build.  After fixing up one or two minor issues with
> >> the elf loader, I believe I now have it to the point where it
> >> jumps correctly to the 32 bit kernel and it crashes there, since
> >> it is 32 bit code still executing on a cpu in 64 bit mode.  The
> >> question is how to return the cpu to 32 bit mode *under xen*?
> > 
> > 
> > IIRC it was already discussed not long ago and it seems to be Xen 
> > limitation. You probably need to ask on xen-devel to be sure.
> > 
> > Is it not more simple to use 32 bit grub with 32 bit kernel to
> > start with?
> 
> The problem with that is that you have to know in advance which kernel
> you are going to boot.  That makes configuring virtual hosts harder;
> they just want one grub image they can use to chainload whatever the
> guest wants to install in their domain.
> 
> Also there must be a way to do this otherwise a 64 bit kernel running
> under xen wouldn't be able to execute a 32 bit binary.  I suppose I'll
> Cc xen-devel.

Running a 32-bit userspace under a 64-bit kernel is a very different
thing to switching a kernel from 64-bit down to 32-bit (or vice versa).

While the former is of course supported just fine the latter is not and
IIRC doing so would be very difficult for a PV kernel due to the way the
interfaces are constructed (e.g. to p2m entries change size, the maximum
addressable machine address changes).

Yes, this is a massive Pain In The A**.

Ian.


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Re: [Xen-devel] Loading a 32 bit kernel from 64 bit grub-xen

[Ian Campbell]

On Tue, 2014-07-01 at 14:27 -0400, Phillip Susi wrote:
> On 7/1/2014 12:18 PM, Andrey Borzenkov wrote:
> > В Tue, 01 Jul 2014 12:06:08 -0400 Phillip Susi <psusi@ubuntu.com>
> > пишет:
> > 
> >> -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1
> > 
> >> I have been trying to fix grub to load a 32 bit kernel from the
> >> 64 bit xen build.  After fixing up one or two minor issues with
> >> the elf loader, I believe I now have it to the point where it
> >> jumps correctly to the 32 bit kernel and it crashes there, since
> >> it is 32 bit code still executing on a cpu in 64 bit mode.  The
> >> question is how to return the cpu to 32 bit mode *under xen*?
> > 
> > 
> > IIRC it was already discussed not long ago and it seems to be Xen 
> > limitation. You probably need to ask on xen-devel to be sure.
> > 
> > Is it not more simple to use 32 bit grub with 32 bit kernel to
> > start with?
> 
> The problem with that is that you have to know in advance which kernel
> you are going to boot.  That makes configuring virtual hosts harder;
> they just want one grub image they can use to chainload whatever the
> guest wants to install in their domain.
> 
> Also there must be a way to do this otherwise a 64 bit kernel running
> under xen wouldn't be able to execute a 32 bit binary.  I suppose I'll
> Cc xen-devel.

Running a 32-bit userspace under a 64-bit kernel is a very different
thing to switching a kernel from 64-bit down to 32-bit (or vice versa).

While the former is of course supported just fine the latter is not and
IIRC doing so would be very difficult for a PV kernel due to the way the
interfaces are constructed (e.g. to p2m entries change size, the maximum
addressable machine address changes).

Yes, this is a massive Pain In The A**.

Ian.

Re: Loading a 32 bit kernel from 64 bit grub-xen

[Ian Campbell]

On Wed, 2014-07-02 at 09:35 +0100, Jan Beulich wrote:
> >>> On 01.07.14 at 20:27, <psusi@ubuntu.com> wrote:
> > -----BEGIN PGP SIGNED MESSAGE-----
> > Hash: SHA1
> > 
> > On 7/1/2014 12:18 PM, Andrey Borzenkov wrote:
> >> В Tue, 01 Jul 2014 12:06:08 -0400 Phillip Susi <psusi@ubuntu.com>
> >> пишет:
> >> 
> >>> -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1
> >> 
> >>> I have been trying to fix grub to load a 32 bit kernel from the
> >>> 64 bit xen build.  After fixing up one or two minor issues with
> >>> the elf loader, I believe I now have it to the point where it
> >>> jumps correctly to the 32 bit kernel and it crashes there, since
> >>> it is 32 bit code still executing on a cpu in 64 bit mode.  The
> >>> question is how to return the cpu to 32 bit mode *under xen*?
> >> 
> >> 
> >> IIRC it was already discussed not long ago and it seems to be Xen 
> >> limitation. You probably need to ask on xen-devel to be sure.
> >> 
> >> Is it not more simple to use 32 bit grub with 32 bit kernel to
> >> start with?
> > 
> > The problem with that is that you have to know in advance which kernel
> > you are going to boot.  That makes configuring virtual hosts harder;
> > they just want one grub image they can use to chainload whatever the
> > guest wants to install in their domain.
> > 
> > Also there must be a way to do this otherwise a 64 bit kernel running
> > under xen wouldn't be able to execute a 32 bit binary.  I suppose I'll
> > Cc xen-devel.
> 
> Perhaps there's some confusion about what "kernel" here means:
> The thing that's the kernel from GrUB's perspective is the hypervisor,
> i.e. xen.gz or xen.efi. While the former expects to be entered in 32-bit
> mode, on the Xen side we all agree that this isn't the way things
> should work under UEFI; Daniel already proposed how to handle this
> case (Xen should be entered in 64-bit mode, with ExitBootServices()
> not having got called yet).
> 
> The thing that you'd traditionally call kernel (e.g. Linux) can be 32-
> or 64-bit irrespective of GrUB's bitness: Xen knows how to load either
> (both for Dom0 and, likely irrelevant here, DomU).

Actually, I read Philips message as refering to launching a domU kernel
from a PV grub-xen, not a dom0 thing.

Ian


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Re: [Xen-devel] Loading a 32 bit kernel from 64 bit grub-xen

[Ian Campbell]

On Wed, 2014-07-02 at 09:35 +0100, Jan Beulich wrote:
> >>> On 01.07.14 at 20:27, <psusi@ubuntu.com> wrote:
> > -----BEGIN PGP SIGNED MESSAGE-----
> > Hash: SHA1
> > 
> > On 7/1/2014 12:18 PM, Andrey Borzenkov wrote:
> >> В Tue, 01 Jul 2014 12:06:08 -0400 Phillip Susi <psusi@ubuntu.com>
> >> пишет:
> >> 
> >>> -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1
> >> 
> >>> I have been trying to fix grub to load a 32 bit kernel from the
> >>> 64 bit xen build.  After fixing up one or two minor issues with
> >>> the elf loader, I believe I now have it to the point where it
> >>> jumps correctly to the 32 bit kernel and it crashes there, since
> >>> it is 32 bit code still executing on a cpu in 64 bit mode.  The
> >>> question is how to return the cpu to 32 bit mode *under xen*?
> >> 
> >> 
> >> IIRC it was already discussed not long ago and it seems to be Xen 
> >> limitation. You probably need to ask on xen-devel to be sure.
> >> 
> >> Is it not more simple to use 32 bit grub with 32 bit kernel to
> >> start with?
> > 
> > The problem with that is that you have to know in advance which kernel
> > you are going to boot.  That makes configuring virtual hosts harder;
> > they just want one grub image they can use to chainload whatever the
> > guest wants to install in their domain.
> > 
> > Also there must be a way to do this otherwise a 64 bit kernel running
> > under xen wouldn't be able to execute a 32 bit binary.  I suppose I'll
> > Cc xen-devel.
> 
> Perhaps there's some confusion about what "kernel" here means:
> The thing that's the kernel from GrUB's perspective is the hypervisor,
> i.e. xen.gz or xen.efi. While the former expects to be entered in 32-bit
> mode, on the Xen side we all agree that this isn't the way things
> should work under UEFI; Daniel already proposed how to handle this
> case (Xen should be entered in 64-bit mode, with ExitBootServices()
> not having got called yet).
> 
> The thing that you'd traditionally call kernel (e.g. Linux) can be 32-
> or 64-bit irrespective of GrUB's bitness: Xen knows how to load either
> (both for Dom0 and, likely irrelevant here, DomU).

Actually, I read Philips message as refering to launching a domU kernel
from a PV grub-xen, not a dom0 thing.

Ian

Re: Loading a 32 bit kernel from 64 bit grub-xen

[Phillip Susi]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

On 7/2/2014 5:26 AM, Ian Campbell wrote:

So you are saying that you could put the cpu into 32 bit mode, but xen
would still believe that the domU is 64 bit, and thus, expect the
hypercalls to be 64 bit?


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Re: [Xen-devel] Loading a 32 bit kernel from 64 bit grub-xen

[Phillip Susi]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

On 7/2/2014 5:26 AM, Ian Campbell wrote:

So you are saying that you could put the cpu into 32 bit mode, but xen
would still believe that the domU is 64 bit, and thus, expect the
hypercalls to be 64 bit?


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Re: Loading a 32 bit kernel from 64 bit grub-xen

[Ian Campbell]

On Tue, 2014-07-08 at 11:37 -0400, Phillip Susi wrote:
> On 7/2/2014 5:26 AM, Ian Campbell wrote:
> 
> So you are saying

I don't know, because you've helpfully trimmed all of the context of
something I said a week ago.

/me consults list archive, I think you are responding to:

> > Running a 32-bit userspace under a 64-bit kernel is a very different
> > thing to switching a kernel from 64-bit down to 32-bit (or vice versa).
> >
> > While the former is of course supported just fine the latter is not and
> > IIRC doing so would be very difficult for a PV kernel due to the way the
> > interfaces are constructed (e.g. to p2m entries change size, the maximum
> > addressable machine address changes).

----

>  that you could put the cpu into 32 bit mode, but xen
> would still believe that the domU is 64 bit, and thus, expect the
> hypercalls to be 64 bit?

Assuming you are asking about 32-bit kernel mode then a Xen PV 64-bit
guest cannot be transitioned to run in kernel mode in anything other
than 64-bit mode. There is simply no way to put the cpu into 32 bit mode
while remaining in kernel mode.

If you meant in userspace then all userspace hypercalls are made via a
kernel driver and thus appear to come from the 64-bit kernel and Xen
sees them as such.

Ian.

Re: [Xen-devel] Loading a 32 bit kernel from 64 bit grub-xen

[Ian Campbell]

On Tue, 2014-07-08 at 11:37 -0400, Phillip Susi wrote:
> On 7/2/2014 5:26 AM, Ian Campbell wrote:
> 
> So you are saying

I don't know, because you've helpfully trimmed all of the context of
something I said a week ago.

/me consults list archive, I think you are responding to:

> > Running a 32-bit userspace under a 64-bit kernel is a very different
> > thing to switching a kernel from 64-bit down to 32-bit (or vice versa).
> >
> > While the former is of course supported just fine the latter is not and
> > IIRC doing so would be very difficult for a PV kernel due to the way the
> > interfaces are constructed (e.g. to p2m entries change size, the maximum
> > addressable machine address changes).

----

>  that you could put the cpu into 32 bit mode, but xen
> would still believe that the domU is 64 bit, and thus, expect the
> hypercalls to be 64 bit?

Assuming you are asking about 32-bit kernel mode then a Xen PV 64-bit
guest cannot be transitioned to run in kernel mode in anything other
than 64-bit mode. There is simply no way to put the cpu into 32 bit mode
while remaining in kernel mode.

If you meant in userspace then all userspace hypercalls are made via a
kernel driver and thus appear to come from the 64-bit kernel and Xen
sees them as such.

Ian.