Re: [Qemu-devel] [RFC PATCH 2/2] spapr: -kernel: allow linking with specified addr

2015-07-21 Thread Andrew Jones 
On Fri, Jul 17, 2015 at 01:56:40PM +0200, Andrew Jones wrote:
 I've started playing with adding ppc support to kvm-unit-tests,
 using spapr for the machine model. I wanted to link the unit test
 at 0x40 to match qemu's load address, making the unit test
 startup code simpler, but ended up with 0x80 instead, due to
 how translate_kernel_address works. The translation makes sense
 for how Linux kernels are linked (always at 0xc000 or
 0xc000), but for the unit test case we need to avoid
 adding the offset.
 
 Signed-off-by: Andrew Jones drjo...@redhat.com
 ---
 Big RFC because I don't know if the always at 0xc... statement
 is 100% true for Linux, nor if this patch would break other stuff...
 
  hw/ppc/spapr.c | 3 +++
  1 file changed, 3 insertions(+)
 
 diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
 index c1cbf3387ae0c..4f1548f5168db 100644
 --- a/hw/ppc/spapr.c
 +++ b/hw/ppc/spapr.c
 @@ -832,6 +832,9 @@ static void spapr_finalize_fdt(sPAPRMachineState *spapr,
  
  static uint64_t translate_kernel_address(void *opaque, uint64_t addr)
  {
 +if ((addr  0x0fff) == addr) {
 +return addr;
 +}
  return (addr  0x0fff) + KERNEL_LOAD_ADDR;
  }


OK, while I'm still not sure if this is a bad idea or not, and I do think
it would be nice to give users more control over the kernel's linked VMAs,
I've decided it would probably be harder to prove this patch is safe, than
to add reloc support to kvm-unit-tests/powerpc. So I added the support
yesterday, at least enough for now, and this patch is no longer necessary.

Thanks,
drew

Re: [Qemu-devel] [RFC PATCH 2/2] spapr: -kernel: allow linking with specified addr

2015-07-21 Thread Thomas Huth 
On 20/07/15 15:09, Andrew Jones wrote:
 On Mon, Jul 20, 2015 at 08:47:53AM +0200, Thomas Huth wrote:
...
 ... or you could try to get the elf_reloc code working for POWER, too
 (see include/hw/elf_ops.h). That way QEMU would take care of relocating
 your program. (you can peek at elf_apply_rela64() in
  https://github.com/aik/SLOF/blob/master/lib/libelf/elf64.c
 if you want to know what basically has to be done for POWER relocations).
 
 kvm-unit-tests doesn't load the unit test elf itself. It relies on QEMU's
 -kernel parameter to get the kernel (the unit test) into memory.

I was talking about the -kernel parameter of QEMU. That triggers the
load_elf() function of QEMU - and this function can provide ELF
relocation, too. It is used for s390x already to relocate the firmware
there, so I think it could be done for ppc64, too.

 Thomas

Re: [Qemu-devel] [RFC PATCH 2/2] spapr: -kernel: allow linking with specified addr

2015-07-21 Thread Andrew Jones 
On Tue, Jul 21, 2015 at 09:10:44AM +0200, Thomas Huth wrote:
 On 20/07/15 15:09, Andrew Jones wrote:
  On Mon, Jul 20, 2015 at 08:47:53AM +0200, Thomas Huth wrote:
 ...
  ... or you could try to get the elf_reloc code working for POWER, too
  (see include/hw/elf_ops.h). That way QEMU would take care of relocating
  your program. (you can peek at elf_apply_rela64() in
   https://github.com/aik/SLOF/blob/master/lib/libelf/elf64.c
  if you want to know what basically has to be done for POWER relocations).
  
  kvm-unit-tests doesn't load the unit test elf itself. It relies on QEMU's
  -kernel parameter to get the kernel (the unit test) into memory.
 
 I was talking about the -kernel parameter of QEMU. That triggers the
 load_elf() function of QEMU - and this function can provide ELF
 relocation, too. It is used for s390x already to relocate the firmware
 there, so I think it could be done for ppc64, too.

Ah, I think I see what you mean now, extend QEMU's elf loader to do the
relocating. Then, when the relocatable kernel starts, it has already
been pre-relocated. That would certainly be do-able, but I think any
kernel that expects to be relocatable will already have it's own code,
and we'd end up doing the relocation twice. Kernels that want to avoid
relocation would typically depend on the linker load addresses (but the
problem here is that spapr has it's own opinion for those...)

I took a quick peek at what s390x does right now for its firmware. afaict
it's similar to what spapr is doing, just overriding the elf's LMA.

drew

Re: [Qemu-devel] [RFC PATCH 2/2] spapr: -kernel: allow linking with specified addr

2015-07-20 Thread Thomas Huth 
On 20/07/15 07:01, David Gibson wrote:
 On Fri, Jul 17, 2015 at 01:56:40PM +0200, Andrew Jones wrote:
 I've started playing with adding ppc support to kvm-unit-tests,
 using spapr for the machine model. I wanted to link the unit test
 at 0x40 to match qemu's load address, making the unit test
 startup code simpler, but ended up with 0x80 instead, due to
 how translate_kernel_address works. The translation makes sense
 for how Linux kernels are linked (always at 0xc000 or
 0xc000), but for the unit test case we need to avoid
 adding the offset.

 Signed-off-by: Andrew Jones drjo...@redhat.com
 ---
 Big RFC because I don't know if the always at 0xc... statement
 is 100% true for Linux, nor if this patch would break other stuff...
 
 Yeah, I'm pretty dubious about this too, especially since I don't
 entirely grasp what the load_elf() translation function is all about
 anyway.

Well, AFAIK it's used to modify the addresses before the ELF loader uses
the address for loading. For example if your ELF binary is linked at
address 0x1000, the translate function would move your binary to
0x401000 instead so that it does not interfere with the SLOF firmware
(which is loaded to address 0 IIRC).

So I also think your fix here is wrong, Andrew. E.g. when you have a
binary that is linked to address 0x1000, you don't want to bypass the
translation step here since it then would clash with the firmware.

 That said, I suspect making your unit test assume a fixed load address
 may not be the best idea - qemu or SLOF could change in future to move
 things about, so it might be more robust to have your test copy itself
 to address it wants to be at before executing.

+1

... or you could try to get the elf_reloc code working for POWER, too
(see include/hw/elf_ops.h). That way QEMU would take care of relocating
your program. (you can peek at elf_apply_rela64() in
 https://github.com/aik/SLOF/blob/master/lib/libelf/elf64.c
if you want to know what basically has to be done for POWER relocations).

 Thomas




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Re: [Qemu-devel] [RFC PATCH 2/2] spapr: -kernel: allow linking with specified addr

2015-07-20 Thread Andrew Jones 
On Mon, Jul 20, 2015 at 08:47:53AM +0200, Thomas Huth wrote:
 On 20/07/15 07:01, David Gibson wrote:
  On Fri, Jul 17, 2015 at 01:56:40PM +0200, Andrew Jones wrote:
  I've started playing with adding ppc support to kvm-unit-tests,
  using spapr for the machine model. I wanted to link the unit test
  at 0x40 to match qemu's load address, making the unit test
  startup code simpler, but ended up with 0x80 instead, due to
  how translate_kernel_address works. The translation makes sense
  for how Linux kernels are linked (always at 0xc000 or
  0xc000), but for the unit test case we need to avoid
  adding the offset.
 
  Signed-off-by: Andrew Jones drjo...@redhat.com
  ---
  Big RFC because I don't know if the always at 0xc... statement
  is 100% true for Linux, nor if this patch would break other stuff...
  
  Yeah, I'm pretty dubious about this too, especially since I don't
  entirely grasp what the load_elf() translation function is all about
  anyway.
 
 Well, AFAIK it's used to modify the addresses before the ELF loader uses
 the address for loading. For example if your ELF binary is linked at
 address 0x1000, the translate function would move your binary to
 0x401000 instead so that it does not interfere with the SLOF firmware
 (which is loaded to address 0 IIRC).

This is correct, but the move isn't just to make sure we don't interfere
with SLOF, it's also to make sure we can load the kernel into main
memory. When the link address is 0xc...,  then we can't use vaddr ==
paddr. The Linux ppc64 kernel, for example, is linked at 0xc000.
So it gets pulled down with the mask, and then the 0x40 offset is added
to get it above SLOF.

 
 So I also think your fix here is wrong, Andrew. E.g. when you have a
 binary that is linked to address 0x1000, you don't want to bypass the
 translation step here since it then would clash with the firmware.

I set the unit test's text segment start at 0x40, so QEMU still
loads it there with this patch, and thus it wouldn't clash with SLOF.
But, anyway, there's no need for SLOF in kvm-unit-tests, and I've
replaced it with a four byte boot loader, b 0x40.

 
  That said, I suspect making your unit test assume a fixed load address
  may not be the best idea - qemu or SLOF could change in future to move
  things about, so it might be more robust to have your test copy itself
  to address it wants to be at before executing.
 

Well, the reason I want vaddr == paddr is to be able to run C code
without the MMU enabled, not to mention to avoid needing to do any sort
of reloc dance in what's supposed to be super simple code. I don't need
to worry about SLOF changing things, since I don't use it. If QEMU changes
the load address, then things will indeed break, but it would be a one line
Makefile fix in kvm-unit-tests to point the text segment to the new offset.

 +1
 
 ... or you could try to get the elf_reloc code working for POWER, too
 (see include/hw/elf_ops.h). That way QEMU would take care of relocating
 your program. (you can peek at elf_apply_rela64() in
  https://github.com/aik/SLOF/blob/master/lib/libelf/elf64.c
 if you want to know what basically has to be done for POWER relocations).
 
  Thomas
 
 

kvm-unit-tests doesn't load the unit test elf itself. It relies on QEMU's
-kernel parameter to get the kernel (the unit test) into memory.

Thanks,
drew

Re: [Qemu-devel] [RFC PATCH 2/2] spapr: -kernel: allow linking with specified addr

2015-07-19 Thread David Gibson 
On Fri, Jul 17, 2015 at 01:56:40PM +0200, Andrew Jones wrote:
 I've started playing with adding ppc support to kvm-unit-tests,
 using spapr for the machine model. I wanted to link the unit test
 at 0x40 to match qemu's load address, making the unit test
 startup code simpler, but ended up with 0x80 instead, due to
 how translate_kernel_address works. The translation makes sense
 for how Linux kernels are linked (always at 0xc000 or
 0xc000), but for the unit test case we need to avoid
 adding the offset.
 
 Signed-off-by: Andrew Jones drjo...@redhat.com
 ---
 Big RFC because I don't know if the always at 0xc... statement
 is 100% true for Linux, nor if this patch would break other stuff...

Yeah, I'm pretty dubious about this too, especially since I don't
entirely grasp what the load_elf() translation function is all about
anyway.

At least 32-bit Linux on powerpc can have a different link offset -
it's actually a config option.  I can't see any practical purpose for
a link address that has any bits in the 0xfff mask though.

That said, I suspect making your unit test assume a fixed load address
may not be the best idea - qemu or SLOF could change in future to move
things about, so it might be more robust to have your test copy itself
to address it wants to be at before executing.

 
  hw/ppc/spapr.c | 3 +++
  1 file changed, 3 insertions(+)
 
 diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
 index c1cbf3387ae0c..4f1548f5168db 100644
 --- a/hw/ppc/spapr.c
 +++ b/hw/ppc/spapr.c
 @@ -832,6 +832,9 @@ static void spapr_finalize_fdt(sPAPRMachineState *spapr,
  
  static uint64_t translate_kernel_address(void *opaque, uint64_t addr)
  {
 +if ((addr  0x0fff) == addr) {
 +return addr;
 +}
  return (addr  0x0fff) + KERNEL_LOAD_ADDR;
  }
  

-- 
David Gibson| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson


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[Qemu-devel] [RFC PATCH 2/2] spapr: -kernel: allow linking with specified addr

2015-07-17 Thread Andrew Jones 
I've started playing with adding ppc support to kvm-unit-tests,
using spapr for the machine model. I wanted to link the unit test
at 0x40 to match qemu's load address, making the unit test
startup code simpler, but ended up with 0x80 instead, due to
how translate_kernel_address works. The translation makes sense
for how Linux kernels are linked (always at 0xc000 or
0xc000), but for the unit test case we need to avoid
adding the offset.

Signed-off-by: Andrew Jones drjo...@redhat.com
---
Big RFC because I don't know if the always at 0xc... statement
is 100% true for Linux, nor if this patch would break other stuff...

 hw/ppc/spapr.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
index c1cbf3387ae0c..4f1548f5168db 100644
--- a/hw/ppc/spapr.c
+++ b/hw/ppc/spapr.c
@@ -832,6 +832,9 @@ static void spapr_finalize_fdt(sPAPRMachineState *spapr,
 
 static uint64_t translate_kernel_address(void *opaque, uint64_t addr)
 {
+if ((addr  0x0fff) == addr) {
+return addr;
+}
 return (addr  0x0fff) + KERNEL_LOAD_ADDR;
 }
 
-- 
2.4.3