LDM/STM alignment fixups on arm64
Hi,
While understanding how the alignment are handled on arm and arm64, we came across the fixups for LDM/LDRD/STM on arm where as these fixups are not present on arm64.
There may be some specific reason why these fixups are not ported to arm64. Can you please help us understand this?
With this difference in how kernel handles 32-bit apps on arm and arm64, there can be apps which are working without abort on arm, but fail on arm64 (SIGBUS). We have tried to get some history on web, but not successful.
If this is indeed missing on arm64, do you see any issue if its ported (does it fail any guidance)?
Thanks,
-- Srinivas R
Signature · 3 lines
Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc., is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.
Quoted 9 lines · Srinivas Ramana
Hi,
While understanding how the alignment are handled on arm and arm64, we came across the fixups for LDM/LDRD/STM on arm where as these fixups are not present on arm64.
There may be some specific reason why these fixups are not ported to arm64. Can you please help us understand this?
Quoted 6 lines
With this difference in how kernel handles 32-bit apps on arm and arm64, there can be apps which are working without abort on arm, but fail on arm64 (SIGBUS). We have tried to get some history on web, but not successful.
If this is indeed missing on arm64, do you see any issue if its ported (does it fail any guidance)?
Do you have an application that fails because of this? Your email makes it sound very theoretical.
Signature · 3 lines
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up according to speedtest.net.
Quoted 14 lines · Russell King - ARM Linux
Quoted 13 lines · Srinivas Ramana
Hi,
While understanding how the alignment are handled on arm and arm64, we came across the fixups for LDM/LDRD/STM on arm where as these fixups are not present on arm64.
There may be some specific reason why these fixups are not ported to arm64. Can you please help us understand this?
With this difference in how kernel handles 32-bit apps on arm and arm64, there can be apps which are working without abort on arm, but fail on arm64 (SIGBUS). We have tried to get some history on web, but not successful.
Quoted 6 lines
Quoted 2 lines
If this is indeed missing on arm64, do you see any issue if its ported (does it fail any guidance)?
Do you have an application that fails because of this? Your email makes it sound very theoretical.
I don't have any application with me right now. But i just tried passing an intentional misaligned address in a test program. When i say intentional, please note this code is buggy and should be fixed.
So, my question is when arm has such fixups to handle such cases and do gracefully, is there any reason why those fixups are not ported to arm64? Again, I do agree that apps has to fix these instances, but we do have fixups in arch/arm.
I do see that the compiler can detect (if its not intentionally induced) such cases and avoiding to generate LDM/STM and generates multiple LDR/STR. So, I just want to know if it is safe to assume that the compiler would take care of all such misaligned addresses passed to LDM/STM?
------------------------>8---------------------------------------
struct locat {
int a;
int b;
int c;
int d;
};int test_func()
{
struct locat *int_pool1;
struct locat int_pool2;
struct locat *int_pool3;
char *ptr;
int_pool1 = malloc(sizeof(struct locat) + 16);
ptr = (char *)int_pool1;
int_pool3 = (struct locat *)(ptr+1);
printf("pool1 addr: 0x%08x pool3 addr: 0x%08x \n", &int_pool1,
int_pool3);
int_pool2 = *int_pool3;
}------------------------8<---------------------------------------
Thanks,
-- Srinivas R
Signature · 3 lines
Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc., is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.
Quoted 3 lines · Srinivas Ramana
I don't have any application with me right now. But i just tried passing an intentional misaligned address in a test program. When i say intentional, please note this code is buggy and should be fixed.
Until there's reports of some application that regresses, there is nothing to fix.
Signature · 3 lines
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up according to speedtest.net.
Quoted 19 lines · Srinivas Ramana
Quoted 12 lines · Russell King - ARM Linux
Quoted 11 lines · Srinivas Ramana
While understanding how the alignment are handled on arm and arm64, we came across the fixups for LDM/LDRD/STM on arm where as these fixups are not present on arm64.
There may be some specific reason why these fixups are not ported to arm64. Can you please help us understand this?
With this difference in how kernel handles 32-bit apps on arm and arm64, there can be apps which are working without abort on arm, but fail on arm64 (SIGBUS). We have tried to get some history on web, but not successful.
Quoted 5 lines
Quoted 2 lines
If this is indeed missing on arm64, do you see any issue if its ported (does it fail any guidance)?
Do you have an application that fails because of this? Your email makes it sound very theoretical.
Quoted 6 lines
I don't have any application with me right now. But i just tried passing an intentional misaligned address in a test program. When i say intentional, please note this code is buggy and should be fixed.
So, my question is when arm has such fixups to handle such cases and do gracefully, is there any reason why those fixups are not ported to arm64?
As Russell said, until we find some application in the wild I wouldn't rush to provide such emulation. Such code is either broken or relying on undefined C behaviour so they should rather be fixed. As with the deprecated/obsolete ARMv7 instructions (SWP, CP15 barriers), we initially decided not to implement the emulation in the arm64 kernel, though we eventually accepted it. But in those cases the instructions were once real and used correctly. The unaligned LDM/STM or LDRD/STRD have never been supported by the ARM architecture. They were added to cope with some unaligned accesses in the Linux kernel network stack (in hindsight, they should have not been provided to user but maybe there were good reasons, I don't know the full history here).
Quoted 2 lines
Again, I do agree that apps has to fix these instances, but we do have fixups in
arch/arm.
I also think the default on arch/arm should be SIGBUS for these instructions on ARMv7 but this was discussed before on the list.
Quoted 4 lines
I do see that the compiler can detect (if its not intentionally induced) such cases and avoiding to generate LDM/STM and generates multiple LDR/STR. So, I just want to know if it is safe to assume that the compiler would take care of all such misaligned addresses passed to LDM/STM?
The compiler won't detect if you break its alignment assumptions (i.e. in your example pointers to struct locat are 64-bit aligned as per the EABI/PCS). If you want the compiler to assume unaligned struct pointers, you'd have to mark the structure with the packed attribute (with the additional padding if necessary, not in your example though).
Signature · 1 line
Catalin
Quoted 27 lines · Catalin Marinas
Quoted 19 lines · Srinivas Ramana
Quoted 12 lines · Russell King - ARM Linux
Quoted 11 lines · Srinivas Ramana
While understanding how the alignment are handled on arm and arm64, we came across the fixups for LDM/LDRD/STM on arm where as these fixups are not present on arm64.
There may be some specific reason why these fixups are not ported to arm64. Can you please help us understand this?
With this difference in how kernel handles 32-bit apps on arm and arm64, there can be apps which are working without abort on arm, but fail on arm64 (SIGBUS). We have tried to get some history on web, but not successful.
Quoted 5 lines
Quoted 2 lines
If this is indeed missing on arm64, do you see any issue if its ported (does it fail any guidance)?
Do you have an application that fails because of this? Your email makes it sound very theoretical.
Quoted 6 lines
I don't have any application with me right now. But i just tried passing an intentional misaligned address in a test program. When i say intentional, please note this code is buggy and should be fixed.
So, my question is when arm has such fixups to handle such cases and do gracefully, is there any reason why those fixups are not ported to arm64?
Quoted 12 lines
As Russell said, until we find some application in the wild I wouldn't rush to provide such emulation. Such code is either broken or relying on undefined C behaviour so they should rather be fixed. As with the deprecated/obsolete ARMv7 instructions (SWP, CP15 barriers), we initially decided not to implement the emulation in the arm64 kernel, though we eventually accepted it. But in those cases the instructions were once real and used correctly. The unaligned LDM/STM or LDRD/STRD have never been supported by the ARM architecture. They were added to cope with some unaligned accesses in the Linux kernel network stack (in hindsight, they should have not been provided to user but maybe there were good reasons, I don't know the full history here).
Sure. Thanks. I think i got some context and guidance now.
Quoted 11 lines
Quoted 2 lines
Again, I do agree that apps has to fix these instances, but we do have fixups in
arch/arm.I also think the default on
arch/armshould be SIGBUS for these instructions on ARMv7 but this was discussed before on the list.Quoted 4 lines
I do see that the compiler can detect (if its not intentionally induced) such cases and avoiding to generate LDM/STM and generates multiple LDR/STR. So, I just want to know if it is safe to assume that the compiler would take care of all such misaligned addresses passed to LDM/STM?
Quoted 6 lines
The compiler won't detect if you break its alignment assumptions (i.e. in your example pointers to struct locat are 64-bit aligned as per the EABI/PCS). If you want the compiler to assume unaligned struct pointers, you'd have to mark the structure with the packed attribute (with the additional padding if necessary, not in your example though).
You are right. with __packed__ attribute compiler detects the unalignment (added a char to the test structure in my program), I could see that compiler generates multiple LDRs instead of LDM.
Thanks for the details.
Thanks,
-- Srinivas R
Signature · 3 lines
Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc., is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.