* Re: [PATCH kvm-unit-tests] arm: prevent compiler from using unaligned accesses
2019-09-05 17:15 [PATCH kvm-unit-tests] arm: prevent compiler from using unaligned accesses Andre Przywara
@ 2019-09-06 6:30 ` Andrew Jones
2019-09-10 17:07 ` Paolo Bonzini
` (2 subsequent siblings)
3 siblings, 0 replies; 7+ messages in thread
From: Andrew Jones @ 2019-09-06 6:30 UTC (permalink / raw)
To: Andre Przywara; +Cc: Paolo Bonzini, kvmarm, kvm
On Thu, Sep 05, 2019 at 06:15:02PM +0100, Andre Przywara wrote:
> The ARM architecture requires all accesses to device memory to be
> naturally aligned[1][2]. Normal memory does not have this strict
> requirement, and in fact many systems do ignore unaligned accesses
> (by the means of clearing the A bit in SCTLR and accessing normal
> memory). So the default behaviour of GCC assumes that unaligned accesses
> are fine, at least if happening on the stack.
>
> Now kvm-unit-tests runs some C code with the MMU off, which degrades the
> whole system memory to device memory. Now every unaligned access will
> fault, regardless of the A bit.
> In fact there is at least one place in lib/printf.c where GCC merges
> two consecutive char* accesses into one "strh" instruction, writing to
> a potentially unaligned address.
> This can be reproduced by configuring kvm-unit-tests for kvmtool, but
> running it on QEMU, which triggers an early printf that exercises this
> particular code path.
>
> Add the -mstrict-align compiler option to the arm64 CFLAGS to fix this
> problem. Also add the respective -mno-unaligned-access flag for arm.
>
> Thanks to Alexandru for helping debugging this.
>
> Signed-off-by: Andre Przywara <andre.przywara@arm.com>
>
> [1] ARMv8 ARM DDI 0487E.a, B2.5.2
> [2] ARMv7 ARM DDI 0406C.d, A3.2.1
> ---
> arm/Makefile.arm | 1 +
> arm/Makefile.arm64 | 1 +
> 2 files changed, 2 insertions(+)
>
> diff --git a/arm/Makefile.arm b/arm/Makefile.arm
> index a625267..43b4be1 100644
> --- a/arm/Makefile.arm
> +++ b/arm/Makefile.arm
> @@ -12,6 +12,7 @@ KEEP_FRAME_POINTER := y
>
> CFLAGS += $(machine)
> CFLAGS += -mcpu=$(PROCESSOR)
> +CFLAGS += -mno-unaligned-access
>
> arch_LDFLAGS = -Ttext=40010000
>
> diff --git a/arm/Makefile.arm64 b/arm/Makefile.arm64
> index 02c24e8..35de5ea 100644
> --- a/arm/Makefile.arm64
> +++ b/arm/Makefile.arm64
> @@ -7,6 +7,7 @@ bits = 64
> ldarch = elf64-littleaarch64
>
> arch_LDFLAGS = -pie -n
> +CFLAGS += -mstrict-align
>
> define arch_elf_check =
> $(if $(shell ! $(OBJDUMP) -R $(1) >&/dev/null && echo "nok"),
> --
> 2.17.1
>
Reviewed-by: Andrew Jones <drjones@redhat.com>
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^ permalink raw reply [flat|nested] 7+ messages in thread* Re: [PATCH kvm-unit-tests] arm: prevent compiler from using unaligned accesses
2019-09-05 17:15 [PATCH kvm-unit-tests] arm: prevent compiler from using unaligned accesses Andre Przywara
2019-09-06 6:30 ` Andrew Jones
@ 2019-09-10 17:07 ` Paolo Bonzini
2019-09-10 18:15 ` Thomas Huth
2019-09-11 8:24 ` Thomas Huth
3 siblings, 0 replies; 7+ messages in thread
From: Paolo Bonzini @ 2019-09-10 17:07 UTC (permalink / raw)
To: Andre Przywara, Andrew Jones; +Cc: kvmarm, kvm
On 05/09/19 19:15, Andre Przywara wrote:
> The ARM architecture requires all accesses to device memory to be
> naturally aligned[1][2]. Normal memory does not have this strict
> requirement, and in fact many systems do ignore unaligned accesses
> (by the means of clearing the A bit in SCTLR and accessing normal
> memory). So the default behaviour of GCC assumes that unaligned accesses
> are fine, at least if happening on the stack.
>
> Now kvm-unit-tests runs some C code with the MMU off, which degrades the
> whole system memory to device memory. Now every unaligned access will
> fault, regardless of the A bit.
> In fact there is at least one place in lib/printf.c where GCC merges
> two consecutive char* accesses into one "strh" instruction, writing to
> a potentially unaligned address.
> This can be reproduced by configuring kvm-unit-tests for kvmtool, but
> running it on QEMU, which triggers an early printf that exercises this
> particular code path.
>
> Add the -mstrict-align compiler option to the arm64 CFLAGS to fix this
> problem. Also add the respective -mno-unaligned-access flag for arm.
>
> Thanks to Alexandru for helping debugging this.
>
> Signed-off-by: Andre Przywara <andre.przywara@arm.com>
>
> [1] ARMv8 ARM DDI 0487E.a, B2.5.2
> [2] ARMv7 ARM DDI 0406C.d, A3.2.1
> ---
> arm/Makefile.arm | 1 +
> arm/Makefile.arm64 | 1 +
> 2 files changed, 2 insertions(+)
>
> diff --git a/arm/Makefile.arm b/arm/Makefile.arm
> index a625267..43b4be1 100644
> --- a/arm/Makefile.arm
> +++ b/arm/Makefile.arm
> @@ -12,6 +12,7 @@ KEEP_FRAME_POINTER := y
>
> CFLAGS += $(machine)
> CFLAGS += -mcpu=$(PROCESSOR)
> +CFLAGS += -mno-unaligned-access
>
> arch_LDFLAGS = -Ttext=40010000
>
> diff --git a/arm/Makefile.arm64 b/arm/Makefile.arm64
> index 02c24e8..35de5ea 100644
> --- a/arm/Makefile.arm64
> +++ b/arm/Makefile.arm64
> @@ -7,6 +7,7 @@ bits = 64
> ldarch = elf64-littleaarch64
>
> arch_LDFLAGS = -pie -n
> +CFLAGS += -mstrict-align
>
> define arch_elf_check =
> $(if $(shell ! $(OBJDUMP) -R $(1) >&/dev/null && echo "nok"),
>
Queued, thanks.
Paolo
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^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: [PATCH kvm-unit-tests] arm: prevent compiler from using unaligned accesses
2019-09-05 17:15 [PATCH kvm-unit-tests] arm: prevent compiler from using unaligned accesses Andre Przywara
2019-09-06 6:30 ` Andrew Jones
2019-09-10 17:07 ` Paolo Bonzini
@ 2019-09-10 18:15 ` Thomas Huth
2019-09-11 8:16 ` Andre Przywara
2019-09-11 8:24 ` Thomas Huth
3 siblings, 1 reply; 7+ messages in thread
From: Thomas Huth @ 2019-09-10 18:15 UTC (permalink / raw)
To: Andre Przywara, Andrew Jones; +Cc: Paolo Bonzini, kvmarm, kvm
On 05/09/2019 19.15, Andre Przywara wrote:
> The ARM architecture requires all accesses to device memory to be
> naturally aligned[1][2]. Normal memory does not have this strict
> requirement, and in fact many systems do ignore unaligned accesses
> (by the means of clearing the A bit in SCTLR and accessing normal
> memory). So the default behaviour of GCC assumes that unaligned accesses
> are fine, at least if happening on the stack.
>
> Now kvm-unit-tests runs some C code with the MMU off, which degrades the
> whole system memory to device memory. Now every unaligned access will
> fault, regardless of the A bit.
> In fact there is at least one place in lib/printf.c where GCC merges
> two consecutive char* accesses into one "strh" instruction, writing to
> a potentially unaligned address.
> This can be reproduced by configuring kvm-unit-tests for kvmtool, but
> running it on QEMU, which triggers an early printf that exercises this
> particular code path.
>
> Add the -mstrict-align compiler option to the arm64 CFLAGS to fix this
> problem. Also add the respective -mno-unaligned-access flag for arm.
>
> Thanks to Alexandru for helping debugging this.
>
> Signed-off-by: Andre Przywara <andre.przywara@arm.com>
>
> [1] ARMv8 ARM DDI 0487E.a, B2.5.2
> [2] ARMv7 ARM DDI 0406C.d, A3.2.1
> ---
> arm/Makefile.arm | 1 +
> arm/Makefile.arm64 | 1 +
> 2 files changed, 2 insertions(+)
>
> diff --git a/arm/Makefile.arm b/arm/Makefile.arm
> index a625267..43b4be1 100644
> --- a/arm/Makefile.arm
> +++ b/arm/Makefile.arm
> @@ -12,6 +12,7 @@ KEEP_FRAME_POINTER := y
>
> CFLAGS += $(machine)
> CFLAGS += -mcpu=$(PROCESSOR)
> +CFLAGS += -mno-unaligned-access
>
> arch_LDFLAGS = -Ttext=40010000
>
> diff --git a/arm/Makefile.arm64 b/arm/Makefile.arm64
> index 02c24e8..35de5ea 100644
> --- a/arm/Makefile.arm64
> +++ b/arm/Makefile.arm64
> @@ -7,6 +7,7 @@ bits = 64
> ldarch = elf64-littleaarch64
>
> arch_LDFLAGS = -pie -n
> +CFLAGS += -mstrict-align
Instead of adding it to both, Makefile.arm and Makefile.arm64, you could
also simply add it to Makefile.common instead.
Thomas
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^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: [PATCH kvm-unit-tests] arm: prevent compiler from using unaligned accesses
2019-09-10 18:15 ` Thomas Huth
@ 2019-09-11 8:16 ` Andre Przywara
2019-09-11 8:19 ` Thomas Huth
0 siblings, 1 reply; 7+ messages in thread
From: Andre Przywara @ 2019-09-11 8:16 UTC (permalink / raw)
To: Thomas Huth; +Cc: Paolo Bonzini, kvmarm, kvm
On Tue, 10 Sep 2019 20:15:19 +0200
Thomas Huth <thuth@redhat.com> wrote:
Hi,
> On 05/09/2019 19.15, Andre Przywara wrote:
> > The ARM architecture requires all accesses to device memory to be
> > naturally aligned[1][2]. Normal memory does not have this strict
> > requirement, and in fact many systems do ignore unaligned accesses
> > (by the means of clearing the A bit in SCTLR and accessing normal
> > memory). So the default behaviour of GCC assumes that unaligned accesses
> > are fine, at least if happening on the stack.
> >
> > Now kvm-unit-tests runs some C code with the MMU off, which degrades the
> > whole system memory to device memory. Now every unaligned access will
> > fault, regardless of the A bit.
> > In fact there is at least one place in lib/printf.c where GCC merges
> > two consecutive char* accesses into one "strh" instruction, writing to
> > a potentially unaligned address.
> > This can be reproduced by configuring kvm-unit-tests for kvmtool, but
> > running it on QEMU, which triggers an early printf that exercises this
> > particular code path.
> >
> > Add the -mstrict-align compiler option to the arm64 CFLAGS to fix this
> > problem. Also add the respective -mno-unaligned-access flag for arm.
> >
> > Thanks to Alexandru for helping debugging this.
> >
> > Signed-off-by: Andre Przywara <andre.przywara@arm.com>
> >
> > [1] ARMv8 ARM DDI 0487E.a, B2.5.2
> > [2] ARMv7 ARM DDI 0406C.d, A3.2.1
> > ---
> > arm/Makefile.arm | 1 +
> > arm/Makefile.arm64 | 1 +
> > 2 files changed, 2 insertions(+)
> >
> > diff --git a/arm/Makefile.arm b/arm/Makefile.arm
> > index a625267..43b4be1 100644
> > --- a/arm/Makefile.arm
> > +++ b/arm/Makefile.arm
> > @@ -12,6 +12,7 @@ KEEP_FRAME_POINTER := y
> >
> > CFLAGS += $(machine)
> > CFLAGS += -mcpu=$(PROCESSOR)
> > +CFLAGS += -mno-unaligned-access
> >
> > arch_LDFLAGS = -Ttext=40010000
> >
> > diff --git a/arm/Makefile.arm64 b/arm/Makefile.arm64
> > index 02c24e8..35de5ea 100644
> > --- a/arm/Makefile.arm64
> > +++ b/arm/Makefile.arm64
> > @@ -7,6 +7,7 @@ bits = 64
> > ldarch = elf64-littleaarch64
> >
> > arch_LDFLAGS = -pie -n
> > +CFLAGS += -mstrict-align
>
> Instead of adding it to both, Makefile.arm and Makefile.arm64, you could
> also simply add it to Makefile.common instead.
But the arguments are not the same (admittedly against intuition)?
I thought about defining arch_CFLAGS in both files, then adding that to Makefile.common, but didn't see the advantage over this straightforward approach here.
Cheers,
Andre.
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^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: [PATCH kvm-unit-tests] arm: prevent compiler from using unaligned accesses
2019-09-11 8:16 ` Andre Przywara
@ 2019-09-11 8:19 ` Thomas Huth
0 siblings, 0 replies; 7+ messages in thread
From: Thomas Huth @ 2019-09-11 8:19 UTC (permalink / raw)
To: Andre Przywara; +Cc: Paolo Bonzini, kvmarm, kvm
On 11/09/2019 10.16, Andre Przywara wrote:
> On Tue, 10 Sep 2019 20:15:19 +0200
> Thomas Huth <thuth@redhat.com> wrote:
>
> Hi,
>
>> On 05/09/2019 19.15, Andre Przywara wrote:
>>> The ARM architecture requires all accesses to device memory to be
>>> naturally aligned[1][2]. Normal memory does not have this strict
>>> requirement, and in fact many systems do ignore unaligned accesses
>>> (by the means of clearing the A bit in SCTLR and accessing normal
>>> memory). So the default behaviour of GCC assumes that unaligned accesses
>>> are fine, at least if happening on the stack.
>>>
>>> Now kvm-unit-tests runs some C code with the MMU off, which degrades the
>>> whole system memory to device memory. Now every unaligned access will
>>> fault, regardless of the A bit.
>>> In fact there is at least one place in lib/printf.c where GCC merges
>>> two consecutive char* accesses into one "strh" instruction, writing to
>>> a potentially unaligned address.
>>> This can be reproduced by configuring kvm-unit-tests for kvmtool, but
>>> running it on QEMU, which triggers an early printf that exercises this
>>> particular code path.
>>>
>>> Add the -mstrict-align compiler option to the arm64 CFLAGS to fix this
>>> problem. Also add the respective -mno-unaligned-access flag for arm.
>>>
>>> Thanks to Alexandru for helping debugging this.
>>>
>>> Signed-off-by: Andre Przywara <andre.przywara@arm.com>
>>>
>>> [1] ARMv8 ARM DDI 0487E.a, B2.5.2
>>> [2] ARMv7 ARM DDI 0406C.d, A3.2.1
>>> ---
>>> arm/Makefile.arm | 1 +
>>> arm/Makefile.arm64 | 1 +
>>> 2 files changed, 2 insertions(+)
>>>
>>> diff --git a/arm/Makefile.arm b/arm/Makefile.arm
>>> index a625267..43b4be1 100644
>>> --- a/arm/Makefile.arm
>>> +++ b/arm/Makefile.arm
>>> @@ -12,6 +12,7 @@ KEEP_FRAME_POINTER := y
>>>
>>> CFLAGS += $(machine)
>>> CFLAGS += -mcpu=$(PROCESSOR)
>>> +CFLAGS += -mno-unaligned-access
>>>
>>> arch_LDFLAGS = -Ttext=40010000
>>>
>>> diff --git a/arm/Makefile.arm64 b/arm/Makefile.arm64
>>> index 02c24e8..35de5ea 100644
>>> --- a/arm/Makefile.arm64
>>> +++ b/arm/Makefile.arm64
>>> @@ -7,6 +7,7 @@ bits = 64
>>> ldarch = elf64-littleaarch64
>>>
>>> arch_LDFLAGS = -pie -n
>>> +CFLAGS += -mstrict-align
>>
>> Instead of adding it to both, Makefile.arm and Makefile.arm64, you could
>> also simply add it to Makefile.common instead.
>
> But the arguments are not the same (admittedly against intuition)?
> I thought about defining arch_CFLAGS in both files, then adding that to Makefile.common, but didn't see the advantage over this straightforward approach here.
D'oh, never mind, I didn't read the patch properly. I somehow thought
that the arguments are the same. It's quite weird that the compiler
developers chose different names here...
Thomas
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^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: [PATCH kvm-unit-tests] arm: prevent compiler from using unaligned accesses
2019-09-05 17:15 [PATCH kvm-unit-tests] arm: prevent compiler from using unaligned accesses Andre Przywara
` (2 preceding siblings ...)
2019-09-10 18:15 ` Thomas Huth
@ 2019-09-11 8:24 ` Thomas Huth
3 siblings, 0 replies; 7+ messages in thread
From: Thomas Huth @ 2019-09-11 8:24 UTC (permalink / raw)
To: Andre Przywara, Andrew Jones; +Cc: Paolo Bonzini, kvmarm, kvm
On 05/09/2019 19.15, Andre Przywara wrote:
> The ARM architecture requires all accesses to device memory to be
> naturally aligned[1][2]. Normal memory does not have this strict
> requirement, and in fact many systems do ignore unaligned accesses
> (by the means of clearing the A bit in SCTLR and accessing normal
> memory). So the default behaviour of GCC assumes that unaligned accesses
> are fine, at least if happening on the stack.
>
> Now kvm-unit-tests runs some C code with the MMU off, which degrades the
> whole system memory to device memory. Now every unaligned access will
> fault, regardless of the A bit.
> In fact there is at least one place in lib/printf.c where GCC merges
> two consecutive char* accesses into one "strh" instruction, writing to
> a potentially unaligned address.
> This can be reproduced by configuring kvm-unit-tests for kvmtool, but
> running it on QEMU, which triggers an early printf that exercises this
> particular code path.
>
> Add the -mstrict-align compiler option to the arm64 CFLAGS to fix this
> problem. Also add the respective -mno-unaligned-access flag for arm.
>
> Thanks to Alexandru for helping debugging this.
>
> Signed-off-by: Andre Przywara <andre.przywara@arm.com>
>
> [1] ARMv8 ARM DDI 0487E.a, B2.5.2
> [2] ARMv7 ARM DDI 0406C.d, A3.2.1
> ---
> arm/Makefile.arm | 1 +
> arm/Makefile.arm64 | 1 +
> 2 files changed, 2 insertions(+)
>
> diff --git a/arm/Makefile.arm b/arm/Makefile.arm
> index a625267..43b4be1 100644
> --- a/arm/Makefile.arm
> +++ b/arm/Makefile.arm
> @@ -12,6 +12,7 @@ KEEP_FRAME_POINTER := y
>
> CFLAGS += $(machine)
> CFLAGS += -mcpu=$(PROCESSOR)
> +CFLAGS += -mno-unaligned-access
>
> arch_LDFLAGS = -Ttext=40010000
>
> diff --git a/arm/Makefile.arm64 b/arm/Makefile.arm64
> index 02c24e8..35de5ea 100644
> --- a/arm/Makefile.arm64
> +++ b/arm/Makefile.arm64
> @@ -7,6 +7,7 @@ bits = 64
> ldarch = elf64-littleaarch64
>
> arch_LDFLAGS = -pie -n
> +CFLAGS += -mstrict-align
>
> define arch_elf_check =
> $(if $(shell ! $(OBJDUMP) -R $(1) >&/dev/null && echo "nok"),
>
FWIW (after finally reading the patch properly ;-)) :
Reviewed-by: Thomas Huth <thuth@redhat.com>
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^ permalink raw reply [flat|nested] 7+ messages in thread