From: Thomas Huth <thuth@redhat.com>
To: Andre Przywara <andre.przywara@arm.com>,
Andrew Jones <drjones@redhat.com>
Cc: kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org,
Alexandru Elisei <alexandru.elisei@arm.com>,
Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH kvm-unit-tests] arm: prevent compiler from using unaligned accesses
Date: Tue, 10 Sep 2019 20:15:19 +0200 [thread overview]
Message-ID: <d41649bc-5061-3c65-146c-d7dff3f086e7@redhat.com> (raw)
In-Reply-To: <20190905171502.215183-1-andre.przywara@arm.com>
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
WARNING: multiple messages have this Message-ID (diff)
From: Thomas Huth <thuth@redhat.com>
To: Andre Przywara <andre.przywara@arm.com>,
Andrew Jones <drjones@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org
Subject: Re: [PATCH kvm-unit-tests] arm: prevent compiler from using unaligned accesses
Date: Tue, 10 Sep 2019 20:15:19 +0200 [thread overview]
Message-ID: <d41649bc-5061-3c65-146c-d7dff3f086e7@redhat.com> (raw)
In-Reply-To: <20190905171502.215183-1-andre.przywara@arm.com>
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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next prev parent reply other threads:[~2019-09-10 18:15 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-05 17:15 [PATCH kvm-unit-tests] arm: prevent compiler from using unaligned accesses Andre Przywara
2019-09-05 17:15 ` Andre Przywara
2019-09-06 6:30 ` Andrew Jones
2019-09-06 6:30 ` Andrew Jones
2019-09-10 17:07 ` Paolo Bonzini
2019-09-10 17:07 ` Paolo Bonzini
2019-09-10 18:15 ` Thomas Huth [this message]
2019-09-10 18:15 ` Thomas Huth
2019-09-11 8:16 ` Andre Przywara
2019-09-11 8:16 ` Andre Przywara
2019-09-11 8:19 ` Thomas Huth
2019-09-11 8:19 ` Thomas Huth
2019-09-11 8:24 ` Thomas Huth
2019-09-11 8:24 ` Thomas Huth
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