Re: [RFC PATCH v3 08/31] KVM: selftests: Require GCC to realign stacks on function entry

Sean Christopherson
2023-01-21T00:29:55Z

On Sat, Jan 21, 2023, Ackerley Tng wrote:
> Some SSE instructions assume a 16-byte aligned stack, and GCC compiles
> assuming the stack is aligned:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=40838. This combination
> results in a #GP in guests.
>
> Adding this compiler flag will generate an alternate prologue and
> epilogue to realign the runtime stack, which makes selftest code
> slower and bigger, but this is okay since we do not need selftest code
> to be extremely performant.

Huh, I had completely forgotten that this is why SSE is problematic. I ran into
this with the base UPM selftests and just disabled SSE. /facepalm.

We should figure out exactly what is causing a misaligned stack. As you've noted,
the x86-64 ABI requires a 16-byte aligned RSP. Unless I'm misreading vm_arch_vcpu_add(),
the starting stack should be page aligned, which means something is causing the
stack to become unaligned at runtime. I'd rather hunt down that something than
paper over it by having the compiler force realignment.

> Similar issue discussed at
> https://lore.kernel.org/all/CAGtprH9yKvuaF5yruh3BupQe4BxDGiBQk3ExtY2m39yP-tppsg@mail.gmail.com/
>
> Signed-off-by: Ackerley Tng <ackerleytng@google.com>
> ---
> tools/testing/selftests/kvm/Makefile | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/tools/testing/selftests/kvm/Makefile b/tools/testing/selftests/kvm/Makefile
> index 317927d9c55bd..5f9cc1e6ee67e 100644
> --- a/tools/testing/selftests/kvm/Makefile
> +++ b/tools/testing/selftests/kvm/Makefile
> @@ -205,7 +205,7 @@ LINUX_TOOL_ARCH_INCLUDE = $(top_srcdir)/tools/arch/x86/include
> else
> LINUX_TOOL_ARCH_INCLUDE = $(top_srcdir)/tools/arch/$(ARCH)/include
> endif
> -CFLAGS += -Wall -Wstrict-prototypes -Wuninitialized -O2 -g -std=gnu99 \
> +CFLAGS += -mstackrealign -Wall -Wstrict-prototypes -Wuninitialized -O2 -g -std=gnu99 \
> -fno-stack-protector -fno-PIE -I$(LINUX_TOOL_INCLUDE) \
> -I$(LINUX_TOOL_ARCH_INCLUDE) -I$(LINUX_HDR_PATH) -Iinclude \
> -I$(<D) -Iinclude/$(UNAME_M) -I ../rseq -I.. $(EXTRA_CFLAGS) \
> --
> 2.39.0.246.g2a6d74b583-goog
>