On Fri, Jan 20, 2023 at 4:28 PM Sean Christopherson seanjc@google.com wrote:
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.
Is not it due to the 32bit execution part of the guest code at boot time. Any push/pop of 32bit registers might make it a 16-byte unaligned stack.
Similar issue discussed at https://lore.kernel.org/all/CAGtprH9yKvuaF5yruh3BupQe4BxDGiBQk3ExtY2m39yP-tp...
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