On Fri, Jan 20, 2023 at 4:28 PM Sean Christopherson <seanjc@xxxxxxxxxx> 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-tppsg@xxxxxxxxxxxxxx/ > > > > Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx> > > --- > > 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 > >
