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

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Date:   Mon, 23 Jan 2023 10:30:30 -0800
From:   Erdem Aktas <erdemaktas@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Ackerley Tng <ackerleytng@...gle.com>,
        linux-kselftest@...r.kernel.org, pbonzini@...hat.com,
        isaku.yamahata@...el.com, sagis@...gle.com, afranji@...gle.com,
        runanwang@...gle.com, shuah@...nel.org, drjones@...hat.com,
        maz@...nel.org, bgardon@...gle.com, jmattson@...gle.com,
        dmatlack@...gle.com, peterx@...hat.com, oupton@...gle.com,
        ricarkol@...gle.com, yang.zhong@...el.com, wei.w.wang@...el.com,
        xiaoyao.li@...el.com, pgonda@...gle.com, marcorr@...gle.com,
        eesposit@...hat.com, borntraeger@...ibm.com, eric.auger@...hat.com,
        wangyanan55@...wei.com, aaronlewis@...gle.com, vkuznets@...hat.com,
        pshier@...gle.com, axelrasmussen@...gle.com,
        zhenzhong.duan@...el.com, maciej.szmigiero@...cle.com,
        like.xu@...ux.intel.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [RFC PATCH v3 08/31] KVM: selftests: Require GCC to realign
 stacks on function entry

On Fri, Jan 20, 2023 at 4:28 PM Sean Christopherson <seanjc@...gle.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-tppsg@mail.gmail.com/
> >
> > Signed-off-by: Ackerley Tng <ackerleytng@...gle.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
> >