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

[Sean Christopherson]

On Mon, Jan 23, 2023, Maciej S. Szmigiero wrote:
> On 23.01.2023 19:30, Erdem Aktas wrote:
> > 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.
> 
> 32-bit stack needs to be 16-byte aligned, too (at function call boundaries) -
> see [1] chapter 2.2.2 "The Stack Frame"

And this showing up in the non-TDX selftests rules that out as the sole problem;
the selftests stuff 64-bit mode, i.e. don't have 32-bit boot code.