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

Maciej S. Szmigiero
2023-02-15T18:46:11Z

On 15.02.2023 01:50, Ackerley Tng wrote:
>
>> On Mon, Jan 23, 2023, Erdem Aktas wrote:
>> > On Mon, Jan 23, 2023 at 10:53 AM Sean Christopherson <seanjc@google.com> wrote:
>> > >
>> > > 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@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.
>> > > >
>> > > > 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.
>> >
>> > Thanks Maciej and Sean for the clarification. I was suspecting the
>> > hand-coded assembly part that we have for TDX tests but  it being
>> > happening in the non-TDX selftests disproves it.
>
>> Not necessarily, it could be both.  Goofs in the handcoded assembly and PEBKAC
>> on my end :-)
>
> I figured it out!
>
> GCC assumes that the stack is 16-byte aligned **before** the call
> instruction. Since call pushes rip to the stack, GCC will compile code
> assuming that on entrance to the function, the stack is -8 from a
> 16-byte aligned address.
>
> Since for TDs we do a ljmp to guest code, providing a function's
> address, the stack was not modified by a call instruction pushing rip to
> the stack, so the stack is 16-byte aligned when the guest code starts
> running, instead of 16-byte aligned -8 that GCC expects.
>
> For VMs, we set rip to a function pointer, and the VM starts running
> with a 16-byte algined stack too.
>
> To fix this, I propose that in vm_arch_vcpu_add(), we align the
> allocated stack address and then subtract 8 from that:
>

Note that if this code is ever used to launch a vCPU with 32-bit entry
point it will need to subtract 4 bytes instead of 8 bytes.

I think it would be worthwhile to at least place a comment mentioning
this near the stack aligning expression so nobody misses this fact.

Thanks,
Maciej