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

Maciej S. Szmigiero
2023-01-23T20:18:59Z

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"

Thanks,
Maciej

[1]: https://www.uclibc.org/docs/psABI-i386.pdf