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

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Message-ID: <Y88ylDFfMQNcUEw7@google.com>
Date:   Tue, 24 Jan 2023 01:21:24 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Erdem Aktas <erdemaktas@...gle.com>
Cc:     "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>,
        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, 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 Mon, Jan 23, 2023, Erdem Aktas wrote:
> On Mon, Jan 23, 2023 at 10:53 AM Sean Christopherson <seanjc@...gle.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@...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.
> > >
> > > 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 :-)