linux-kernel - Re: [PATCH 1/2] KVM: VMX: Move IRQ invocation to assembly subroutine

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Date:   Mon, 14 Sep 2020 14:54:11 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Uros Bizjak <ubizjak@...il.com>,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 1/2] KVM: VMX: Move IRQ invocation to assembly subroutine

On Mon, Sep 14, 2020 at 04:38:13PM -0500, Josh Poimboeuf wrote:
> On Mon, Sep 14, 2020 at 02:07:19PM -0700, Sean Christopherson wrote:
> > > RSP needs to be aligned to what?  How would this align the stack, other
> > > than by accident?
> > 
> > Ah, yeah, that's lacking info.
> > 
> > 16-byte aligned to correctly mimic CPU behavior when vectoring an IRQ/NMI.
> > When not changing stack, the CPU aligns RSP before pushing the frame.
> > 
> > The above shenanigans work because the x86-64 ABI also requires RSP to be
> > 16-byte aligned prior to CALL.  RSP is thus 8-byte aligned due to CALL
> > pushing the return IP, and so creating the stack frame by pushing RBP makes
> > it 16-byte aliagned again.
> 
> As Uros mentioned, the kernel doesn't do this.

Argh, apparently I just got lucky with my compiles then.  I added explicit
checks on RSP being properly aligned and thought that confirmed the kernel
played nice.  Bummer.