linux-kernel - Re: [PATCH v2] KVM/x86: Move definition of _

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Message-ID: <X+D1TPXRuTggnvHv@google.com>
Date:   Mon, 21 Dec 2020 11:19:40 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Uros Bizjak <ubizjak@...il.com>
Cc:     kvm@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Krish Sadhukhan <krish.sadhukhan@...cle.com>
Subject: Re: [PATCH v2] KVM/x86: Move definition of __ex to x86.h

On Mon, Dec 21, 2020, Uros Bizjak wrote:
> On Mon, Dec 21, 2020 at 7:19 PM Sean Christopherson <seanjc@...gle.com> wrote:
> >
> > On Sun, Dec 20, 2020, Uros Bizjak wrote:
> > > Merge __kvm_handle_fault_on_reboot with its sole user
> >
> > There's also a comment in vmx.c above kvm_cpu_vmxoff() that should be updated.
> > Alternatively, and probably preferably for me, what about keeping the long
> > __kvm_handle_fault_on_reboot() name for the macro itself and simply moving the
> > __ex() macro?
> >
> > That would also allow keeping kvm_spurious_fault() and
> > __kvm_handle_fault_on_reboot() where they are (for no reason other than to avoid
> > code churn).  Though I'm also ok if folks would prefer to move everything to
> > x86.h.
> 
> The new patch is vaguely based on our correspondence on the prototype patch:
> 
> --q--
> Moving this to asm/kvm_host.h is a bit sketchy as __ex() isn't exactly the
> most unique name.  arch/x86/kvm/x86.h would probably be a better
> destination as it's "private".  __ex() is only used in vmx.c, nested.c and
> svm.c, all of which already include x86.h.
> --/q--
> 
> where you mentioned that x86.h would be a better destination for
> __ex().

Ya, thankfully I still agree with my past self on this one :-)

> IMO, __kvm_handle_fault_on_reboot also belongs in x86.h, as it
> deals with a low-level access to the processor, and there is really no
> reason for this #define to be available for the whole x86 architecture
> directory. I remember looking for the __kvm_handle_falult_on_reboot,
> and was surprised to find it in a global x86 include directory.

Works for me.  If you have a strong preference for moving everything to x86.h,
then let's do that.

> I tried to keep __ex as a redefine to __kvm_hanlde_fault_on_reboot in
> x86.h, but it just looked weird, since __ex is the only user and the
> introductory document explains in detail, what
> __kvm_hanlde_fault_on_reboot (aka __ex) does.

I like the verbose name because it very quickly reminds what the macro does; I
somehow manage to forget every few months.  I agree it's a bit superfluous since
the comment explains exactly what goes on.  And I can see how
__kvm_handle_fault_on_reboot() would be misleading as it also "handles" faults
at all other times as well.

What if we add a one-line synopsis in the comment to state the (very) high-level
purpose of the function?  We could also opportunistically clean up the
formatting in the existing comment to save a line, e.g.:

/*
 * Handle a fault on a hardware virtualization (VMX or SVM) instruction.
 *
 * Hardware virtualization extension instructions may fault if a reboot turns
 * off virtualization while processes are running.  Usually after catching the
 * fault we just panic; during reboot instead the instruction is ignored.
 */