linux-kernel - RE: [PATCH v9 4/6] KVM: x86: Introduce untag_addr() in kvm_x86

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Date:   Thu, 29 Jun 2023 08:30:55 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Binbin Wu' <binbin.wu@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.com>
CC:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "chao.gao@...el.com" <chao.gao@...el.com>,
        "kai.huang@...el.com" <kai.huang@...el.com>,
        "robert.hu@...ux.intel.com" <robert.hu@...ux.intel.com>
Subject: RE: [PATCH v9 4/6] KVM: x86: Introduce untag_addr() in kvm_x86_ops

From: Binbin Wu
> Sent: 29 June 2023 07:12
...
> >> +void vmx_untag_addr(struct kvm_vcpu *vcpu, gva_t *gva, u32 flags)
> >
> > Rather than modify the pointer, return the untagged address.  That's more flexible
> > as it allows using the result in if-statements and whatnot.  That might not ever
> > come into play, but there's no good reason to use an in/out param in a void
> > function.
>
> In earlier version, it did return the untagged address.
> In this version, I changed it as an in/out param to make the interface
> conditional and avoid to add a dummy one in SVM.
> Is it can be a reason?

You are always going to need a 'dummy' version.
If it ends up being 'x = x' the compiler will just optimise
it away.

But for a real function you'll get much better code from:
	x = fn(x);
than
	fn(&x);

It also lets you used 'void *' (etc) to avoid casts which
can easily hide bugs.

	David

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