linux-kernel - RE: [PATCH v3 17/35] x86/pci/xen: Use msi

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Date:   Sun, 25 Oct 2020 13:20:16 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'David Woodhouse' <dwmw2@...radead.org>,
        "x86@...nel.org" <x86@...nel.org>
CC:     kvm <kvm@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "joro@...tes.org" <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paolo Bonzini <pbonzini@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "maz@...terjones.org" <maz@...terjones.org>,
        Dexuan Cui <decui@...rosoft.com>
Subject: RE: [PATCH v3 17/35] x86/pci/xen: Use msi_msg shadow structs

From: David Woodhouse
> Sent: 25 October 2020 10:26
> To: David Laight <David.Laight@...LAB.COM>; x86@...nel.org
> 
> On Sun, 2020-10-25 at 09:49 +0000, David Laight wrote:
> > Just looking at a random one of these patches...
> >
> > Does the compiler manage to optimise that reasonably?
> > Or does it generate a lot of shifts and masks as each
> > bitfield is set?
> >
> > The code generation for bitfields is often a lot worse
> > that that for |= setting bits in a word.
> 
> Indeed, it appears to be utterly appalling. That was one of my
> motivations for doing it with masks and shifts in the first place.

I thought it would be.

I'm not even sure using bitfields to map hardware registers
makes the code any more readable (or fool proof).

I suspect using #define constants and explicit |= and &= ~
is actually best - provided the names are descriptive enough.

If you set all the fields the compiler will merge the values
and do a single write - provided nothing you read might
alias the target.

The only way to get strongly typed integers is to cast them
to a structure pointer type.
Together with a static inline function to remove the casts
and | the values together it might make things fool proof.
But I've never tried it.

ISTR once doing something like that with error codes, but
it didn't ever make source control.

	David

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