linux-kernel - Re: [PATCH V3 08/10] vfio/pci: Probe and store ability to support dynamic MSI-X

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Date:   Tue, 25 Apr 2023 11:51:51 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Reinette Chatre <reinette.chatre@...el.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>, yishaih@...dia.com,
        shameerali.kolothum.thodi@...wei.com, kevin.tian@...el.com,
        tglx@...utronix.de, darwi@...utronix.de, kvm@...r.kernel.org,
        dave.jiang@...el.com, jing2.liu@...el.com, ashok.raj@...el.com,
        fenghua.yu@...el.com, tom.zanussi@...ux.intel.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3 08/10] vfio/pci: Probe and store ability to support
 dynamic MSI-X

On Mon, Apr 24, 2023 at 04:52:08PM -0700, Reinette Chatre wrote:
> Hi Jason,
> 
> On 4/24/2023 10:43 AM, Jason Gunthorpe wrote:
> > On Wed, Apr 19, 2023 at 11:11:48AM -0700, Reinette Chatre wrote:
> >> On 4/18/2023 3:38 PM, Alex Williamson wrote:
> >>> On Tue, 18 Apr 2023 10:29:19 -0700
> >>> Reinette Chatre <reinette.chatre@...el.com> wrote:
> 
> ...
> 
> >> diff --git a/include/linux/vfio_pci_core.h b/include/linux/vfio_pci_core.h
> >> index 4f070f2d6fde..d730d78754a2 100644
> >> --- a/include/linux/vfio_pci_core.h
> >> +++ b/include/linux/vfio_pci_core.h
> >> @@ -67,8 +67,8 @@ struct vfio_pci_core_device {
> >>  	u8			msix_bar;
> >>  	u16			msix_size;
> >>  	u32			msix_offset;
> >> -	bool			has_dyn_msix;
> >>  	u32			rbar[7];
> >> +	bool			has_dyn_msix;
> >>  	bool			pci_2_3;
> >>  	bool			virq_disabled;
> >>  	bool			reset_works;
> > 
> > Also, Linus on record as strongly disliking these lists of bools
> 
> This looks like an example:
> https://lkml.org/lkml/2017/11/21/384
> 
> > 
> > If they don't need read_once/etc stuff then use a list of bitfields
> 
> I do not see any direct usage of read_once in the driver, but it is not
> clear to me what falls under the "etc" umbrella.

Anything that might assume atomicity, smp_store_release, set_bit, and others

>  Do you consider all the bools in struct vfio_pci_core_device to be
> candidates for transition?

Yes, group them ito into a bitfield.

> I think a base type of unsigned int since it appears to be the custom
> and (if I understand correctly) was preferred at the time Linus wrote
> the message I found.

It doesn't matter a lot, using "bool" means the compiler adds extra
code to ensure "foo = 4" stores true, and the underyling size is not
well defined (but we don't care here)
 
> Looking ahead there seems be be a bigger task here. A quick search
> revealed a few other instances of vfio using "bool" in a struct. It
> does not all qualify for your "lists of bools" comment, but they
> may need a closer look because of the "please don't use "bool" in
> structures at all" comment made by Linus in the email I found.

IMHO bool is helpful for clarity, it says it is a flag. In these cases
we won't gain anything by using u8 instead

Lists of bools however start to get a little silly when we use maybe 4
bytes per bool (though x86-64 is using 1 byte in structs)

Jason