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

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Date:   Mon, 24 Apr 2023 14:43:09 -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 Wed, Apr 19, 2023 at 11:11:48AM -0700, Reinette Chatre wrote:
> Hi Alex,
> 
> 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 148fd1ae6c1c..4f070f2d6fde 100644
> >> --- a/include/linux/vfio_pci_core.h
> >> +++ b/include/linux/vfio_pci_core.h
> >> @@ -67,6 +67,7 @@ struct vfio_pci_core_device {
> >>  	u8			msix_bar;
> >>  	u16			msix_size;
> >>  	u32			msix_offset;
> >> +	bool			has_dyn_msix;
> >>  	u32			rbar[7];
> >>  	bool			pci_2_3;
> >>  	bool			virq_disabled;
> > 
> > Nit, the whole data structure probably needs to be sorted with pahole,
> > but creating a hole here for locality to other msix fields should
> > probably be secondary to keeping the structure well packed, which
> > suggests including this new field among the bools below.  Thanks,
> 
> Thanks for catching this. Moving it one field lower as shown in the
> delta patch below seems to improve the layout:
> 
> 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

If they don't need read_once/etc stuff then use a list of bitfields

bool abc:1;
bool xyz:1;

Jason