linux-kernel - Re: [PATCH] KVM: x86: Use MMCONFIG for all PCI config space accesses

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Message-ID: <CAHp75Vfp8aabZo_NW78kM-OLLKDgK1CwvLZNwmPZyQgaw6bXtQ@mail.gmail.com>
Date:   Fri, 31 Jul 2020 12:41:06 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Julia Suvorova <jusual@...hat.com>,
        "open list:VFIO DRIVER" <kvm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Bjorn Helgaas <helgaas@...nel.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] KVM: x86: Use MMCONFIG for all PCI config space accesses

On Fri, Jul 31, 2020 at 12:22 PM Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
> Andy Shevchenko <andy.shevchenko@...il.com> writes:
> > On Thu, Jul 30, 2020 at 10:37 PM Julia Suvorova <jusual@...hat.com> wrote:

...

> >> +static int __init kvm_pci_arch_init(void)
> >> +{
> >> +       if (raw_pci_ext_ops &&

> >> +               return 0;
> >> +       }
> >
> >> +       return 1;
> >
> > Hmm... I don't remember what positive codes means there. Perhaps you
> > need to return a rather error code?
>
> If I'm reading the code correctly,
>
> pci_arch_init() has the following:
>
>         if (x86_init.pci.arch_init && !x86_init.pci.arch_init())
>                 return 0;
>
>
> so returning '1' here means 'continue' and this seems to be
> correct. (E.g. Hyper-V's hv_pci_init() does the same). What I'm not sure
> about is 'return 0' above as this will result in skipping the rest of
> pci_arch_init(). Was this desired or should we return '1' in both cases?

I think it depends what you want. In complex cases we recognize three
possibilities

-ERRNO: function failed, we have to stop and bailout with error from callee
0: function OK, stop and return 0
1: function OK, continue the rest in callee

Do we have needs in this or is the current enough for all (exist) callees?

-- 
With Best Regards,
Andy Shevchenko