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

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Date:   Fri, 31 Jul 2020 20:23:37 +0200
From:   Julia Suvorova <jusual@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Andy Shevchenko <andy.shevchenko@...il.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 11:22 AM 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:
> >>
> >> Using MMCONFIG instead of I/O ports cuts the number of config space
> >> accesses in half, which is faster on KVM and opens the door for
> >> additional optimizations such as Vitaly's "[PATCH 0/3] KVM: x86: KVM
> >> MEM_PCI_HOLE memory":
> >
> >> https://lore.kernel.org/kvm/20200728143741.2718593-1-vkuznets@redhat.com
> >
> > You may use Link: tag for this.
> >
> >> However, this change will not bring significant performance improvement
> >> unless it is running on x86 within a hypervisor. Moreover, allowing
> >> MMCONFIG access for addresses < 256 can be dangerous for some devices:
> >> see commit a0ca99096094 ("PCI x86: always use conf1 to access config
> >> space below 256 bytes"). That is why a special feature flag is needed.
> >>
> >> Introduce KVM_FEATURE_PCI_GO_MMCONFIG, which can be enabled when the
> >> configuration is known to be safe (e.g. in QEMU).
> >
> > ...
> >
> >> +static int __init kvm_pci_arch_init(void)
> >> +{
> >> +       if (raw_pci_ext_ops &&
> >> +           kvm_para_has_feature(KVM_FEATURE_PCI_GO_MMCONFIG)) {
> >
> > Better to use traditional pattern, i.e.
> >   if (not_supported)
> >     return bail_out;
> >
> >   ...do useful things...
> >   return 0;
> >
> >> +               pr_info("PCI: Using MMCONFIG for base access\n");
> >> +               raw_pci_ops = 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?

This is intentional because pci_direct_init() is about to overwrite
raw_pci_ops. And since QEMU doesn't have anything in
pciprobe_dmi_table, it is safe to skip it.

Best regards, Julia Suvorova.