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

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Message-ID: <873658kpj2.fsf@vitty.brq.redhat.com>
Date:   Fri, 31 Jul 2020 11:22:09 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Julia Suvorova <jusual@...hat.com>
Cc:     "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

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?

-- 
Vitaly