linux-kernel - Re: [PATCH] x86/PCI: Use MMCONFIG by default for KVM guests

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Message-ID: <87lfj5lzp0.fsf@vitty.brq.redhat.com>
Date:   Mon, 27 Jul 2020 17:55:55 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Julia Suvorova <jusual@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        "open list\:VFIO DRIVER" <kvm@...r.kernel.org>,
        linux-pci <linux-pci@...r.kernel.org>
Subject: Re: [PATCH] x86/PCI: Use MMCONFIG by default for KVM guests

Andy Shevchenko <andy.shevchenko@...il.com> writes:

> On Wed, Jul 22, 2020 at 12:47 PM Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>> Julia Suvorova <jusual@...hat.com> writes:
>
>> > Scanning for PCI devices at boot takes a long time for KVM guests. It
>> > can be reduced if KVM will handle all configuration space accesses for
>> > non-existent devices without going to userspace [1]. But for this to
>> > work, all accesses must go through MMCONFIG.
>> > This change allows to use pci_mmcfg as raw_pci_ops for 64-bit KVM
>> > guests making MMCONFIG the default access method.
>
> I'm not sure it won't break anything.

It likely will as it's really hard to check all possible KVM
configurations in existence and that's why we are converging on adding a
feature bit which KVM userspace (e.g. QEMU) will set when the
configuration is known to be good.

>
>> > [1] https://lkml.org/lkml/2020/5/14/936
>
> use Link: tag and better to use lore.kernel.org.
>
>> This implies mmconfig access method is always functional (when present)
>> for all KVM guests, regardless of hypervisor version/which KVM userspace
>> is is use/... In case the assumption is true the patch looks good (to
>> me) but in case it isn't or if we think that more control over this
>> is needed we may want to introduce a PV feature bit for KVM.
>>
>> Also, I'm thinking about moving this to arch/x86/kernel/kvm.c: we can
>> override x86_init.pci.arch_init and reassign raw_pci_ops after doing
>> pci_arch_init().
>
> % git grep -n -w x86_init.pci.arch_init -- arch/x86/
> arch/x86/hyperv/hv_init.c:400:  x86_init.pci.arch_init = hv_pci_init;
> arch/x86/kernel/apic/apic_numachip.c:203:       x86_init.pci.arch_init
> = pci_numachip_init;
> arch/x86/kernel/jailhouse.c:207:        x86_init.pci.arch_init
>  = jailhouse_pci_arch_init;
> arch/x86/pci/init.c:20: if (x86_init.pci.arch_init && !x86_init.pci.arch_init())
> arch/x86/platform/intel-mid/intel-mid.c:172:    x86_init.pci.arch_init
> = intel_mid_pci_init;
> arch/x86/platform/olpc/olpc.c:309:              x86_init.pci.arch_init
> = pci_olpc_init;
> arch/x86/xen/enlighten_pv.c:1411:
> x86_init.pci.arch_init = pci_xen_init;
>
> Are you going to update all these? Or how this is supposed to work (I
> may be missing something)?

My suggestion was to do exactly the same for KVM guests instead of
switching ops in pci_mmcfg_arch_init() depending on kvm_para_available()
output. Basically, keep all KVM-related tunings in one place
(arch/x86/kernel/kvm.c).

-- 
Vitaly