linux-kernel - Re: [PATCH v2] x86/PCI: Prefer MMIO over PIO on all hypervisor

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Message-ID: <87lepw9ejx.fsf@redhat.com>
Date:   Tue, 04 Oct 2022 10:30:10 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Nadav Amit <namit@...are.com>, Alexander Graf <graf@...zon.com>,
        Ajay Kaher <akaher@...are.com>
Cc:     "x86@...nel.org" <x86@...nel.org>, "hpa@...or.com" <hpa@...or.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        Srivatsa Bhat <srivatsab@...are.com>,
        "srivatsa@...il.mit.edu" <srivatsa@...il.mit.edu>,
        Alexey Makhalov <amakhalov@...are.com>,
        Vasavi Sirnapalli <vsirnapalli@...are.com>,
        "er.ajay.kaher@...il.com" <er.ajay.kaher@...il.com>,
        "willy@...radead.org" <willy@...radead.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "jailhouse-dev@...glegroups.com" <jailhouse-dev@...glegroups.com>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "acrn-dev@...ts.projectacrn.org" <acrn-dev@...ts.projectacrn.org>,
        "helgaas@...nel.org" <helgaas@...nel.org>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v2] x86/PCI: Prefer MMIO over PIO on all hypervisor

Nadav Amit <namit@...are.com> writes:

> On Oct 3, 2022, at 8:03 AM, Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>
>> Not my but rather PCI maintainer's call but IMHO dropping 'const' is
>> better, introducing a new global var is our 'last resort' and should be
>> avoided whenever possible. Alternatively, you can add a
>> raw_pci_ext_ops_preferred() function checking somethin within 'struct
>> hypervisor_x86' but I'm unsure if it's better.
>> 
>> Also, please check Alex' question/suggestion.
>
> Here is my take (and Ajay knows probably more than me):
>
> Looking briefly on MCFG, I do not see a clean way of using the ACPI table.
> The two options are either to use a reserved field (which who knows, might
> be used one day) or some OEM ID. I am also not familiar with
> PCI_COMMAND.MEMORY=0, so Ajay can hopefully give some answer about that.
>
> Anyhow, I understand (although not relate) to the objection for a new global
> variable. How about explicitly calling this hardware bug a “bug” and using
> the proper infrastructure? Calling it explicitly a bug may even push whoever
> can to resolve it.
>
> IOW, how about doing something along the lines of (not tested):
>

Works for me. Going forward, the intention shoud be to also clear the
bug on other x86 hypervisors, e.g. we test modern Hyper-V versions and
if MMIO works well we clear it, we test modern QEMU/KVM setups and if
MMIO works introduce a feature bit somewhere and also clear the bug in
the guest when the bit is set.

-- 
Vitaly