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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Nadav Amit <namit@vmware.com>, Alexander Graf <graf@amazon.com>,
        Ajay Kaher <akaher@vmware.com>
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Subject: Re: [PATCH v2] x86/PCI: Prefer MMIO over PIO on all hypervisor
In-Reply-To: <04F550C5-786A-4B8E-9A88-EBFBD8872F16@vmware.com>
References: <9FEC6622-780D-41E6-B7CA-8D39EDB2C093@vmware.com>
 <87zgf3pfd1.fsf@redhat.com>
 <B64FD502-E794-4E94-A267-D690476C57EE@vmware.com>
 <87tu4l9cfm.fsf@redhat.com>
 <04F550C5-786A-4B8E-9A88-EBFBD8872F16@vmware.com>
Date:   Tue, 04 Oct 2022 10:30:10 +0200
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Nadav Amit <namit@vmware.com> writes:

> On Oct 3, 2022, at 8:03 AM, Vitaly Kuznetsov <vkuznets@redhat.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.
>>=20
>> 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=3D0, so Ajay can hopefully give some answer about that.
>
> Anyhow, I understand (although not relate) to the objection for a new glo=
bal
> variable. How about explicitly calling this hardware bug a =E2=80=9Cbug=
=E2=80=9D and using
> the proper infrastructure? Calling it explicitly a bug may even push whoe=
ver
> 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.

--=20
Vitaly