Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Ajay Kaher <akaher@vmware.com>
Cc:     "x86@kernel.org" <x86@kernel.org>, "hpa@zytor.com" <hpa@zytor.com>,
        "linux-pci@vger.kernel.org" <linux-pci@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "rostedt@goodmis.org" <rostedt@goodmis.org>,
        Srivatsa Bhat <srivatsab@vmware.com>,
        "srivatsa@csail.mit.edu" <srivatsa@csail.mit.edu>,
        Alexey Makhalov <amakhalov@vmware.com>,
        Vasavi Sirnapalli <vsirnapalli@vmware.com>,
        "er.ajay.kaher@gmail.com" <er.ajay.kaher@gmail.com>,
        "willy@infradead.org" <willy@infradead.org>,
        Nadav Amit <namit@vmware.com>,
        "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "jailhouse-dev@googlegroups.com" <jailhouse-dev@googlegroups.com>,
        "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
        "acrn-dev@lists.projectacrn.org" <acrn-dev@lists.projectacrn.org>,
        "helgaas@kernel.org" <helgaas@kernel.org>,
        "bhelgaas@google.com" <bhelgaas@google.com>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "bp@alien8.de" <bp@alien8.de>,
        "dave.hansen@linux.intel.com" <dave.hansen@linux.intel.com>,
        Alexander Graf <graf@amazon.com>
Subject: Re: [PATCH v2] x86/PCI: Prefer MMIO over PIO on all hypervisor
In-Reply-To: <B64FD502-E794-4E94-A267-D690476C57EE@vmware.com>
References: <9FEC6622-780D-41E6-B7CA-8D39EDB2C093@vmware.com>
 <87zgf3pfd1.fsf@redhat.com>
 <B64FD502-E794-4E94-A267-D690476C57EE@vmware.com>
Date:   Mon, 03 Oct 2022 17:03:41 +0200
Message-ID: <87tu4l9cfm.fsf@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

Ajay Kaher <akaher@vmware.com> writes:

>> =EF=BB=BFOn 13/09/22, 7:05 PM, "Vitaly Kuznetsov" <vkuznets@redhat.com> =
wrote:
>>>
>>> Thanks Vitaly for your response.
>>>
>>> 1. we have multiple objects of struct pci_raw_ops, 2. adding 'priority'=
 field to struct pci_raw_ops
>>> doesn't seems to be appropriate as need to take decision which object o=
f struct pci_raw_ops has
>>> to be used, not something with-in struct pci_raw_ops.
>>
>> I'm not sure I follow, you have two instances of 'struct pci_raw_ops'
>> which are called 'raw_pci_ops' and 'raw_pci_ext_ops'. What if you do
>> something like (completely untested):
>>
>> diff --git a/arch/x86/include/asm/pci_x86.h b/arch/x86/include/asm/pci_x=
86.h
>> index 70533fdcbf02..fb8270fa6c78 100644
>> --- a/arch/x86/include/asm/pci_x86.h
>> +++ b/arch/x86/include/asm/pci_x86.h
>> @@ -116,6 +116,7 @@ extern void (*pcibios_disable_irq)(struct pci_dev *d=
ev);
>> extern bool mp_should_keep_irq(struct device *dev);
>>
>> struct pci_raw_ops {
>> +       int rating;
>>          int (*read)(unsigned int domain, unsigned int bus, unsigned int=
 devfn,
>>                                                int reg, int len, u32 *va=
l);
>>          int (*write)(unsigned int domain, unsigned int bus, unsigned in=
t devfn,
>> diff --git a/arch/x86/pci/common.c b/arch/x86/pci/common.c
>> index ddb798603201..e9965fd11576 100644
>> --- a/arch/x86/pci/common.c
>> +++ b/arch/x86/pci/common.c
>> @@ -40,7 +40,8 @@ const struct pci_raw_ops *__read_mostly raw_pci_ext_op=
s;
>>  int raw_pci_read(unsigned int domain, unsigned int bus, unsigned int de=
vfn,
>>                                                 int reg, int len, u32 *v=
al)
>> {
>> -       if (domain =3D=3D 0 && reg < 256 && raw_pci_ops)
>> +       if (domain =3D=3D 0 && reg < 256 && raw_pci_ops &&
>> +           (!raw_pci_ext_ops || raw_pci_ext_ops->rating <=3D raw_pci_op=
s->rating))
>>                 return raw_pci_ops->read(domain, bus, devfn, reg, len, v=
al);
>>         if (raw_pci_ext_ops)
>>                 return raw_pci_ext_ops->read(domain, bus, devfn, reg, le=
n, val);
>> @@ -50,7 +51,8 @@ int raw_pci_read(unsigned int domain, unsigned int bus=
, unsigned int devfn,
>>  int raw_pci_write(unsigned int domain, unsigned int bus, unsigned int d=
evfn,
>>                                                 int reg, int len, u32 va=
l)
>> {
>> -       if (domain =3D=3D 0 && reg < 256 && raw_pci_ops)
>> +       if (domain =3D=3D 0 && reg < 256 && raw_pci_ops &&
>> +           (!raw_pci_ext_ops || raw_pci_ext_ops->rating <=3D raw_pci_op=
s->rating))
>>                 return raw_pci_ops->write(domain, bus, devfn, reg, len, =
val);
>>          if (raw_pci_ext_ops)
>>                 return raw_pci_ext_ops->write(domain, bus, devfn, reg, l=
en, val);
>>
>> and then somewhere in Vmware hypervisor initialization code
>> (arch/x86/kernel/cpu/vmware.c) you do
>>
>>  raw_pci_ext_ops->rating =3D 100;
>
> Thanks Vitaly, for your review and helping us to improve the code.
>
> I was working to make changes as you suggested, but before sending v3 wou=
ld like to
> discuss on following:
>
> If we add rating with-in struct pci_raw_ops then we can't have pci_mmcfg =
as const,
> and following change is must in arch/x86/pci/mmconfig_64.c:
>
> -const struct pci_raw_ops pci_mmcfg =3D {
> +struct pci_raw_ops pci_mmcfg =3D {
>  	.read =3D		pci_mmcfg_read,
>  	.write =3D	pci_mmcfg_write,
> };
>
> So to avoid this change, is it fine to have global bool prefer_raw_pci_ex=
t_ops?
>
> And raw_pci_read() will have following change:
>
> -	if (domain =3D=3D 0 && reg < 256 && raw_pci_ops)
> +	if (domain =3D=3D 0 && reg < 256 && raw_pci_ops &&
> +	     (!prefer_raw_pci_ext_ops ||  !raw_pci_ext_ops)
>

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.

...

--=20
Vitaly