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

["H. Peter Anvin" <hpa@xxxxxxxxx>]

On October 3, 2022 8:03:41 AM PDT, Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote:
>Ajay Kaher <akaher@xxxxxxxxxx> writes:
>
>>> On 13/09/22, 7:05 PM, "Vitaly Kuznetsov" <vkuznets@xxxxxxxxxx> 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 of 
>>>> 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_x86.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 *dev);
>>> 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 *val);
>>>          int (*write)(unsigned int domain, unsigned int bus, unsigned int 
>>> 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_ops;
>>>  int raw_pci_read(unsigned int domain, unsigned int bus, unsigned int devfn,
>>>                                                 int reg, int len, u32 *val)
>>> {
>>> -       if (domain == 0 && reg < 256 && raw_pci_ops)
>>> +       if (domain == 0 && reg < 256 && raw_pci_ops &&
>>> +           (!raw_pci_ext_ops || raw_pci_ext_ops->rating <= 
>>> raw_pci_ops->rating))
>>>                 return raw_pci_ops->read(domain, bus, devfn, reg, len, val);
>>>         if (raw_pci_ext_ops)
>>>                 return raw_pci_ext_ops->read(domain, bus, devfn, reg, len, 
>>> 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 
>>> devfn,
>>>                                                 int reg, int len, u32 val)
>>> {
>>> -       if (domain == 0 && reg < 256 && raw_pci_ops)
>>> +       if (domain == 0 && reg < 256 && raw_pci_ops &&
>>> +           (!raw_pci_ext_ops || raw_pci_ext_ops->rating <= 
>>> raw_pci_ops->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, len, 
>>> val);
>>>
>>> and then somewhere in Vmware hypervisor initialization code
>>> (arch/x86/kernel/cpu/vmware.c) you do
>>>
>>>  raw_pci_ext_ops->rating = 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 would 
>> 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 = {
>> +struct pci_raw_ops pci_mmcfg = {
>>      .read =         pci_mmcfg_read,
>>      .write =        pci_mmcfg_write,
>> };
>>
>> So to avoid this change, is it fine to have global bool 
>> prefer_raw_pci_ext_ops?
>>
>> And raw_pci_read() will have following change:
>>
>> -    if (domain == 0 && reg < 256 && raw_pci_ops)
>> +    if (domain == 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.
>
>...
>

Could this be ro_after_init?