Re: [PATCH v4 05/11] vpci/header: implement guest BAR register handlers

[Jan Beulich <jbeulich@suse.com>]

On 19.11.2021 13:10, Oleksandr Andrushchenko wrote:
> On 19.11.21 13:58, Jan Beulich wrote:
>> On 05.11.2021 07:56, Oleksandr Andrushchenko wrote:
>>> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
>>>
>>> Add relevant vpci register handlers when assigning PCI device to a domain
>>> and remove those when de-assigning. This allows having different
>>> handlers for different domains, e.g. hwdom and other guests.
>>>
>>> Emulate guest BAR register values: this allows creating a guest view
>>> of the registers and emulates size and properties probe as it is done
>>> during PCI device enumeration by the guest.
>>>
>>> ROM BAR is only handled for the hardware domain and for guest domains
>>> there is a stub: at the moment PCI expansion ROM is x86 only, so it
>>> might not be used by other architectures without emulating x86. Other
>>> use-cases may include using that expansion ROM before Xen boots, hence
>>> no emulation is needed in Xen itself. Or when a guest wants to use the
>>> ROM code which seems to be rare.
>> At least in the initial days of EFI there was the concept of EFI byte
>> code, for ROM code to be compiled to such that it would be arch-
>> independent. While I don't mean this to be an argument against leaving
>> out ROM BAR handling for now, this may want mentioning here to avoid
>> giving the impression that it's only x86 which might be affected by
>> this deliberate omission.
> I can put:
> at the moment PCI expansion ROM handling is supported for x86 only
> and it might not be used by other architectures without emulating x86.

Sounds at least somewhat better to me.

>>> --- a/xen/drivers/vpci/header.c
>>> +++ b/xen/drivers/vpci/header.c
>>> @@ -408,6 +408,48 @@ static void bar_write(const struct pci_dev *pdev, unsigned int reg,
>>>       pci_conf_write32(pdev->sbdf, reg, val);
>>>   }
>>>   
>>> +static void guest_bar_write(const struct pci_dev *pdev, unsigned int reg,
>>> +                            uint32_t val, void *data)
>>> +{
>>> +    struct vpci_bar *bar = data;
>>> +    bool hi = false;
>>> +
>>> +    if ( bar->type == VPCI_BAR_MEM64_HI )
>>> +    {
>>> +        ASSERT(reg > PCI_BASE_ADDRESS_0);
>>> +        bar--;
>>> +        hi = true;
>>> +    }
>>> +    else
>>> +    {
>>> +        val &= PCI_BASE_ADDRESS_MEM_MASK;
>>> +        val |= bar->type == VPCI_BAR_MEM32 ? PCI_BASE_ADDRESS_MEM_TYPE_32
>>> +                                           : PCI_BASE_ADDRESS_MEM_TYPE_64;
>>> +        val |= bar->prefetchable ? PCI_BASE_ADDRESS_MEM_PREFETCH : 0;
>>> +    }
>>> +
>>> +    bar->guest_addr &= ~(0xffffffffull << (hi ? 32 : 0));
>>> +    bar->guest_addr |= (uint64_t)val << (hi ? 32 : 0);
>>> +
>>> +    bar->guest_addr &= ~(bar->size - 1) | ~PCI_BASE_ADDRESS_MEM_MASK;
>>> +}
>>> +
>>> +static uint32_t guest_bar_read(const struct pci_dev *pdev, unsigned int reg,
>>> +                               void *data)
>>> +{
>>> +    const struct vpci_bar *bar = data;
>>> +    bool hi = false;
>>> +
>>> +    if ( bar->type == VPCI_BAR_MEM64_HI )
>>> +    {
>>> +        ASSERT(reg > PCI_BASE_ADDRESS_0);
>>> +        bar--;
>>> +        hi = true;
>>> +    }
>>> +
>>> +    return bar->guest_addr >> (hi ? 32 : 0);
>> I'm afraid "guest_addr" then isn't the best name; maybe "guest_val"?
>> This would make more obvious that there is a meaningful difference
>> from "addr" besides the guest vs host aspect.
> I am not sure I can agree here:
> bar->addr and bar->guest_addr make it clear what are these while
> bar->addr and bar->guest_val would make someone go look for
> additional information about what that val is for.

Feel free to replace "val" with something more suitable. "guest_bar"
maybe? The value definitely is not an address, so "addr" seems
inappropriate / misleading to me.

Jan