Re: [PATCH v7 1/2] xen/vpci: header: status register handler

[Stewart Hildebrand <stewart.hildebrand@xxxxxxx>]

On 11/21/23 10:18, Roger Pau Monné wrote:
> On Tue, Nov 21, 2023 at 10:03:01AM -0500, Stewart Hildebrand wrote:
>> On 11/21/23 09:45, Roger Pau Monné wrote:
>>> On Wed, Sep 13, 2023 at 10:35:46AM -0400, Stewart Hildebrand wrote:
>>>> @@ -407,26 +439,25 @@ uint32_t vpci_read(pci_sbdf_t sbdf, unsigned int 
>>>> reg, unsigned int size)
>>>>  
>>>>  /*
>>>>   * Perform a maybe partial write to a register.
>>>> - *
>>>> - * Note that this will only work for simple registers, if Xen needs to
>>>> - * trap accesses to rw1c registers (like the status PCI header register)
>>>> - * the logic in vpci_write will have to be expanded in order to correctly
>>>> - * deal with them.
>>>>   */
>>>>  static void vpci_write_helper(const struct pci_dev *pdev,
>>>>                                const struct vpci_register *r, unsigned int 
>>>> size,
>>>>                                unsigned int offset, uint32_t data)
>>>>  {
>>>> +    uint32_t val = 0;
>>>> +
>>>>      ASSERT(size <= r->size);
>>>>  
>>>> -    if ( size != r->size )
>>>> +    if ( (size != r->size) || r->ro_mask )
>>>>      {
>>>> -        uint32_t val;
>>>> -
>>>>          val = r->read(pdev, r->offset, r->private);
>>>> +        val &= ~r->rw1c_mask;
>>>>          data = merge_result(val, data, size, offset);
>>>>      }
>>>>  
>>>> +    data &= ~(r->rsvdz_mask | r->ro_mask);
>>>> +    data |= val & r->ro_mask;
>>>
>>> I've been thinking about this, and the way the ro_mask is implemented
>>> (and the way we want to handle ro bits) is the same behavior as RsvdP.
>>> I would suggest to rename the ro_mask to rsvdp_mask and note
>>> that for resilience reasons we will handle RO bits as RsvdP.
>>
>> But the reads behave differently. RO should return the value, RsvdP should 
>> return 0 when read (according to the PCIe Base Spec 4.0).
> 
> Hm, right, sorry for the wrong suggestion.  We should force bits to 0
> for guests reads, but make sure that for writes the value on the
> hardware is preserved.
> 
> So we need the separate mask for RsvdP, which will be handled like
> ro_mask in vpci_write_helper() and like rsvdz_mask in vpci_read().

Agreed. The only reason I didn't add RsvdP support in this patch was that it 
wasn't needed for the status register... Since RsvdP will be needed for the 
command register, I think I may as well implement it as part of this series, 
perhaps in a separate patch following this one. Stay tuned for v8.