Re: [PATCH v2 1/1] vpci: Add resizable bar support

["Chen, Jiqian" <Jiqian.Chen@xxxxxxx>]

On 2024/12/10 19:25, Roger Pau Monné wrote:
> On Tue, Dec 10, 2024 at 10:54:43AM +0100, Jan Beulich wrote:
>> On 10.12.2024 08:57, Chen, Jiqian wrote:
>>> On 2024/12/10 15:17, Jan Beulich wrote:
>>>> On 10.12.2024 08:07, Chen, Jiqian wrote:
>>>>> On 2024/12/9 21:59, Jan Beulich wrote:
>>>>>> On 02.12.2024 07:09, Jiqian Chen wrote:
>>>>>>> +static void cf_check rebar_ctrl_write(const struct pci_dev *pdev,
>>>>>>> +                                      unsigned int reg,
>>>>>>> +                                      uint32_t val,
>>>>>>> +                                      void *data)
>>>>>>> +{
>>>>>>> +    uint64_t size;
>>>>>>> +    unsigned int index;
>>>>>>> +    struct vpci_bar *bars = data;
>>>>>>> +
>>>>>>> +    if ( pci_conf_read16(pdev->sbdf, PCI_COMMAND) & PCI_COMMAND_MEMORY 
>>>>>>> )
>>>>>>> +        return;
>>>>>>
>>>>>> I don't think something like this can go uncommented. I don't think the
>>>>>> spec mandates to drop writes in this situation?
>>>>> Spec says: Software must clear the Memory Space Enable bit in the Command 
>>>>> register before writing the BAR Size field.
>>>>> This check is suggested by Roger and it really helps to prevent erroneous 
>>>>> writes in this case,
>>>>> such as the result of debugging with Roger in the previous version.
>>>>> I will add the spec's sentences as comments here in next version.
>>>>
>>>> What you quote from the spec may not be enough as a comment here. There's
>>>> no direct implication that the write would simply be dropped on the floor
>>>> if the bit is still set. So I think you want to go a little beyond just
>>>> quoting from the spec.
>>> How about quoting Roger's previous words: " The memory decoding must be 
>>> disabled before writing the BAR size field.
>>> Otherwise changing the BAR size will lead to the active p2m mappings 
>>> getting out of sync w.r.t. the new BAR size." ?
>>
>> That'll be better, but imo still not enough to explain the outright ignoring
>> of the write.
> 
> I think we might want to do something along the lines of:
> 
> uint64_t size = PCI_REBAR_CTRL_SIZE(val);
> struct vpci_bar *bar = data;
> 
> if ( bar->enabled )
> {
>     if ( size == bar->size )
>         return;
> 
>     /*
>      * Refuse to resize a BAR while memory decoding is enabled, as
>      * otherwise the size of the mapped region in the p2m would become
>      * stale with the newly set BAR size, and the position of the BAR
>      * would be reset to undefined.  Note the PCIe specification also
>      * forbids resizing a BAR with memory decoding enabled.
>      */
>     gprintk(XENLOG_ERR,
>             "%pp: refuse to resize BAR with memory decoding enabled\n",
>           &pci->sbdf);
>     return;
> }
Thank you very much!

> 
> Note this requires that the data parameter points to the BAR that
> matches the ReBAR control register, this needs adjusting in
> init_rebar().
I think I can keep current implementation of init_rebar() and use bars[index] 
to get the corresponding BAR.

-- 
Best regards,
Jiqian Chen.