Re: [PATCH v5 03/14] vpci: move lock outside of struct vpci

[Oleksandr Andrushchenko <Oleksandr_Andrushchenko@xxxxxxxx>]

Hi, Roger, Jan!

On 13.01.22 10:58, Roger Pau Monné wrote:
> On Wed, Jan 12, 2022 at 04:52:51PM +0100, Jan Beulich wrote:
>> On 12.01.2022 16:42, Roger Pau Monné wrote:
>>> On Wed, Jan 12, 2022 at 03:57:36PM +0100, Jan Beulich wrote:
>>>> On 25.11.2021 12:02, Oleksandr Andrushchenko wrote:
>>>>> @@ -379,7 +396,6 @@ uint32_t vpci_read(pci_sbdf_t sbdf, unsigned int reg, 
>>>>> unsigned int size)
>>>>>   
>>>>>           data = merge_result(data, tmp_data, size - data_offset, 
>>>>> data_offset);
>>>>>       }
>>>>> -    spin_unlock(&pdev->vpci->lock);
>>>>>   
>>>>>       return data & (0xffffffff >> (32 - 8 * size));
>>>>>   }
>>>> Here and ...
>>>>
>>>>> @@ -475,13 +498,12 @@ void vpci_write(pci_sbdf_t sbdf, unsigned int reg, 
>>>>> unsigned int size,
>>>>>               break;
>>>>>           ASSERT(data_offset < size);
>>>>>       }
>>>>> +    spin_unlock(&pdev->vpci_lock);
>>>>>   
>>>>>       if ( data_offset < size )
>>>>>           /* Tailing gap, write the remaining. */
>>>>>           vpci_write_hw(sbdf, reg + data_offset, size - data_offset,
>>>>>                         data >> (data_offset * 8));
>>>>> -
>>>>> -    spin_unlock(&pdev->vpci->lock);
>>>>>   }
>>>> ... even more so here I'm not sure of the correctness of the moving
>>>> you do: While pdev->vpci indeed doesn't get accessed, I wonder
>>>> whether there wasn't an intention to avoid racing calls to
>>>> vpci_{read,write}_hw() this way. In any event I think such movement
>>>> would need justification in the description.
>>> I agree about the need for justification in the commit message, or
>>> even better this being split into a pre-patch, as it's not related to
>>> the lock switching done here.
>>>
>>> I do think this is fine however, as racing calls to
>>> vpci_{read,write}_hw() shouldn't be a problem. Those are just wrappers
>>> around pci_conf_{read,write} functions, and the required locking (in
>>> case of using the IO ports) is already taken care in
>>> pci_conf_{read,write}.
>> IOW you consider it acceptable for a guest (really: Dom0) read racing
>> a write to read back only part of what was written (so far)? I would
>> think individual multi-byte reads and writes should appear atomic to
>> the guest.
> We split 64bit writes into two 32bit ones without taking the lock for
> the whole duration of the access, so it's already possible to see a
> partially updated state as a result of a 64bit write.
>
> I'm going over the PCI(e) spec but I don't seem to find anything about
> whether the ECAM is allowed to split memory transactions into multiple
> Configuration Requests, and whether those could then interleave with
> requests from a different CPU.
So, with the above is it still fine for you to have the change as is or
you want this optimization to go into a dedicated patch before this one?
>
> Thanks, Roger.
Thank you,
Oleksandr