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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, Jan 28, 2022 at 02:15:08PM +0000, Oleksandr Andrushchenko wrote:
> 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?

The change seems slightly controversial, so I think it would be best
if it was split into a separate patch with a proper reasoning in the
commit message.

Thanks, Roger.