Re: [Xen-devel] vpci: deferral of register write until p2m changes are done

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 28.11.18 at 11:09, <roger.pau@xxxxxxxxxx> wrote:
> Hello,
> 
> While doing the recent vPCI fixes and also working on SR-IOV support
> I've been thinking about how vPCI handles writes to PCI registers that
> imply modifications to the p2m for PVH Dom0.
> 
> When memory decoding or ROM BARs are enabled Xen performs the
> following flow:
> 
> 1. Create a rangeset with the memory regions that need to be
> mapped/unmapped.
> 2. Block the vCPU and perform the p2m changes in a preemptive way.
> 3. After the p2m changes have been applied (or in case of error) write
> to the register in order to enable/disable memory decoding or the ROM
> BAR and mark the BARs as enabled.
> 
> I'm unsure about the benefit of deferring the register write (step 3)
> for a PVH Dom0, so I would like to perform the register write before
> applying the changes to the p2m.

As expressed while reviewing respective patches, I'm not sure either.
Being not sure, putting ourselves on the safe side by disabling decode
early and enabling decode late seems best to me though. Any
deviation from this would imo require a conclusive discussion of why
it is safe. In the interest of later enabling of the code for DomU, any
such discussion should, as far as possible, avoid argumentation along
the Dom0-only line.

Jan



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