On Thu, Mar 03, 2022 at 12:59:30PM -0700, Alex Williamson wrote: > > > If it's an abuse, then let's not do it. It was never my > > > impression or intention So maybe abuse is the wrong word, but I don't want to mess up this interface, which is intended to support real pre-copy devices, just because devices that don't actually implement true precopy might do silly things. The vGPU case you imagine will still work and qemu will switch to STOP_COPY with a huge trailer and be slow. That is unavoidable and I think it is fine. > > > Furthermore the acc driver was explicitly directed not to indicate any degree > > > of trailing data size in dirty_bytes, so while trailing data may be small for acc, > > > this interface is explicitly not intended to provide any indication of trailing > > > data size. Thanks, Yes, trailing data is not what this is for. This is only to help decide when to switch from PRE_COPY to STOP_COPY. If the device can execute STOP_COPY in the right time is a completely different discussion/interface. > > Just to clarify, so the suggestion here is not to use PRE_COPY for compatibility > > check at all and have a different proper infrastructure for that later as Jason > > suggested? > > > > If so, I will remove this patch from this series and go back to the old revision > > where we only have STOP_COPY and do the compatibility check during the final > > load data operation. > > Hi Shameer, > > I think NVIDIA has a company long weekend, so I'm not sure how quickly > we'll hear a rebuttal from Jason, but at this point I'd rather not > move Yes, company long weekend. > forward with using PRE_COPY exclusively for compatibility testing if > that is seen as an abuse of the interface, regardless of the size of > the remaining STOP_COPY data. It might be most expedient to respin > without PRE_COPY and we'll revisit methods to perform early > compatibility testing in the future. Thanks, Shameerali has talked about wanting this compat check early from the start, and done all the work to implement it. I think it is pretty extreme to blow up his series over trailing_data. To me acc is fine to use it this way until we get a better solution for compatability. We all need this, but I expect it to be complicated to define. Jason
