On Thu, 3 Mar 2022 18:05:53 +0000 Shameerali Kolothum Thodi <shameerali.kolothum.thodi@xxxxxxxxxx> wrote: > > -----Original Message----- > > From: Alex Williamson [mailto:alex.williamson@xxxxxxxxxx] > > Sent: 03 March 2022 15:21 > > To: Jason Gunthorpe <jgg@xxxxxxxxxx> > > Cc: Shameerali Kolothum Thodi <shameerali.kolothum.thodi@xxxxxxxxxx>; > > kvm@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; > > linux-crypto@xxxxxxxxxxxxxxx; linux-pci@xxxxxxxxxxxxxxx; cohuck@xxxxxxxxxx; > > mgurtovoy@xxxxxxxxxx; yishaih@xxxxxxxxxx; Linuxarm > > <linuxarm@xxxxxxxxxx>; liulongfang <liulongfang@xxxxxxxxxx>; Zengtao (B) > > <prime.zeng@xxxxxxxxxxxxx>; Jonathan Cameron > > <jonathan.cameron@xxxxxxxxxx>; Wangzhou (B) <wangzhou1@xxxxxxxxxxxxx> > > Subject: Re: [PATCH v7 07/10] vfio: Extend the device migration protocol with > > PRE_COPY > > > > On Thu, 3 Mar 2022 09:01:24 -0400 > > Jason Gunthorpe <jgg@xxxxxxxxxx> wrote: > > > > > On Wed, Mar 02, 2022 at 08:47:52PM -0700, Alex Williamson wrote: > > > > On Wed, 2 Mar 2022 20:05:28 -0400 > > > > Jason Gunthorpe <jgg@xxxxxxxxxx> wrote: > > > > > > > > > On Wed, Mar 02, 2022 at 01:31:59PM -0700, Alex Williamson wrote: > > > > > > > + * initial_bytes reflects the estimated remaining size of any > > > > > > > + initial mandatory > > > > > > > + * precopy data transfer. When initial_bytes returns as zero > > > > > > > + then the initial > > > > > > > + * phase of the precopy data is completed. Generally initial_bytes > > should start > > > > > > > + * out as approximately the entire device state. > > > > > > > > > > > > What is "mandatory" intended to mean here? The user isn't required > > to > > > > > > collect any data from the device in the PRE_COPY states. > > > > > > > > > > If the data is split into initial,dirty,trailer then mandatory > > > > > means that first chunk. > > > > > > > > But there's no requirement to read anything in PRE_COPY, so initial > > > > becomes indistinguishable from trailer and dirty doesn't exist. > > > > > > It is still mandatory to read that data out, it doesn't matter if it > > > is read during PRE_COPY or STOP_COPY. > > > > Not really, PRE_COPY -> RUNNING is a valid arc. > > > > > > > > "The vfio_precopy_info data structure returned by this ioctl > > > > > > provides estimates of data available from the device during the > > PRE_COPY states. > > > > > > This estimate is split into two categories, initial_bytes and > > > > > > dirty_bytes. > > > > > > > > > > > > The initial_bytes field indicates the amount of static data > > > > > > available from the device. This field should have a non-zero initial > > value and > > > > > > decrease as migration data is read from the device. > > > > > > > > > > static isn't great either, how about just say 'minimum data available' > > > > > > > > 'initial precopy data-set'? > > > > > > Sure > > > > > > > We have no basis to make that assertion. We've agreed that precopy > > > > can be used for nothing more than a compatibility test, so we could > > > > have a vGPU with a massive framebuffer and no ability to provide > > > > dirty tracking implement precopy only to include the entire > > > > framebuffer in the trailing STOP_COPY data set. Per my > > > > understanding and the fact that we cannot enforce any heuristics > > > > regarding the size of the tailer relative to the pre-copy data set, > > > > I think the above strongly phrased sentence is necessary to > > > > understand the limitations of what this ioctl is meant to convey. > > > > Thanks, > > > > > > This is why abusing precopy for compatability is not a great idea. It > > > is OK for acc because its total state is tiny, but I would not agree > > > to a vGPU driver being merged working like you describe. It distorts > > > the entire purpose of PRE_COPY and this whole estimation mechanism. > > > > > > The ioctl is intended to convey when to switch to STOP_COPY, and the > > > driver should provide a semantic where the closer the reported length > > > is to 0 then the faster the STOP_COPY will go. > > > > If it's an abuse, then let's not do it. It was never my impression or intention > > that this was ok for acc only due to the minimal trailing data size. My > > statement was that use of PRE_COPY for compatibility testing only had been a > > previously agreed valid use case of the original migration interface. > > > > 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, > > 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 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, Alex
