On 18.07.2013, at 09:41, Kevin Wolf wrote: > Am 17.07.2013 um 22:12 hat Mark Cave-Ayland geschrieben: >> On 17/07/13 14:35, Kevin Wolf wrote: >> >>> Okay, so I've had a quick look at that DMA controller, and it seems that >>> for a complete emulation, there's no way around using a bounce buffer >>> (and calling directly into the block layer instead of using >>> dma-helpers.c) for the general case. >>> >>> You can have a fast path that is triggered if one or more directly >>> following INPUT/OUTPUT commands cover the whole IDE command, and that >>> creates an QEMUSGList as described above and uses dma-helpers.c to >>> implement zero-copy requests. I suspect that your Darwin requests would >>> actually fall into this category. >>> >>> Essentially I think Alex' patches are doing something similar, just not >>> implementing the complete DMA controller feature set and with the >>> regular slow path hacked as additional code into the fast path. So the >>> code could be cleaner, it could use asynchronous block layer functions >>> and handle errors, and it could be more complete, but at the end of >>> the day you'd still have some fast-path zero-copy I/O and some calls >>> into the block layer using bounce buffers. >> >> I think the key concept to understand here is at what point does the >> data hit the disk? From the comments in various parts of >> Darwin/Linux, it could be understood that the DMA transfers are >> between memory and the ATA drive *buffer*, so for writes especially >> there is no guarantee that they even hit the disk until some point >> in the future, unless of course the FLUSH flag is set in the control >> register. >> >> So part of me makes me think that maybe we are over-thinking this >> and we should just go with Kevin's original suggestion: what about >> if we start a new QEMUSGList for each IDE transfer, and just keep >> appending QEMUSGList entries until we find an OUTPUT_LAST/INPUT_LAST >> command? >> >> Why is this valid? We can respond with a complete status for the >> intermediate INPUT_MORE/OUTPUT_MORE commands without touching the >> disk because all that guarantees is that data has been passed >> between memory and the drive *buffer* - not that it has actually hit >> the disk. And what is the point of having explicit _LAST commands if >> they aren't used to signify completion of the whole transfer between >> drive and memory? > > I don't think there is even a clear relation between the DMA controller > status and whether the data is on disk or not. It's the IDE register's > job to tell the driver when a request has completed. The DMA controller > is only responsible for getting the data from the RAM to the device, > which might start doing a write only after it has received all data and > completed the DMA operation. (cf. PIO operation in core.c where the > bytes are gathered in a bounce buffer and only when the last byte > arrives, the whole sector is written out) > > What I would do, however, is to complete even the INPUT/OUTPUT_MORE > commands only at the end of the whole request. This is definitely > allowed behaviour, and it ensures that a memory region isn't already > reused by the OS while e.g. a write request is still running and taking > data from this memory. We should only complete the DMA command as > soon as we don't touch the memory any more.
Yes, that's the version that I described as "throw away almost all of today's code and rewrite it" :). Keep in mind that the same DMA controller can be used for Ethernet, so coupling it very tightly with IDE doesn't sound overly appealing to me either. Alex
