[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH V3 13/13] HV/Storvsc: Add Isolation VM support for storvsc driver
On Thu, Aug 19, 2021 at 06:17:40PM +0000, Michael Kelley wrote: > > +#define storvsc_dma_map(dev, page, offset, size, dir) \ > > + dma_map_page(dev, page, offset, size, dir) > > + > > +#define storvsc_dma_unmap(dev, dma_range, dir) \ > > + dma_unmap_page(dev, dma_range.dma, \ > > + dma_range.mapping_size, \ > > + dir ? DMA_FROM_DEVICE : DMA_TO_DEVICE) > > + > > Each of these macros is used only once. IMHO, they don't > add a lot of value. Just coding dma_map/unmap_page() > inline would be fine and eliminate these lines of code. Yes, I had the same thought when looking over the code. Especially as macros tend to further obsfucate the code (compared to actual helper functions). > > + for (i = 0; i < request->hvpg_count; i++) > > + storvsc_dma_unmap(&device->device, > > + request->dma_range[i], > > + > > request->vstor_packet.vm_srb.data_in == READ_TYPE); > > I think you can directly get the DMA direction as > request->cmd->sc_data_direction. Yes. > > > > @@ -1824,6 +1848,13 @@ static int storvsc_queuecommand(struct Scsi_Host > > *host, struct scsi_cmnd *scmnd) > > payload->range.len = length; > > payload->range.offset = offset_in_hvpg; > > > > + cmd_request->dma_range = kcalloc(hvpg_count, > > + sizeof(*cmd_request->dma_range), > > + GFP_ATOMIC); > > With this patch, it appears that storvsc_queuecommand() is always > doing bounce buffering, even when running in a non-isolated VM. > The dma_range is always allocated, and the inner loop below does > the dma mapping for every I/O page. The corresponding code in > storvsc_on_channel_callback() that does the dma unmap allows for > the dma_range to be NULL, but that never happens. Maybe I'm missing something in the hyperv code, but I don't think dma_map_page would bounce buffer for the non-isolated case. It will just return the physical address. > > + if (!cmd_request->dma_range) { > > + ret = -ENOMEM; > > The other memory allocation failure in this function returns > SCSI_MLQUEUE_DEVICE_BUSY. It may be debatable as to whether > that's the best approach, but that's a topic for a different patch. I > would suggest being consistent and using the same return code > here. Independent of if SCSI_MLQUEUE_DEVICE_BUSY is good (it it a common pattern in SCSI drivers), ->queuecommand can't return normal negative errnos. It must return the SCSI_MLQUEUE_* codes or 0. We should probably change the return type of the method definition to a suitable enum to make this more clear. > > + if (offset_in_hvpg) { > > + payload->range.offset = dma & > > ~HV_HYP_PAGE_MASK; > > + offset_in_hvpg = 0; > > + } > > I'm not clear on why payload->range.offset needs to be set again. > Even after the dma mapping is done, doesn't the offset in the first > page have to be the same? If it wasn't the same, Hyper-V wouldn't > be able to process the PFN list correctly. In fact, couldn't the above > code just always set offset_in_hvpg = 0? Careful. DMA mapping is supposed to keep the offset in the page, but for that the DMA mapping code needs to know what the device considers a "page". For that the driver needs to set the min_align_mask field in struct device_dma_parameters. > > The whole approach here is to do dma remapping on each individual page > of the I/O buffer. But wouldn't it be possible to use dma_map_sg() to map > each scatterlist entry as a unit? Each scatterlist entry describes a range of > physically contiguous memory. After dma_map_sg(), the resulting dma > address must also refer to a physically contiguous range in the swiotlb > bounce buffer memory. So at the top of the "for" loop over the scatterlist > entries, do dma_map_sg() if we're in an isolated VM. Then compute the > hvpfn value based on the dma address instead of sg_page(). But everything > else is the same, and the inner loop for populating the pfn_arry is > unmodified. > Furthermore, the dma_range array that you've added is not needed, since > scatterlist entries already have a dma_address field for saving the mapped > address, and dma_unmap_sg() uses that field. Yes, I think dma_map_sg is the right thing to use here, probably even for the non-isolated case so that we can get the hv drivers out of their little corner and into being more like a normal kernel driver. That is, use the scsi_dma_map/scsi_dma_unmap helpers, and then iterate over the dma addresses one page at a time using for_each_sg_dma_page. > > One thing: There's a maximum swiotlb mapping size, which I think works > out to be 256 Kbytes. See swiotlb_max_mapping_size(). We need to make > sure that we don't get a scatterlist entry bigger than this size. But I think > this already happens because you set the device->dma_mask field in > Patch 11 of this series. __scsi_init_queue checks for this setting and > sets max_sectors to limits transfers to the max mapping size. Indeed.
|
Lists.xenproject.org is hosted with RackSpace, monitoring our |