From: hch@xxxxxx <hch@xxxxxx> Sent: Thursday, August 19, 2021 9:33 PM
>
> On Thu, Aug 19, 2021 at 06:17:40PM +0000, Michael Kelley wrote:
> > >
> > > @@ -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.
OK, right. In the isolated VM case, the swiotlb is in force mode
and will do bounce buffering. In the non-isolated case,
dma_map_page_attrs() -> dma_direct_map_page() does a lot of
checking but eventually just returns the physical address. As this
patch is currently coded, it adds a fair amount of overhead
here in storvsc_queuecommand(), plus the overhead of the dma
mapping function deciding to use the identity mapping. But if
dma_map_sg() is used and the code is simplified a bit, the overhead
will be less in general and will be per sgl entry instead of per page.
>
> > > + 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.
>
I see that the swiotlb code gets and uses the min_align_mask field. But
the NVME driver is the only driver that ever sets it, so the value is zero
in all other cases. Does swiotlb just use PAGE_SIZE in that that case? I
couldn't tell from a quick glance at the swiotlb code.
> >
> > 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.
>
Doing some broader revisions to the Hyper-V storvsc driver is up next on
my to-do list. Rather than significantly modifying the non-isolated case in
this patch set, I'd suggest factoring it into my broader revisions.
Michael
