Date:   Fri, 20 Aug 2021 15:40:08 +0000
From:   Michael Kelley <mikelley@...rosoft.com>
To:     "hch@....de" <hch@....de>
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Subject: RE: [PATCH V3 13/13] HV/Storvsc: Add Isolation VM support for storvsc
 driver

From: hch@....de <hch@....de> 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

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