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Re: [PATCH V3 13/13] HV/Storvsc: Add Isolation VM support for storvsc driver

On 8/20/2021 12:32 PM, hch@xxxxxx wrote:
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

+                               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 


@@ -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.

Yes, the swiotlb_force mode isn't enabled in non-isolated VM and so
dma_page_page() returns the physical address directly.

+               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

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.

Yes, will update. Thanks.

+                               if (offset_in_hvpg) {
+                                       payload->range.offset = dma & 
+                                       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 default allocate unit of swiotlb bounce is IO_TLB_SIZE(2k).
Otherwise, I find some scsi request cmd's length is less than 100byte.
Keep a small unit can avoid wasting bounce buffer and just need to
update the offset.

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.

I wonder whether we may introduce a new API scsi_dma_map_with_callback. Caller provides a callback and run callback in sg loop of dma_direct_map_sg(). Caller need to update some data structure in the sg loop. Here is such case that driver needs to populate payload->range.pfn_array[]. This is why I don't use dma_map_sg() here.

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.




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