Re: [PATCH v2 15/21] xen-blkfront: Make use of the new sg_map helper function
On Thu, Apr 27, 2017 at 05:03:45PM -0600, Logan Gunthorpe wrote:
On 27/04/17 04:11 PM, Jason Gunthorpe wrote:
> On Thu, Apr 27, 2017 at 03:53:37PM -0600, Logan Gunthorpe wrote:
> Well, that is in the current form, with more users it would make sense
> to optimize for the single page case, eg by providing the existing
> call, providing a faster single-page-only variant of the copy, perhaps
> even one that is inlined.
Ok, does it make sense then to have an sg_copy_page_to_buffer (or some
such... I'm having trouble thinking of a sane name that isn't too long).
That just does k(un)map_atomic and memcpy? I could try that if it makes
sense to people.
It seems the most robust: test for iomem, and jump to a slow path
copy, otherwise inline the kmap and memcpy
Every place doing memcpy from sgl will need that pattern to be
correct.
> sg_miter will still fail when the sg contains __iomem, however I
would
> expect that the sg_copy will work with iomem, by using the __iomem
> memcpy variant.
Yes, that's true. Any sg_miters that ever see iomem will need to be
converted to support it. This isn't much different than the other
kmap(sg_page()) users I was converting that will also fail if they see
iomem. Though, I suspect an sg_miter user would be easier to convert to
iomem than a random kmap user.
How? sg_miter seems like the next nightmare down this path, what is
sg_miter_next supposed to do when something hits an iomem sgl?
miter.addr is supposed to be a kernel pointer that must not be
__iomem..
Jason