Re: [RFC][PATCH] Fix superpage unmap on Intel IOMMU

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



On Fri, 3 Jun 2011, Alex Williamson wrote:

> On Fri, 2011-06-03 at 20:31 +0100, David Woodhouse wrote:
> > Tell me it isn't so...
> 
> Looks accurate to me, in fact, with hugetlbfs it seems like it's doing
> exactly what it should do.  The non-hugetlbfs case isn't efficient, but
> it isn't wrong either.  Our only other option is to figure out what's
> contiguous, map those chunks and try to keep track of all that. 

That's not hard. You're already iterating over the pages. Instead of 
unconditionally mapping one page at a time, you have simple logic in the 
loop to do something like:
	if (this_hpa == first_hpa + nr_pages * page_size)
		nr_pages++;
	else {
		iommu_map(first_hpa, nr_pages);
		first_hpa = this_hpa;
		nr_pages = 1;
	}

This gives you a fairly simple way to spot contiguous ranges, avoid *some* 
of the extra cache and IOTLB flushes, and perhaps give us a chance to 
*opportunistically* use superpages. (You could avoid all the extra flushes 
if you give us a sglist, but that's probably more hassle than it's worth.)

Of course, if we do opportunistically use superpages, we're going to have 
to be able to break them. Currently your API just says "unmap whatever 
page size you happen to find in the PTE here, and tell me what it was", 
which will hurt if you really only mean to unmap 4KiB for ballooning, when 
we'd spotted that we could map a whole 2MiB page there.

(Unless the IOMMU keeps track *separately* of the page size used when 
mapping each range, somewhere outside the page tables themselves. But no. 
You *have* that information, in kvm_host_page_size(vma). So give it.)

-- 
dwmw2

--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html


[Index of Archives]     [KVM ARM]     [KVM ia64]     [KVM ppc]     [Virtualization Tools]     [Spice Development]     [Libvirt]     [Libvirt Users]     [Linux USB Devel]     [Linux Audio Users]     [Yosemite Questions]     [Linux Kernel]     [Linux SCSI]     [XFree86]
  Powered by Linux