[PATCH kernel v2 2/2] KVM: PPC: Check if IOMMU page is contained in the pinned physical page
David Gibson
david at gibson.dropbear.id.au
Mon Jul 2 14:52:43 AEST 2018
On Mon, Jul 02, 2018 at 02:33:30PM +1000, Alexey Kardashevskiy wrote:
> On Mon, 2 Jul 2018 14:08:52 +1000
> David Gibson <david at gibson.dropbear.id.au> wrote:
>
> > On Fri, Jun 29, 2018 at 05:07:47PM +1000, Alexey Kardashevskiy wrote:
> > > On Fri, 29 Jun 2018 15:18:20 +1000
> > > Alexey Kardashevskiy <aik at ozlabs.ru> wrote:
> > >
> > > > On Fri, 29 Jun 2018 14:57:02 +1000
> > > > David Gibson <david at gibson.dropbear.id.au> wrote:
> > > >
> > > > > On Fri, Jun 29, 2018 at 02:51:21PM +1000, Alexey Kardashevskiy wrote:
> > > > > > On Fri, 29 Jun 2018 14:12:41 +1000
> > > > > > David Gibson <david at gibson.dropbear.id.au> wrote:
> > > > > >
> > > > > > > On Tue, Jun 26, 2018 at 03:59:26PM +1000, Alexey Kardashevskiy wrote:
> > > > > > > > We already have a check in drivers/vfio/vfio_iommu_spapr_tce.c that
> > > > > > > > an IOMMU page is contained in the physical page so the PCI hardware won't
> > > > > > > > get access to unassigned host memory.
> > > > > > > >
> > > > > > > > However we do not have this check in KVM fastpath (H_PUT_TCE accelerated
> > > > > > > > code) so the user space can pin memory backed with 64k pages and create
> > > > > > > > a hardware TCE table with a bigger page size. We were lucky so far and
> > > > > > > > did not hit this yet as the very first time the mapping happens
> > > > > > > > we do not have tbl::it_userspace allocated yet and fall back to
> > > > > > > > the userspace which in turn calls VFIO IOMMU driver and that fails
> > > > > > > > because of the check in vfio_iommu_spapr_tce.c which is really
> > > > > > > > sustainable solution.
> > > > > > > >
> > > > > > > > This stores the smallest preregistered page size in the preregistered
> > > > > > > > region descriptor and changes the mm_iommu_xxx API to check this against
> > > > > > > > the IOMMU page size.
> > > > > > > >
> > > > > > > > Signed-off-by: Alexey Kardashevskiy <aik at ozlabs.ru>
> > > > > > > > ---
> > > > > > > > Changes:
> > > > > > > > v2:
> > > > > > > > * explicitly check for compound pages before calling compound_order()
> > > > > > > >
> > > > > > > > ---
> > > > > > > > The bug is: run QEMU _without_ hugepages (no -mempath) and tell it to
> > > > > > > > advertise 16MB pages to the guest; a typical pseries guest will use 16MB
> > > > > > > > for IOMMU pages without checking the mmu pagesize and this will fail
> > > > > > > > at https://git.qemu.org/?p=qemu.git;a=blob;f=hw/vfio/common.c;h=fb396cf00ac40eb35967a04c9cc798ca896eed57;hb=refs/heads/master#l256
> > > > > > > >
> > > > > > > > With the change, mapping will fail in KVM and the guest will print:
> > > > > > > >
> > > > > > > > mlx5_core 0000:00:00.0: ibm,create-pe-dma-window(2027) 0 8000000 20000000 18 1f returned 0 (liobn = 0x80000001 starting addr = 8000000 0)
> > > > > > > > mlx5_core 0000:00:00.0: created tce table LIOBN 0x80000001 for /pci at 800000020000000/ethernet at 0
> > > > > > > > mlx5_core 0000:00:00.0: failed to map direct window for
> > > > > > > > /pci at 800000020000000/ethernet at 0: -1
> > > > > > >
> > > > > > > [snip]
> > > > > > > > @@ -124,7 +125,7 @@ long mm_iommu_get(struct mm_struct *mm, unsigned long ua, unsigned long entries,
> > > > > > > > struct mm_iommu_table_group_mem_t **pmem)
> > > > > > > > {
> > > > > > > > struct mm_iommu_table_group_mem_t *mem;
> > > > > > > > - long i, j, ret = 0, locked_entries = 0;
> > > > > > > > + long i, j, ret = 0, locked_entries = 0, pageshift;
> > > > > > > > struct page *page = NULL;
> > > > > > > >
> > > > > > > > mutex_lock(&mem_list_mutex);
> > > > > > > > @@ -166,6 +167,8 @@ long mm_iommu_get(struct mm_struct *mm, unsigned long ua, unsigned long entries,
> > > > > > > > goto unlock_exit;
> > > > > > > > }
> > > > > > > >
> > > > > > > > + mem->pageshift = 30; /* start from 1G pages - the biggest we have */
> > > > > > >
> > > > > > > What about 16G pages on an HPT system?
> > > > > >
> > > > > >
> > > > > > Below in the loop mem->pageshift will reduce to the biggest actual size
> > > > > > which will be 16mb/64k/4k. Or remain 1GB if no memory is actually
> > > > > > pinned, no loss there.
> > > > >
> > > > > Are you saying that 16G IOMMU pages aren't supported? Or that there's
> > > > > some reason a guest can never use them?
> > > >
> > > >
> > > > ah, 16_G_, not _M_. My bad. I just never tried such huge pages, I will
> > > > lift the limit up to 64 then, easier this way.
> > >
> > >
> > > Ah, no, rather this as the upper limit:
> > >
> > > mem->pageshift = ilog2(entries) + PAGE_SHIFT;
> >
> > I can't make sense of this comment in context. I see how you're
> > computing the minimum page size in the reserved region.
> >
> > My question is about what the "maximum minimum" is - the starting
> > value from which you calculate. Currently it's 1G, but I can't
> > immediately see a reason that 16G is impossible here.
>
>
> 16GB is impossible if the chunk we are preregistering here is smaller
> than that, for example, the entire guest ram is 4GB.
Of course. Just like it was for 1GiB if you had a 512MiB guest, for
example. I'm talking about a case where you have a guest that's
>=16GiB and you *have* allocated 16GiB hugepages to back it.
> If that is the
> case and we try mapping a 16GB IOMMU page, this should fail as I do not
> really know what happens to the memory between 4GB..16GB.
>
> imho if not that, than 1<<64 would make a good upper limit.
>
>
>
> >
> > > @entries here is a number of system pages being pinned in that
> > > function.
> > >
> > >
> > >
> > > >
> > > > >
> > > > > > > > for (i = 0; i < entries; ++i) {
> > > > > > > > if (1 != get_user_pages_fast(ua + (i << PAGE_SHIFT),
> > > > > > > > 1/* pages */, 1/* iswrite */, &page)) {
> > > > > > > > @@ -199,6 +202,11 @@ long mm_iommu_get(struct mm_struct *mm, unsigned long ua, unsigned long entries,
> > > > > > > > }
> > > > > > > > }
> > > > > > > > populate:
> > > > > > > > + pageshift = PAGE_SHIFT;
> > > > > > > > + if (PageCompound(page))
> > > > > > > > + pageshift += compound_order(compound_head(page));
> > > > > > > > + mem->pageshift = min_t(unsigned int, mem->pageshift, pageshift);
> > > > > > >
> > > > > > > Why not make mem->pageshift and pageshift local the same type to avoid
> > > > > > > the min_t() ?
> > > > > >
> > > > > > I was under impression min() is deprecated (misinterpret checkpatch.pl
> > > > > > may be) and therefore did not pay attention to it. I can fix this and
> > > > > > repost if there is no other question.
> > > > >
> > > > > Hm, it's possible.
> > > >
> > > > Nah, tried min(), compiles fine.
> > >
> > >
> > >
> >
> >
> >
>
>
>
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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