On Fri, Nov 02, 2018 at 10:26:37AM +0000, Suzuki K Poulose wrote:
> Hi Christoffer,
>
> On 02/11/18 07:53, Christoffer Dall wrote:
> >There are two things we need to take care of when we create block
> >mappings in the stage 2 page tables:
> >
> > (1) The alignment within a PMD between the host address range and the
> > guest IPA range must be the same, since otherwise we end up mapping
> > pages with the wrong offset.
> >
> > (2) The head and tail of a memory slot may not cover a full block
> > size, and we have to take care to not map those with block
> > descriptors, since we could expose memory to the guest that the host
> > did not intend to expose.
> >
> >So far, we have been taking care of (1), but not (2), and our commentary
> >describing (1) was somewhat confusing.
> >
> >This commit attempts to factor out the checks of both into a common
> >function, and if we don't pass the check, we won't attempt any PMD
> >mappings for neither hugetlbfs nor THP.
> >
> >Note that we used to only check the alignment for THP, not for
> >hugetlbfs, but as far as I can tell the check needs to be applied to
> >both scenarios.
> >
> >Cc: Ralph Palutke <[email protected]>
> >Cc: Lukas Braun <[email protected]>
> >Reported-by: Lukas Braun <[email protected]>
> >Signed-off-by: Christoffer Dall <[email protected]>
> >---
> > virt/kvm/arm/mmu.c | 79 +++++++++++++++++++++++++++++++++++++---------
> > 1 file changed, 64 insertions(+), 15 deletions(-)
> >
> >diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
> >index 5eca48bdb1a6..4e7572656b5c 100644
> >--- a/virt/kvm/arm/mmu.c
> >+++ b/virt/kvm/arm/mmu.c
> >@@ -1470,6 +1470,63 @@ static void kvm_send_hwpoison_signal(unsigned long
> >address,
> > send_sig_mceerr(BUS_MCEERR_AR, (void __user *)address, lsb, current);
> > }
> >+static bool fault_supports_stage2_pmd_mappings(struct kvm_memory_slot
> >*memslot,
> >+ unsigned long hva)
> >+{
> >+ gpa_t gpa_start, gpa_end;
> >+ hva_t uaddr_start, uaddr_end;
> >+ size_t size;
> >+
> >+ size = memslot->npages * PAGE_SIZE;
> >+
> >+ gpa_start = memslot->base_gfn << PAGE_SHIFT;
> >+ gpa_end = gpa_start + size;
> >+
> >+ uaddr_start = memslot->userspace_addr;
> >+ uaddr_end = uaddr_start + size;
> >+
> >+ /*
> >+ * Pages belonging to memslots that don't have the same alignment
> >+ * within a PMD for userspace and IPA cannot be mapped with stage-2
> >+ * PMD entries, because we'll end up mapping the wrong pages.
> >+ *
> >+ * Consider a layout like the following:
> >+ *
> >+ * memslot->userspace_addr:
> >+ * +-----+--------------------+--------------------+---+
> >+ * |abcde|fgh Stage-1 PMD | Stage-1 PMD tv|xyz|
> >+ * +-----+--------------------+--------------------+---+
> >+ *
> >+ * memslot->base_gfn << PAGE_SIZE:
> >+ * +---+--------------------+--------------------+-----+
> >+ * |abc|def Stage-2 PMD | Stage-2 PMD |tvxyz|
> >+ * +---+--------------------+--------------------+-----+
> >+ *
> >+ * If we create those stage-2 PMDs, we'll end up with this incorrect
> >+ * mapping:
> >+ * d -> f
> >+ * e -> g
> >+ * f -> h
> >+ */
>
> Nice ! The comment makes it so easy to understand the problem.
>
> >+ if ((gpa_start & ~S2_PMD_MASK) != (uaddr_start & ~S2_PMD_MASK))
> >+ return false;
> >+
> >+ /*
> >+ * Next, let's make sure we're not trying to map anything not covered
> >+ * by the memslot. This means we have to prohibit PMD size mappings
> >+ * for the beginning and end of a non-PMD aligned and non-PMD sized
> >+ * memory slot (illustrated by the head and tail parts of the
> >+ * userspace view above containing pages 'abcde' and 'xyz',
> >+ * respectively).
> >+ *
> >+ * Note that it doesn't matter if we do the check using the
> >+ * userspace_addr or the base_gfn, as both are equally aligned (per
> >+ * the check above) and equally sized.
> >+ */
> >+ return (hva & S2_PMD_MASK) >= uaddr_start &&
> >+ (hva & S2_PMD_MASK) + S2_PMD_SIZE <= uaddr_end;
>
> Shouldn't this be :
>
> (hva & S2_PMD_MASK) + S2_PMD_SIZE < uaddr_end;
>
> or
>
> (hva & S2_PMD_MASK) + S2_PMD_SIZE <= (uaddr_end - 1);
Let's consider an example. Imagine a 4MB region (1024 4K pages) start
at 0, and we want to ensure that the address is within one of the 2MB
pages.
That means, size will be:
size = 1024 * 4096 = 4194304
uaddr_start = 0
uaddr_end = 0 + 4194304 = 4194304
and the comparison check for anything in the first 2MB region becomes
return 0 >= 0 && 2097152 <= 4194304;
which is what we want.
The comparison check for anything in the second 2MB region becomes
return 2097152 >= 0 && 4194304 <= 4194304;
which is also what we want.
If we had done a stricly less than or subtracted one from uaddr_end this
check would fail, which we don't want.
Am I missing something?
>
> >+}
> >+
> > static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> > struct kvm_memory_slot *memslot, unsigned long hva,
> > unsigned long fault_status)
> >@@ -1495,6 +1552,12 @@ static int user_mem_abort(struct kvm_vcpu *vcpu,
> >phys_addr_t fault_ipa,
> > return -EFAULT;
> > }
> >+ if (!fault_supports_stage2_pmd_mappings(memslot, hva))
> >+ force_pte = true;
> >+
> >+ if (logging_active)
> >+ force_pte = true;
>
> super minor nit: If we combine the checks, may be we could skip checking
> the alignments, when logging is active.
>
> if (logging_active ||
> !fault_supports_stage2_pmd_mappings(memslot, hva))
> force_pte = true;
>
I prefer the way the code is written now, because logging active is a
logically very separate concept from checking the boundaries, and this
is realy the slow path anyway, so not sure there's any measurable
benefit.
More generally, I'd like user_mem_abort() to become more like:
if (check1())
conditionr1 = value;
if (check2())
condition2 = value;
if (check3())
condition3 = value;
manipulate_page_tables(condition1, condition2, condition3, ...);
And I'll have a got at that following Punit's PUD huge pages.
> >+
> > /* Let's check if we will get back a huge page backed by hugetlbfs */
> > down_read(¤t->mm->mmap_sem);
> > vma = find_vma_intersection(current->mm, hva, hva + 1);
> >@@ -1504,22 +1567,9 @@ static int user_mem_abort(struct kvm_vcpu *vcpu,
> >phys_addr_t fault_ipa,
> > return -EFAULT;
> > }
> >- if (vma_kernel_pagesize(vma) == PMD_SIZE && !logging_active) {
> >+ if (vma_kernel_pagesize(vma) == PMD_SIZE && !force_pte) {
> > hugetlb = true;
> > gfn = (fault_ipa & PMD_MASK) >> PAGE_SHIFT;
> >- } else {
> >- /*
> >- * Pages belonging to memslots that don't have the same
> >- * alignment for userspace and IPA cannot be mapped using
> >- * block descriptors even if the pages belong to a THP for
> >- * the process, because the stage-2 block descriptor will
> >- * cover more than a single THP and we loose atomicity for
> >- * unmapping, updates, and splits of the THP or other pages
> >- * in the stage-2 block range.
> >- */
> >- if ((memslot->userspace_addr & ~PMD_MASK) !=
> >- ((memslot->base_gfn << PAGE_SHIFT) & ~PMD_MASK))
> >- force_pte = true;
> > }
> > up_read(¤t->mm->mmap_sem);
> >@@ -1558,7 +1608,6 @@ static int user_mem_abort(struct kvm_vcpu *vcpu,
> >phys_addr_t fault_ipa,
> > * should not be mapped with huge pages (it introduces churn
> > * and performance degradation), so force a pte mapping.
> > */
> >- force_pte = true;
> > flags |= KVM_S2_FLAG_LOGGING_ACTIVE;
>
> Otherwise looks good to me.
>
Thanks!
Christoffer
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