Re: [PATCH v10 7/9] KVM: Update lpage info when private/shared memory are mixed

[Chao Peng]

On Tue, Dec 06, 2022 at 10:42:24PM -0800, Isaku Yamahata wrote:
> On Tue, Dec 06, 2022 at 08:02:24PM +0800,
> Chao Peng <chao.p.peng@linux.intel.com> wrote:
> 
> > On Mon, Dec 05, 2022 at 02:49:59PM -0800, Isaku Yamahata wrote:
> > > On Fri, Dec 02, 2022 at 02:13:45PM +0800,
> > > Chao Peng <chao.p.peng@linux.intel.com> wrote:
> > > 
> > > > A large page with mixed private/shared subpages can't be mapped as large
> > > > page since its sub private/shared pages are from different memory
> > > > backends and may also treated by architecture differently. When
> > > > private/shared memory are mixed in a large page, the current lpage_info
> > > > is not sufficient to decide whether the page can be mapped as large page
> > > > or not and additional private/shared mixed information is needed.
> > > > 
> > > > Tracking this 'mixed' information with the current 'count' like
> > > > disallow_lpage is a bit challenge so reserve a bit in 'disallow_lpage'
> > > > to indicate a large page has mixed private/share subpages and update
> > > > this 'mixed' bit whenever the memory attribute is changed between
> > > > private and shared.
> > > > 
> > > > Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> > > > ---
> > > >  arch/x86/include/asm/kvm_host.h |   8 ++
> > > >  arch/x86/kvm/mmu/mmu.c          | 134 +++++++++++++++++++++++++++++++-
> > > >  arch/x86/kvm/x86.c              |   2 +
> > > >  include/linux/kvm_host.h        |  19 +++++
> > > >  virt/kvm/kvm_main.c             |   9 ++-
> > > >  5 files changed, 169 insertions(+), 3 deletions(-)
> > > > 
> > > > diff --git a/arch/x86/include/asm/kvm_host.h 
> > > > b/arch/x86/include/asm/kvm_host.h
> > > > index 283cbb83d6ae..7772ab37ac89 100644
> > > > --- a/arch/x86/include/asm/kvm_host.h
> > > > +++ b/arch/x86/include/asm/kvm_host.h
> > > > @@ -38,6 +38,7 @@
> > > >  #include <asm/hyperv-tlfs.h>
> > > >  
> > > >  #define __KVM_HAVE_ARCH_VCPU_DEBUGFS
> > > > +#define __KVM_HAVE_ARCH_SET_MEMORY_ATTRIBUTES
> > > >  
> > > >  #define KVM_MAX_VCPUS 1024
> > > >  
> > > > @@ -1011,6 +1012,13 @@ struct kvm_vcpu_arch {
> > > >  #endif
> > > >  };
> > > >  
> > > > +/*
> > > > + * Use a bit in disallow_lpage to indicate private/shared pages mixed 
> > > > at the
> > > > + * level. The remaining bits are used as a reference count.
> > > > + */
> > > > +#define KVM_LPAGE_PRIVATE_SHARED_MIXED         (1U << 31)
> > > > +#define KVM_LPAGE_COUNT_MAX                    ((1U << 31) - 1)
> > > > +
> > > >  struct kvm_lpage_info {
> > > >         int disallow_lpage;
> > > >  };
> > > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > > > index e2c70b5afa3e..2190fd8c95c0 100644
> > > > --- a/arch/x86/kvm/mmu/mmu.c
> > > > +++ b/arch/x86/kvm/mmu/mmu.c
> > > > @@ -763,11 +763,16 @@ static void update_gfn_disallow_lpage_count(const 
> > > > struct kvm_memory_slot *slot,
> > > >  {
> > > >         struct kvm_lpage_info *linfo;
> > > >         int i;
> > > > +       int disallow_count;
> > > >  
> > > >         for (i = PG_LEVEL_2M; i <= KVM_MAX_HUGEPAGE_LEVEL; ++i) {
> > > >                 linfo = lpage_info_slot(gfn, slot, i);
> > > > +
> > > > +               disallow_count = linfo->disallow_lpage & 
> > > > KVM_LPAGE_COUNT_MAX;
> > > > +               WARN_ON(disallow_count + count < 0 ||
> > > > +                       disallow_count > KVM_LPAGE_COUNT_MAX - count);
> > > > +
> > > >                 linfo->disallow_lpage += count;
> > > > -               WARN_ON(linfo->disallow_lpage < 0);
> > > >         }
> > > >  }
> > > >  
> > > > @@ -6986,3 +6991,130 @@ void kvm_mmu_pre_destroy_vm(struct kvm *kvm)
> > > >         if (kvm->arch.nx_huge_page_recovery_thread)
> > > >                 kthread_stop(kvm->arch.nx_huge_page_recovery_thread);
> > > >  }
> > > > +
> > > > +static bool linfo_is_mixed(struct kvm_lpage_info *linfo)
> > > > +{
> > > > +       return linfo->disallow_lpage & KVM_LPAGE_PRIVATE_SHARED_MIXED;
> > > > +}
> > > > +
> > > > +static void linfo_set_mixed(gfn_t gfn, struct kvm_memory_slot *slot,
> > > > +                           int level, bool mixed)
> > > > +{
> > > > +       struct kvm_lpage_info *linfo = lpage_info_slot(gfn, slot, 
> > > > level);
> > > > +
> > > > +       if (mixed)
> > > > +               linfo->disallow_lpage |= KVM_LPAGE_PRIVATE_SHARED_MIXED;
> > > > +       else
> > > > +               linfo->disallow_lpage &= 
> > > > ~KVM_LPAGE_PRIVATE_SHARED_MIXED;
> > > > +}
> > > > +
> > > > +static bool is_expected_attr_entry(void *entry, unsigned long 
> > > > expected_attrs)
> > > > +{
> > > > +       bool expect_private = expected_attrs & 
> > > > KVM_MEMORY_ATTRIBUTE_PRIVATE;
> > > > +
> > > > +       if (xa_to_value(entry) & KVM_MEMORY_ATTRIBUTE_PRIVATE) {
> > > > +               if (!expect_private)
> > > > +                       return false;
> > > > +       } else if (expect_private)
> > > > +               return false;
> > > > +
> > > > +       return true;
> > > > +}
> > > > +
> > > > +static bool mem_attrs_mixed_2m(struct kvm *kvm, unsigned long attrs,
> > > > +                              gfn_t start, gfn_t end)
> > > > +{
> > > > +       XA_STATE(xas, &kvm->mem_attr_array, start);
> > > > +       gfn_t gfn = start;
> > > > +       void *entry;
> > > > +       bool mixed = false;
> > > > +
> > > > +       rcu_read_lock();
> > > > +       entry = xas_load(&xas);
> > > > +       while (gfn < end) {
> > > > +               if (xas_retry(&xas, entry))
> > > > +                       continue;
> > > > +
> > > > +               KVM_BUG_ON(gfn != xas.xa_index, kvm);
> > > > +
> > > > +               if (!is_expected_attr_entry(entry, attrs)) {
> > > > +                       mixed = true;
> > > > +                       break;
> > > > +               }
> > > > +
> > > > +               entry = xas_next(&xas);
> > > > +               gfn++;
> > > > +       }
> > > > +
> > > > +       rcu_read_unlock();
> > > > +       return mixed;
> > > > +}
> > > > +
> > > > +static bool mem_attrs_mixed(struct kvm *kvm, struct kvm_memory_slot 
> > > > *slot,
> > > > +                           int level, unsigned long attrs,
> > > > +                           gfn_t start, gfn_t end)
> > > > +{
> > > > +       unsigned long gfn;
> > > > +
> > > > +       if (level == PG_LEVEL_2M)
> > > > +               return mem_attrs_mixed_2m(kvm, attrs, start, end);
> > > > +
> > > > +       for (gfn = start; gfn < end; gfn += KVM_PAGES_PER_HPAGE(level - 
> > > > 1))
> > > > +               if (linfo_is_mixed(lpage_info_slot(gfn, slot, level - 
> > > > 1)) ||
> > > > +                   
> > > > !is_expected_attr_entry(xa_load(&kvm->mem_attr_array, gfn),
> > > > +                                           attrs))
> > > > +                       return true;
> > > > +       return false;
> > > > +}
> > > > +
> > > > +static void kvm_update_lpage_private_shared_mixed(struct kvm *kvm,
> > > > +                                                 struct 
> > > > kvm_memory_slot *slot,
> > > > +                                                 unsigned long attrs,
> > > > +                                                 gfn_t start, gfn_t 
> > > > end)
> > > > +{
> > > > +       unsigned long pages, mask;
> > > > +       gfn_t gfn, gfn_end, first, last;
> > > > +       int level;
> > > > +       bool mixed;
> > > > +
> > > > +       /*
> > > > +        * The sequence matters here: we set the higher level basing on 
> > > > the
> > > > +        * lower level's scanning result.
> > > > +        */
> > > > +       for (level = PG_LEVEL_2M; level <= KVM_MAX_HUGEPAGE_LEVEL; 
> > > > level++) {
> > > > +               pages = KVM_PAGES_PER_HPAGE(level);
> > > > +               mask = ~(pages - 1);
> > > > +               first = start & mask;
> > > > +               last = (end - 1) & mask;
> > > > +
> > > > +               /*
> > > > +                * We only need to scan the head and tail page, for 
> > > > middle pages
> > > > +                * we know they will not be mixed.
> > > > +                */
> > > > +               gfn = max(first, slot->base_gfn);
> > > > +               gfn_end = min(first + pages, slot->base_gfn + 
> > > > slot->npages);
> > > > +               mixed = mem_attrs_mixed(kvm, slot, level, attrs, gfn, 
> > > > gfn_end);
> > > > +               linfo_set_mixed(gfn, slot, level, mixed);
> > > > +
> > > > +               if (first == last)
> > > > +                       return;
> > > 
> > > 
> > > continue.
> > 
> > Ya!
> > 
> > > 
> > > > +
> > > > +               for (gfn = first + pages; gfn < last; gfn += pages)
> > > > +                       linfo_set_mixed(gfn, slot, level, false);
> > > > +
> > > > +               gfn = last;
> > > > +               gfn_end = min(last + pages, slot->base_gfn + 
> > > > slot->npages);
> > > 
> > > if (gfn == gfn_end) continue.
> > 
> > Do you see a case where gfn can equal to gfn_end? Though it does not
> > hurt to add a check.
> 
> If last == base_gfn + npages, gfn == gfn_end can occur.

'end' is guaranteed <=  base_gfn + npages in kvm_unmap_mem_range():
        gfn_range.end = min(end, slot->base_gfn + slot->npages);

And 'last' is defined as:
        last = (end - 1) & mask;

Then the math is:
        last = (end - 1) & mask
             <= end - 1
             <= base_gfn + npages - 1
             <  base_gfn + npages

Thanks,
Chao
> 
> 
> > > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > > index 9a07380f8d3c..5aefcff614d2 100644
> > > > --- a/arch/x86/kvm/x86.c
> > > > +++ b/arch/x86/kvm/x86.c
> > > > @@ -12362,6 +12362,8 @@ static int kvm_alloc_memslot_metadata(struct 
> > > > kvm *kvm,
> > > >                 if ((slot->base_gfn + npages) & 
> > > > (KVM_PAGES_PER_HPAGE(level) - 1))
> > > >                         linfo[lpages - 1].disallow_lpage = 1;
> > > >                 ugfn = slot->userspace_addr >> PAGE_SHIFT;
> > > > +               if (kvm_slot_can_be_private(slot))
> > > > +                       ugfn |= slot->restricted_offset >> PAGE_SHIFT;
> > > 
> > > Is there any alignment restriction? If no, It should be +=.
> > > In practice, alignment will hold though.
> > 
> > All we need here is checking whether both userspace_addr and
> > restricted_offset are aligned to HPAGE_SIZE or not. '+=' actually can
> > yield wrong value in cases when userspace_addr + restricted_offset is
> > aligned to HPAGE_SIZE but individually they may not align to HPAGE_SIZE.
> 
> Ah, got it. The blow comment explains it.
> 
> > Thanks,
> > Chao
> > > 
> > > Thanks,
> > > 
> > > >                 /*
> > > >                  * If the gfn and userspace address are not aligned wrt 
> > > > each
> > > >                  * other, disable large page support for this slot.
> -- 
> Isaku Yamahata <isaku.yamahata@gmail.com>