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

[Chao Peng]

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@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx>
> > ---
> >  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.

> 
> 
> > +		mixed = mem_attrs_mixed(kvm, slot, level, attrs, gfn, gfn_end);
> > +		linfo_set_mixed(gfn, slot, level, mixed);
> > +	}
> > +}
> > +
> > +void kvm_arch_set_memory_attributes(struct kvm *kvm,
> > +				    struct kvm_memory_slot *slot,
> > +				    unsigned long attrs,
> > +				    gfn_t start, gfn_t end)
> > +{
> > +	if (kvm_slot_can_be_private(slot))
> > +		kvm_update_lpage_private_shared_mixed(kvm, slot, attrs,
> > +						      start, end);
> > +}
> > 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.

Thanks,
Chao
> 
> Thanks,
> 
> >  		/*
> >  		 * If the gfn and userspace address are not aligned wrt each
> >  		 * other, disable large page support for this slot.
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index 3331c0c92838..25099c94e770 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -592,6 +592,11 @@ struct kvm_memory_slot {
> >  	struct restrictedmem_notifier notifier;
> >  };
> >  
> > +static inline bool kvm_slot_can_be_private(const struct kvm_memory_slot *slot)
> > +{
> > +	return slot && (slot->flags & KVM_MEM_PRIVATE);
> > +}
> > +
> >  static inline bool kvm_slot_dirty_track_enabled(const struct kvm_memory_slot *slot)
> >  {
> >  	return slot->flags & KVM_MEM_LOG_DIRTY_PAGES;
> > @@ -2316,4 +2321,18 @@ static inline void kvm_account_pgtable_pages(void *virt, int nr)
> >  /* Max number of entries allowed for each kvm dirty ring */
> >  #define  KVM_DIRTY_RING_MAX_ENTRIES  65536
> >  
> > +#ifdef __KVM_HAVE_ARCH_SET_MEMORY_ATTRIBUTES
> > +void kvm_arch_set_memory_attributes(struct kvm *kvm,
> > +				    struct kvm_memory_slot *slot,
> > +				    unsigned long attrs,
> > +				    gfn_t start, gfn_t end);
> > +#else
> > +static inline void kvm_arch_set_memory_attributes(struct kvm *kvm,
> > +						  struct kvm_memory_slot *slot,
> > +						  unsigned long attrs,
> > +						  gfn_t start, gfn_t end)
> > +{
> > +}
> > +#endif /* __KVM_HAVE_ARCH_SET_MEMORY_ATTRIBUTES */
> > +
> >  #endif
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 4e1e1e113bf0..e107afea32f0 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -2354,7 +2354,8 @@ static u64 kvm_supported_mem_attributes(struct kvm *kvm)
> >  	return 0;
> >  }
> >  
> > -static void kvm_unmap_mem_range(struct kvm *kvm, gfn_t start, gfn_t end)
> > +static void kvm_unmap_mem_range(struct kvm *kvm, gfn_t start, gfn_t end,
> > +				unsigned long attrs)
> >  {
> >  	struct kvm_gfn_range gfn_range;
> >  	struct kvm_memory_slot *slot;
> > @@ -2378,6 +2379,10 @@ static void kvm_unmap_mem_range(struct kvm *kvm, gfn_t start, gfn_t end)
> >  			gfn_range.slot = slot;
> >  
> >  			r |= kvm_unmap_gfn_range(kvm, &gfn_range);
> > +
> > +			kvm_arch_set_memory_attributes(kvm, slot, attrs,
> > +						       gfn_range.start,
> > +						       gfn_range.end);
> >  		}
> >  	}
> >  
> > @@ -2427,7 +2432,7 @@ static int kvm_vm_ioctl_set_mem_attributes(struct kvm *kvm,
> >  		idx = srcu_read_lock(&kvm->srcu);
> >  		KVM_MMU_LOCK(kvm);
> >  		if (i > start)
> > -			kvm_unmap_mem_range(kvm, start, i);
> > +			kvm_unmap_mem_range(kvm, start, i, attrs->attributes);
> >  		kvm_mmu_invalidate_end(kvm);
> >  		KVM_MMU_UNLOCK(kvm);
> >  		srcu_read_unlock(&kvm->srcu, idx);
> > -- 
> > 2.25.1
> > 
> 
> -- 
> Isaku Yamahata <isaku.yamahata@xxxxxxxxx>