Re: [PATCH v9 6/8] KVM: Update lpage info when private/shared memory are mixed

[Chao Peng]

On Tue, Nov 08, 2022 at 08:08:05PM +0800, Yuan Yao wrote:
> On Tue, Oct 25, 2022 at 11:13:42PM +0800, Chao Peng wrote:
> > When private/shared memory are mixed in a large page, the lpage_info may
> > not be accurate and should be updated with this mixed info. A large page
> > has mixed pages can't be really mapped as large page since its
> > private/shared pages are from different physical memory.
> >
> > Update lpage_info when private/shared memory attribute is changed. If
> > both private and shared pages are within a large page region, it can't
> > be mapped as large page. It's a bit challenge to track the mixed
> > info in a 'count' like variable, this patch instead reserves a bit in
> > 'disallow_lpage' to indicate a large page has mixed private/share pages.
> >
> > Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> > ---
> >  arch/x86/include/asm/kvm_host.h |   8 +++
> >  arch/x86/kvm/mmu/mmu.c          | 112 +++++++++++++++++++++++++++++++-
> >  arch/x86/kvm/x86.c              |   2 +
> >  include/linux/kvm_host.h        |  19 ++++++
> >  virt/kvm/kvm_main.c             |  16 +++--
> >  5 files changed, 152 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index 7551b6f9c31c..db811a54e3fd 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -37,6 +37,7 @@
> >  #include <asm/hyperv-tlfs.h>
> >
> >  #define __KVM_HAVE_ARCH_VCPU_DEBUGFS
> > +#define __KVM_HAVE_ARCH_UPDATE_MEM_ATTR
> >
> >  #define KVM_MAX_VCPUS 1024
> >
> > @@ -952,6 +953,13 @@ 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 33b1aec44fb8..67a9823a8c35 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -762,11 +762,16 @@ 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);
> >  	}
> >  }
> >
> > @@ -6910,3 +6915,108 @@ void kvm_mmu_pre_destroy_vm(struct kvm *kvm)
> >  	if (kvm->arch.nx_lpage_recovery_thread)
> >  		kthread_stop(kvm->arch.nx_lpage_recovery_thread);
> >  }
> > +
> > +static inline bool linfo_is_mixed(struct kvm_lpage_info *linfo)
> > +{
> > +	return linfo->disallow_lpage & KVM_LPAGE_PRIVATE_SHARED_MIXED;
> > +}
> > +
> > +static inline void linfo_update_mixed(struct kvm_lpage_info *linfo, bool mixed)
> > +{
> > +	if (mixed)
> > +		linfo->disallow_lpage |= KVM_LPAGE_PRIVATE_SHARED_MIXED;
> > +	else
> > +		linfo->disallow_lpage &= ~KVM_LPAGE_PRIVATE_SHARED_MIXED;
> > +}
> > +
> > +static bool mem_attr_is_mixed_2m(struct kvm *kvm, unsigned int attr,
> > +				 gfn_t start, gfn_t end)
> > +{
> > +	XA_STATE(xas, &kvm->mem_attr_array, start);
> > +	gfn_t gfn = start;
> > +	void *entry;
> > +	bool shared = attr == KVM_MEM_ATTR_SHARED;
> > +	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 ((entry && !shared) || (!entry && shared)) {
> > +			mixed = true;
> > +			goto out;
> > +		}
> > +
> > +		entry = xas_next(&xas);
> > +		gfn++;
> > +	}
> > +out:
> > +	rcu_read_unlock();
> > +	return mixed;
> > +}
> > +
> > +static bool mem_attr_is_mixed(struct kvm *kvm, struct kvm_memory_slot *slot,
> > +			      int level, unsigned int attr,
> > +			      gfn_t start, gfn_t end)
> > +{
> > +	unsigned long gfn;
> > +	void *entry;
> > +
> > +	if (level == PG_LEVEL_2M)
> > +		return mem_attr_is_mixed_2m(kvm, attr, start, end);
> > +
> > +	entry = xa_load(&kvm->mem_attr_array, start);
> > +	for (gfn = start; gfn < end; gfn += KVM_PAGES_PER_HPAGE(level - 1)) {
> > +		if (linfo_is_mixed(lpage_info_slot(gfn, slot, level - 1)))
> > +			return true;
> > +		if (xa_load(&kvm->mem_attr_array, gfn) != entry)
> > +			return true;
> > +	}
> > +	return false;
> > +}
> > +
> > +void kvm_arch_update_mem_attr(struct kvm *kvm, struct kvm_memory_slot *slot,
> > +			      unsigned int attr, gfn_t start, gfn_t end)
> > +{
> > +
> > +	unsigned long lpage_start, lpage_end;
> > +	unsigned long gfn, pages, mask;
> > +	int level;
> > +
> > +	WARN_ONCE(!(attr & (KVM_MEM_ATTR_PRIVATE | KVM_MEM_ATTR_SHARED)),
> > +			"Unsupported mem attribute.\n");
> > +
> > +	/*
> > +	 * The sequence matters here: we update 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);
> > +		lpage_start = max(start & mask, slot->base_gfn);
> > +		lpage_end = (end - 1) & mask;
> > +
> > +		/*
> > +		 * We only need to scan the head and tail page, for middle pages
> > +		 * we know they are not mixed.
> > +		 */
> > +		linfo_update_mixed(lpage_info_slot(lpage_start, slot, level),
> > +				   mem_attr_is_mixed(kvm, slot, level, attr,
> > +						     lpage_start, start));
> 
> Looks only query the lpage_start, start is not enough:
> 
> A and B are private gfns from same lpage_start as below, A > B :
> lpage_start
>        |---------A
>        |----B
> 
> Convert A to shared, this makes the upper 2M page to MIX.
> Convert B to shared, this also makes the upper 2M page to MIX.
> Convert B to private, this makes the upper 2M page to Non-MIX, but
> it's incorrect, due to A is shared.

In previous versions this is actually "lpage_start, lpage_start +
pages", e.g. covers the whole large page. While fixing another issue[*]
in v8 this was wrongly changed to "lpage_start, start", at that time I
made an assumption that "end > lpage_start + pages" so the remaining
scanning in the same large page is useless if we know what attribute we
will set, this is definitely not true though.

[*]
https://lore.kernel.org/linux-mm/20220930085914.GA2799703@xxxxxxxxxxxxxxxxxx/

Thanks,
Chao
> 
> Same to tail case.
> 
> > +
> > +		if (lpage_start == lpage_end)
> > +			return;
> > +
> > +		for (gfn = lpage_start + pages; gfn < lpage_end; gfn += pages)
> > +			linfo_update_mixed(lpage_info_slot(gfn, slot, level),
> > +					   false);
> > +
> > +		linfo_update_mixed(lpage_info_slot(lpage_end, slot, level),
> > +				   mem_attr_is_mixed(kvm, slot, level, attr,
> > +						     end, lpage_end + pages));
> > +	}
> > +}
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 02ad31f46dd7..4276ca73bd7b 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -12563,6 +12563,8 @@ 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;
> >  		/*
> >  		 * 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 4ce98fa0153c..6ce36065532c 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -2284,4 +2284,23 @@ 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 CONFIG_KVM_GENERIC_PRIVATE_MEM
> > +
> > +#define KVM_MEM_ATTR_SHARED	0x0001
> > +#define KVM_MEM_ATTR_PRIVATE	0x0002
> > +
> > +#ifdef __KVM_HAVE_ARCH_UPDATE_MEM_ATTR
> > +void kvm_arch_update_mem_attr(struct kvm *kvm, struct kvm_memory_slot *slot,
> > +			      unsigned int attr, gfn_t start, gfn_t end);
> > +#else
> > +static inline void kvm_arch_update_mem_attr(struct kvm *kvm,
> > +					    struct kvm_memory_slot *slot,
> > +					    unsigned int attr,
> > +					    gfn_t start, gfn_t end)
> > +{
> > +}
> > +#endif
> > +
> > +#endif /* CONFIG_KVM_GENERIC_PRIVATE_MEM */
> > +
> >  #endif
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index fc3835826ace..13a37b4d9e97 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -939,7 +939,8 @@ static int kvm_init_mmu_notifier(struct kvm *kvm)
> >
> >  #ifdef CONFIG_KVM_GENERIC_PRIVATE_MEM
> >
> > -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 int attr)
> >  {
> >  	struct kvm_gfn_range gfn_range;
> >  	struct kvm_memory_slot *slot;
> > @@ -963,6 +964,7 @@ 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_update_mem_attr(kvm, slot, attr, start, end);
> >  		}
> >  	}
> >
> > @@ -970,7 +972,6 @@ static void kvm_unmap_mem_range(struct kvm *kvm, gfn_t start, gfn_t end)
> >  		kvm_flush_remote_tlbs(kvm);
> >  }
> >
> > -#define KVM_MEM_ATTR_SHARED	0x0001
> >  static int kvm_vm_ioctl_set_mem_attr(struct kvm *kvm, gpa_t gpa, gpa_t size,
> >  				     bool is_private)
> >  {
> > @@ -979,6 +980,7 @@ static int kvm_vm_ioctl_set_mem_attr(struct kvm *kvm, gpa_t gpa, gpa_t size,
> >  	void *entry;
> >  	int idx;
> >  	int r = 0;
> > +	unsigned int attr;
> >
> >  	if (size == 0 || gpa + size < gpa)
> >  		return -EINVAL;
> > @@ -992,7 +994,13 @@ static int kvm_vm_ioctl_set_mem_attr(struct kvm *kvm, gpa_t gpa, gpa_t size,
> >  	 * Guest memory defaults to private, kvm->mem_attr_array only stores
> >  	 * shared memory.
> >  	 */
> > -	entry = is_private ? NULL : xa_mk_value(KVM_MEM_ATTR_SHARED);
> > +	if (is_private) {
> > +		attr = KVM_MEM_ATTR_PRIVATE;
> > +		entry = NULL;
> > +	} else {
> > +		attr = KVM_MEM_ATTR_SHARED;
> > +		entry = xa_mk_value(KVM_MEM_ATTR_SHARED);
> > +	}
> >
> >  	idx = srcu_read_lock(&kvm->srcu);
> >  	KVM_MMU_LOCK(kvm);
> > @@ -1005,7 +1013,7 @@ static int kvm_vm_ioctl_set_mem_attr(struct kvm *kvm, gpa_t gpa, gpa_t size,
> >  			goto err;
> >  	}
> >
> > -	kvm_unmap_mem_range(kvm, start, end);
> > +	kvm_unmap_mem_range(kvm, start, end, attr);
> >
> >  	goto ret;
> >  err:
> > --
> > 2.25.1
> >