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Date: Wed, 28 Aug 2013 11:12:05 +0300
From: Gleb Natapov <gleb@...hat.com>
To: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc: avi.kivity@...il.com, mtosatti@...hat.com, pbonzini@...hat.com,
linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH 07/12] KVM: MMU: redesign the algorithm of pte_list
On Tue, Jul 30, 2013 at 09:02:05PM +0800, Xiao Guangrong wrote:
> Change the algorithm to:
> 1) always add new desc to the first desc (pointed by parent_ptes/rmap)
> that is good to implement rcu-nulls-list-like lockless rmap
> walking
>
> 2) always move the entry in first desc to the the position we want
> to remove when remove a spte in the parent_ptes/rmap （backward-move).
> It is good for us to implement lockless rmap walk since in the current
> code, when a spte is deleted from the "desc", another spte in the last
> "desc" will be moved to this position to replace the deleted one. If the
> deleted one has been accessed and we do not access the replaced one, the
> replaced one is missed when we do lockless walk.
> To fix this case, we do not backward move the spte, instead, we forward
> move the entry: when a spte is deleted, we move the entry in the first
> desc to that position
>
> Both of these also can reduce cache miss
>
> Signed-off-by: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
> ---
> arch/x86/kvm/mmu.c | 182 ++++++++++++++++++++++++++++++++++++-----------------
> 1 file changed, 125 insertions(+), 57 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 5a40564..3013bb1 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -918,6 +918,50 @@ static int mapping_level(struct kvm_vcpu *vcpu, gfn_t large_gfn)
> return level - 1;
> }
>
> +static int __find_first_free(struct pte_list_desc *desc)
> +{
> + int i;
> +
> + for (i = 0; i < PTE_LIST_EXT; i++)
> + if (!desc->sptes[i])
> + break;
> + return i;
> +}
> +
> +static int find_first_free(struct pte_list_desc *desc)
> +{
> + int free = __find_first_free(desc);
> +
> + WARN_ON(free >= PTE_LIST_EXT);
> + return free;
> +}
> +
> +static int find_last_used(struct pte_list_desc *desc)
> +{
> + int used = __find_first_free(desc) - 1;
> +
> + WARN_ON(used < 0 || used >= PTE_LIST_EXT);
> + return used;
> +}
> +
> +/*
> + * TODO: we can encode the desc number into the rmap/parent_ptes
> + * since at least 10 physical/virtual address bits are reserved
> + * on x86. It is worthwhile if it shows that the desc walking is
> + * a performance issue.
> + */
> +static int count_spte_number(struct pte_list_desc *desc)
> +{
> + int first_free, desc_num;
> +
> + first_free = __find_first_free(desc);
> +
> + for (desc_num = 0; desc->more; desc = desc->more)
> + desc_num++;
> +
> + return first_free + desc_num * PTE_LIST_EXT;
> +}
> +
> /*
> * Pte mapping structures:
> *
> @@ -934,92 +978,116 @@ static int pte_list_add(struct kvm_vcpu *vcpu, u64 *spte,
> unsigned long *pte_list)
> {
> struct pte_list_desc *desc;
> - int i, count = 0;
> + int free_pos;
>
> if (!*pte_list) {
> rmap_printk("pte_list_add: %p %llx 0->1\n", spte, *spte);
> *pte_list = (unsigned long)spte;
> - } else if (!(*pte_list & 1)) {
> + return 0;
> + }
> +
> + if (!(*pte_list & 1)) {
> rmap_printk("pte_list_add: %p %llx 1->many\n", spte, *spte);
> desc = mmu_alloc_pte_list_desc(vcpu);
> desc->sptes[0] = (u64 *)*pte_list;
> desc->sptes[1] = spte;
> *pte_list = (unsigned long)desc | 1;
> - ++count;
> - } else {
> - rmap_printk("pte_list_add: %p %llx many->many\n", spte, *spte);
> - desc = (struct pte_list_desc *)(*pte_list & ~1ul);
> - while (desc->sptes[PTE_LIST_EXT-1] && desc->more) {
> - desc = desc->more;
> - count += PTE_LIST_EXT;
> - }
> - if (desc->sptes[PTE_LIST_EXT-1]) {
> - desc->more = mmu_alloc_pte_list_desc(vcpu);
> - desc = desc->more;
> - }
> - for (i = 0; desc->sptes[i]; ++i)
> - ++count;
> - desc->sptes[i] = spte;
> + return 1;
> + }
> +
> + rmap_printk("pte_list_add: %p %llx many->many\n", spte, *spte);
> + desc = (struct pte_list_desc *)(*pte_list & ~1ul);
> +
> + /* No empty position in the desc. */
> + if (desc->sptes[PTE_LIST_EXT - 1]) {
> + struct pte_list_desc *new_desc;
> + new_desc = mmu_alloc_pte_list_desc(vcpu);
> + new_desc->more = desc;
> + desc = new_desc;
> + *pte_list = (unsigned long)desc | 1;
> }
> - return count;
> +
> + free_pos = find_first_free(desc);
> + desc->sptes[free_pos] = spte;
> + return count_spte_number(desc);
Should it be count_spte_number(desc) - 1? The function should returns
the number of pte entries before the spte was added.
> }
>
> static void
> -pte_list_desc_remove_entry(unsigned long *pte_list, struct pte_list_desc *desc,
> - int i, struct pte_list_desc *prev_desc)
> +pte_list_desc_remove_entry(unsigned long *pte_list,
> + struct pte_list_desc *desc, int i)
> {
> - int j;
> + struct pte_list_desc *first_desc;
> + int last_used;
> +
> + first_desc = (struct pte_list_desc *)(*pte_list & ~1ul);
> + last_used = find_last_used(first_desc);
>
> - for (j = PTE_LIST_EXT - 1; !desc->sptes[j] && j > i; --j)
> - ;
> - desc->sptes[i] = desc->sptes[j];
> - desc->sptes[j] = NULL;
> - if (j != 0)
> + /*
> + * Move the entry from the first desc to this position we want
> + * to remove.
> + */
> + desc->sptes[i] = first_desc->sptes[last_used];
> + first_desc->sptes[last_used] = NULL;
> +
What if desc == first_desc and i < last_used. You still move spte
backwards so lockless walk may have already examined entry at i and
will miss spte that was moved there from last_used position, no?
> + /* No valid entry in this desc, we can free this desc now. */
> + if (!first_desc->sptes[0]) {
> + struct pte_list_desc *next_desc = first_desc->more;
> +
> + /*
> + * Only one entry existing but still use a desc to store it?
> + */
> + WARN_ON(!next_desc);
> +
> + mmu_free_pte_list_desc(first_desc);
> + first_desc = next_desc;
> + *pte_list = (unsigned long)first_desc | 1ul;
> return;
> - if (!prev_desc && !desc->more)
> - *pte_list = (unsigned long)desc->sptes[0];
> - else
> - if (prev_desc)
> - prev_desc->more = desc->more;
> - else
> - *pte_list = (unsigned long)desc->more | 1;
> - mmu_free_pte_list_desc(desc);
> + }
> +
> + WARN_ON(!first_desc->sptes[0]);
> +
> + /*
> + * Only one entry in this desc, move the entry to the head
> + * then the desc can be freed.
> + */
> + if (!first_desc->sptes[1] && !first_desc->more) {
> + *pte_list = (unsigned long)first_desc->sptes[0];
> + mmu_free_pte_list_desc(first_desc);
> + }
> }
>
> static void pte_list_remove(u64 *spte, unsigned long *pte_list)
> {
> struct pte_list_desc *desc;
> - struct pte_list_desc *prev_desc;
> int i;
>
> if (!*pte_list) {
> - printk(KERN_ERR "pte_list_remove: %p 0->BUG\n", spte);
> - BUG();
> - } else if (!(*pte_list & 1)) {
> + WARN(1, KERN_ERR "pte_list_remove: %p 0->BUG\n", spte);
Why change BUG() to WARN() here and below?
> + return;
> + }
> +
> + if (!(*pte_list & 1)) {
> rmap_printk("pte_list_remove: %p 1->0\n", spte);
> if ((u64 *)*pte_list != spte) {
> - printk(KERN_ERR "pte_list_remove: %p 1->BUG\n", spte);
> - BUG();
> + WARN(1, KERN_ERR "pte_list_remove: %p 1->BUG\n", spte);
> }
Remove {} since only one statement left in the if(). Or better yet why
not:
WARN ((u64 *)*pte_list != spte, ....)?
But again why not BUG()?
> *pte_list = 0;
> - } else {
> - rmap_printk("pte_list_remove: %p many->many\n", spte);
> - desc = (struct pte_list_desc *)(*pte_list & ~1ul);
> - prev_desc = NULL;
> - while (desc) {
> - for (i = 0; i < PTE_LIST_EXT && desc->sptes[i]; ++i)
> - if (desc->sptes[i] == spte) {
> - pte_list_desc_remove_entry(pte_list,
> - desc, i,
> - prev_desc);
> - return;
> - }
> - prev_desc = desc;
> - desc = desc->more;
> - }
> - pr_err("pte_list_remove: %p many->many\n", spte);
> - BUG();
> + return;
> }
> +
> + rmap_printk("pte_list_remove: %p many->many\n", spte);
> + desc = (struct pte_list_desc *)(*pte_list & ~1ul);
> + while (desc) {
> + for (i = 0; i < PTE_LIST_EXT && desc->sptes[i]; ++i)
> + if (desc->sptes[i] == spte) {
> + pte_list_desc_remove_entry(pte_list,
> + desc, i);
> + return;
> + }
> + desc = desc->more;
> + }
> +
> + WARN(1, "pte_list_remove: %p many->many\n", spte);
> }
>
> typedef void (*pte_list_walk_fn) (u64 *spte);
> --
> 1.8.1.4
--
Gleb.
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