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SubjectRe: [PATCH v2 9/9] KVM: X86: Optimize zapping rmap
On Fri, Jun 25, 2021, Peter Xu wrote:
> Using rmap_get_first() and rmap_remove() for zapping a huge rmap list could be
> slow. The easy way is to travers the rmap list, collecting the a/d bits and
> free the slots along the way.
>
> Provide a pte_list_destroy() and do exactly that.
>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
> arch/x86/kvm/mmu/mmu.c | 45 +++++++++++++++++++++++++++++++-----------
> 1 file changed, 33 insertions(+), 12 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index ba0258bdebc4..45aac78dcabc 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -1014,6 +1014,38 @@ unsigned int pte_list_count(struct kvm_rmap_head *rmap_head)
> return count;
> }
>
> +/* Return true if rmap existed and callback called, false otherwise */
> +static bool pte_list_destroy(struct kvm_rmap_head *rmap_head,
> + int (*callback)(u64 *sptep))
> +{
> + struct pte_list_desc *desc, *next;
> + int i;
> +
> + if (!rmap_head->val)
> + return false;
> +
> + if (!(rmap_head->val & 1)) {
> + if (callback)
> + callback((u64 *)rmap_head->val);
> + goto out;
> + }
> +
> + desc = (struct pte_list_desc *)(rmap_head->val & ~1ul);
> +
> + while (desc) {
> + if (callback)
> + for (i = 0; i < desc->spte_count; i++)
> + callback(desc->sptes[i]);
> + next = desc->more;
> + mmu_free_pte_list_desc(desc);
> + desc = next;

Alternatively,

desc = (struct pte_list_desc *)(rmap_head->val & ~1ul);
for ( ; desc; desc = next) {
for (i = 0; i < desc->spte_count; i++)
mmu_spte_clear_track_bits((u64 *)rmap_head->val);
next = desc->more;
mmu_free_pte_list_desc(desc);
}

> + }
> +out:
> + /* rmap_head is meaningless now, remember to reset it */
> + rmap_head->val = 0;
> + return true;

Why implement this as a generic method with a callback? gcc is suprisingly
astute in optimizing callback(), but I don't see the point of adding a complex
helper that has a single caller, and is extremely unlikely to gain new callers.
Or is there another "zap everything" case I'm missing?

E.g. why not this?

static bool kvm_zap_rmapp(struct kvm *kvm, struct kvm_rmap_head *rmap_head,
const struct kvm_memory_slot *slot)
{
struct pte_list_desc *desc, *next;
int i;

if (!rmap_head->val)
return false;

if (!(rmap_head->val & 1)) {
mmu_spte_clear_track_bits((u64 *)rmap_head->val);
goto out;
}

desc = (struct pte_list_desc *)(rmap_head->val & ~1ul);
for ( ; desc; desc = next) {
for (i = 0; i < desc->spte_count; i++)
mmu_spte_clear_track_bits(desc->sptes[i]);
next = desc->more;
mmu_free_pte_list_desc(desc);
}
out:
/* rmap_head is meaningless now, remember to reset it */
rmap_head->val = 0;
return true;
}

> +}
> +
> static struct kvm_rmap_head *__gfn_to_rmap(gfn_t gfn, int level,
> struct kvm_memory_slot *slot)
> {
> @@ -1403,18 +1435,7 @@ static bool rmap_write_protect(struct kvm_vcpu *vcpu, u64 gfn)
> static bool kvm_zap_rmapp(struct kvm *kvm, struct kvm_rmap_head *rmap_head,
> struct kvm_memory_slot *slot)
> {
> - u64 *sptep;
> - struct rmap_iterator iter;
> - bool flush = false;
> -
> - while ((sptep = rmap_get_first(rmap_head, &iter))) {
> - rmap_printk("spte %p %llx.\n", sptep, *sptep);
> -
> - pte_list_remove(rmap_head, sptep);
> - flush = true;
> - }
> -
> - return flush;
> + return pte_list_destroy(rmap_head, mmu_spte_clear_track_bits);
> }
>
> static bool kvm_unmap_rmapp(struct kvm *kvm, struct kvm_rmap_head *rmap_head,
> --
> 2.31.1
>

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