Re: [PATCH 0/3] kvm support for ksm

[ewandevelop]

On 2009/3/31 08:00, Izik Eidus wrote:

> apply it against Avi git tree.
>
> Izik Eidus (3):
>    kvm: dont hold pagecount reference for mapped sptes pages.
>    kvm: add SPTE_HOST_WRITEABLE flag to the shadow ptes.
>    kvm: add support for change_pte mmu notifiers
>
>   arch/x86/include/asm/kvm_host.h |    1 +
>   arch/x86/kvm/mmu.c              |   89 ++++++++++++++++++++++++++++++++-------
>   arch/x86/kvm/paging_tmpl.h      |   16 ++++++-
>   virt/kvm/kvm_main.c             |   14 ++++++
>   4 files changed, 101 insertions(+), 19 deletions(-)
>
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Hi, I'm learning kvm-mmu codes, when I was reading codes from this patch,

I can't understand why we need to do special process for "writable pte".

> +static int kvm_set_pte_rmapp(struct kvm *kvm, unsigned long *rmapp,
> +                 unsigned long data)
> +{
> +    int need_flush = 0;
> +    u64 *spte, new_spte;
> +    pte_t *ptep = (pte_t *)data;
> +    pfn_t new_pfn;
> +
> +    new_pfn = pte_pfn(ptep_val(ptep));
> +    spte = rmap_next(kvm, rmapp, NULL);
> +    while (spte) {
> +        BUG_ON(!is_shadow_present_pte(*spte));
> +        rmap_printk("kvm_set_pte_rmapp: spte %p %llx\n", spte, *spte);
> +        need_flush = 1;
> +        if (pte_write(ptep_val(ptep))) {
> +            rmap_remove(kvm, spte);
> +            set_shadow_pte(spte, shadow_trap_nonpresent_pte);
> +            spte = rmap_next(kvm, rmapp, NULL);
> +        } else {
> +            new_spte = *spte &~ (PT64_BASE_ADDR_MASK);
> +            new_spte |= new_pfn << PAGE_SHIFT;
> +
> +            if (!pte_write(ptep_val(ptep))) {
> +                new_spte &= ~PT_WRITABLE_MASK;
> +                new_spte &= ~SPTE_HOST_WRITEABLE;
> +                if (is_writeble_pte(*spte))
> +                    kvm_set_pfn_dirty(spte_to_pfn(*spte));
> +            }
> +            set_shadow_pte(spte, new_spte);
> +            spte = rmap_next(kvm, rmapp, spte);
> +        }
> +    }
> +    if (need_flush)
> +        kvm_flush_remote_tlbs(kvm);
> +
> +    return 0;
> +}
> +

In my opinion, we can just regard writable pte same as readable/executable,

all the corresponding sptes will be set as write-protect, and when guest

access them, an EPT-violation occurs and we do this #PF in kvm.

Shall anyone has some hint ?