Re: [PATCH 2/5] KVM: x86/mmu: Fix wrong gfn range of tlb flushing in kvm_set_pte_rmapp()

[Sean Christopherson]

On Fri, Jun 24, 2022, Hou Wenlong wrote:
> When the spte of hupe page is dropped in kvm_set_pte_rmapp(),
> the whole gfn range covered by the spte should be flushed.
> However, rmap_walk_init_level() doesn't align down the gfn
> for new level like tdp iterator does, then the gfn used in
> kvm_set_pte_rmapp() is not the base gfn of huge page. And
> the size of gfn range is wrong too for huge page. Since
> the base gfn of huge page is more meaningful during the
> rmap walking, so align down the gfn for new level and use
> the correct size of huge page for tlb flushing in
> kvm_set_pte_rmapp().

It's also worth noting that kvm_set_pte_rmapp() is the other user of the rmap
iterators that consumes @gfn, i.e. modifying iterator->gfn is safe-ish.

> Fixes: c3134ce240eed ("KVM: Replace old tlb flush function with new one to flush a specified range.")
> Signed-off-by: Hou Wenlong <houwenlong.hwl@xxxxxxxxxxxx>
> ---
>  arch/x86/kvm/mmu/mmu.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index b8a1f5b46b9d..37bfc88ea212 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -1427,7 +1427,7 @@ static bool kvm_set_pte_rmapp(struct kvm *kvm, struct kvm_rmap_head *rmap_head,
>  	}
>  
>  	if (need_flush && kvm_available_flush_tlb_with_range()) {
> -		kvm_flush_remote_tlbs_with_address(kvm, gfn, 1);
> +		kvm_flush_remote_tlbs_with_address(kvm, gfn, KVM_PAGES_PER_HPAGE(level));
>  		return false;
>  	}
>  
> @@ -1455,7 +1455,7 @@ static void
>  rmap_walk_init_level(struct slot_rmap_walk_iterator *iterator, int level)
>  {
>  	iterator->level = level;
> -	iterator->gfn = iterator->start_gfn;
> +	iterator->gfn = iterator->start_gfn & -KVM_PAGES_PER_HPAGE(level);

Hrm, arguably this be done on start_gfn in slot_rmap_walk_init().  Having iter->gfn
be less than iter->start_gfn will be odd.

>  	iterator->rmap = gfn_to_rmap(iterator->gfn, level, iterator->slot);
>  	iterator->end_rmap = gfn_to_rmap(iterator->end_gfn, level, iterator->slot);
>  }
> -- 
> 2.31.1
>