linux-kernel - Re: [PATCH] KVM:X86 simply update A/D bits route

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Date:   Thu, 12 Oct 2017 16:18:47 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peng Hao <peng.hao2@....com.cn>, rkrcmar@...hat.com
Cc:     x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM:X86 simply update A/D bits route

On 12/10/2017 23:17, Peng Hao wrote:
> update_accessed_dirty_bits return 0 when dirty/accessed bits are
> not supported. So walk_addr_generic just call update_accessed_dirty_bits
> with supporting dirty/accessed bits.
> 
> Signed-off-by: Peng Hao <peng.hao2@....com.cn>
> ---
>  arch/x86/kvm/paging_tmpl.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
> index 86b68dc..b40f23e 100644
> --- a/arch/x86/kvm/paging_tmpl.h
> +++ b/arch/x86/kvm/paging_tmpl.h
> @@ -422,7 +422,7 @@ static int FNAME(walk_addr_generic)(struct guest_walker *walker,
>  		accessed_dirty &= pte >>
>  			(PT_GUEST_DIRTY_SHIFT - PT_GUEST_ACCESSED_SHIFT);
>  
> -	if (unlikely(!accessed_dirty)) {
> +	if (unlikely(!accessed_dirty && have_ad)) {
>  		ret = FNAME(update_accessed_dirty_bits)(vcpu, mmu, walker, write_fault);
>  		if (unlikely(ret < 0))
>  			goto error;
> 

At least you would have to remove the corresponding conditional in
update_accessed_dirty_bits, or change it to a WARN.

But I don't see the point really... why is it _better_ to check in
walk_addr_generic instead of update_accessed_dirty_bits?

Paolo