linux-kernel - Re: [PATCH] KVM: dirty all pages in kvm_write_guest

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Date:	Wed, 08 Apr 2015 10:49:59 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Radim Krčmář <rkrcmar@...hat.com>,
	linux-kernel@...r.kernel.org
CC:	kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: dirty all pages in kvm_write_guest_cached()



On 07/04/2015 22:34, Radim Krčmář wrote:
> We dirtied only one page because writes originally couldn't span more.
> Use improved syntax for '>> PAGE_SHIFT' while at it.
> 
> Fixes: 8f964525a121 ("KVM: Allow cross page reads and writes from cached translations.")
> Signed-off-by: Radim Krčmář <rkrcmar@...hat.com>

Cross-page reads and writes should never get here; they have
ghc->memslot set to NULL and go through the slow path in kvm_write_guest.

What am I missing?

Paolo

> ---
>  The function handles cross memslot writes in a different path.
> 
>  I think we should dirty pages after partial writes too (r < len),
>  but it probably won't happen and I already started refactoring :)
> 
>  virt/kvm/kvm_main.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index aadef264bed1..863df9dcab6f 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1665,6 +1665,7 @@ int kvm_write_guest_cached(struct kvm *kvm, struct gfn_to_hva_cache *ghc,
>  {
>  	struct kvm_memslots *slots = kvm_memslots(kvm);
>  	int r;
> +	gfn_t gfn;
>  
>  	BUG_ON(len > ghc->len);
>  
> @@ -1680,7 +1681,10 @@ int kvm_write_guest_cached(struct kvm *kvm, struct gfn_to_hva_cache *ghc,
>  	r = __copy_to_user((void __user *)ghc->hva, data, len);
>  	if (r)
>  		return -EFAULT;
> -	mark_page_dirty_in_slot(kvm, ghc->memslot, ghc->gpa >> PAGE_SHIFT);
> +
> +	for (gfn =  gpa_to_gfn(ghc->gpa);
> +	     gfn <= gpa_to_gfn(ghc->gpa + len - 1); gfn++)
> +		mark_page_dirty_in_slot(kvm, ghc->memslot, gfn);
>  
>  	return 0;
>  }
> 
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