Re: [kvm-devel] [PATCH 1/1] Enble a guest to access a device's memory mapped I/O regions directly.

[Avi K. <av...@qu...>]

be...@il... wrote:
> From: Ben-Ami Yassour <be...@il...>
>
> Signed-off-by: Ben-Ami Yassour <be...@il...>
> Signed-off-by: Muli Ben-Yehuda <mu...@il...>
> ---
>  arch/x86/kvm/mmu.c         |   59 +++++++++++++++++++++++++++++--------------
>  arch/x86/kvm/paging_tmpl.h |   19 +++++++++----
>  include/linux/kvm_host.h   |    2 +-
>  virt/kvm/kvm_main.c        |   17 +++++++++++-
>  4 files changed, 69 insertions(+), 28 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 078a7f1..c89029d 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -112,6 +112,8 @@ static int dbg = 1;
>  #define PT_FIRST_AVAIL_BITS_SHIFT 9
>  #define PT64_SECOND_AVAIL_BITS_SHIFT 52
>  
> +#define PT_SHADOW_IO_MARK (1ULL << PT_FIRST_AVAIL_BITS_SHIFT)
> +
>   

Please rename this PT_SHADOW_MMIO_MASK.


>  #define VALID_PAGE(x) ((x) != INVALID_PAGE)
>  
>  #define PT64_LEVEL_BITS 9
> @@ -237,6 +239,9 @@ static int is_dirty_pte(unsigned long pte)
>  
>  static int is_rmap_pte(u64 pte)
>  {
> +	if (pte & PT_SHADOW_IO_MARK)
> +		return false;
> +
>  	return is_shadow_present_pte(pte);
>  }
>   

Why avoid rmap on mmio pages?  Sure it's unnecessary work, but having 
less cases improves overall reliability.

You can use pfn_valid() in gfn_to_pfn() and kvm_release_pfn_*() to 
conditionally update the page refcounts.

-- 
Any sufficiently difficult bug is indistinguishable from a feature.