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

[Muli Ben-Yehuda <muli@il.ibm.com>]

On Fri, Apr 18, 2008 at 06:50:03PM +0300, Avi Kivity wrote:
> benami@il.ibm.com wrote:
> > From: Ben-Ami Yassour <benami@il.ibm.com>
> >
> > Signed-off-by: Ben-Ami Yassour <benami@il.ibm.com>
> > Signed-off-by: Muli Ben-Yehuda <muli@il.ibm.com>
> > ---
> >  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.

Sure thing.

> >  #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.

The rmap functions already have a check to bail out if the pte is not
an rmap pte, so in that sense, we aren't adding a new case for the
code to handle, just adding direct MMIO ptes to the existing list of
non-rmap ptes.

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

Since rmap isn't useful for direct MMIO ptes, doesn't it make more
sense to "bail out" early rather than in the bowls of the rmap code?

Chag Same'ach
Muli


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