Re: [PATCH v6 6/9] KVM: MMU: fast path of handling guest page fault

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Jun 14, 2012 at 11:00:14AM +0800, Xiao Guangrong wrote:
> On 06/14/2012 06:40 AM, Marcelo Tosatti wrote:
> 
> > On Tue, May 29, 2012 at 02:50:32PM +0800, Xiao Guangrong wrote:
> >> If the the present bit of page fault error code is set, it indicates
> >> the shadow page is populated on all levels, it means what we do is
> >> only modify the access bit which can be done out of mmu-lock
> >>
> >> Currently, in order to simplify the code, we only fix the page fault
> >> caused by write-protect on the fast path
> >>
> >> Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
> >> ---
> >>  arch/x86/kvm/mmu.c |  126 +++++++++++++++++++++++++++++++++++++++++++++++-----
> >>  1 files changed, 114 insertions(+), 12 deletions(-)
> >>
> >> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> >> index 150c5ad..d6101a8 100644
> >> --- a/arch/x86/kvm/mmu.c
> >> +++ b/arch/x86/kvm/mmu.c
> >> @@ -445,6 +445,11 @@ static bool __check_direct_spte_mmio_pf(u64 spte)
> >>  }
> >>  #endif
> >>
> >> +static bool spte_can_be_writable(u64 spte)
> >> +{
> >> +	return !(~spte & (SPTE_HOST_WRITEABLE | SPTE_MMU_WRITEABLE));
> >> +}
> >> +
> > 
> > spte_is_locklessly_modifiable(). Its easy to confuse
> > "spte_can_be_writable" with different things.
> > 
> 
> 
> Yes. Will update it.
> 
> >>  static bool spte_has_volatile_bits(u64 spte)
> >>  {
> >>  	if (!shadow_accessed_mask)
> >> @@ -454,7 +459,7 @@ static bool spte_has_volatile_bits(u64 spte)
> >>  		return false;
> >>
> >>  	if ((spte & shadow_accessed_mask) &&
> >> -	      (!is_writable_pte(spte) || (spte & shadow_dirty_mask)))
> >> +	      (!spte_can_be_writable(spte) || (spte & shadow_dirty_mask)))
> >>  		return false;
> > 
> > mmu_spte_update is handling several different cases. Please rewrite
> > it, add a comment on top of it (or spread comments on top of each
> > significant code line) with all cases it is handling (also recheck it
> > regarding new EPT accessed/dirty bits code).
> > 
> 
> 
> Okay.
> 
> > For one thing, if spte can be updated locklessly the update must be
> > atomic:
> > 
> > if spte can be locklessly updated
> > 	read-and-modify must be atomic.
> 
> 
> Actually, i did it in the v5, Avi has some comments on that. Please
> see https://lkml.org/lkml/2012/5/24/55
> 
> What the reason we should locklessly update spte here? So far i know
> is for volatile bit lost and getting a stable is_writable_spte()?

Yes.

> But this two cases can be avoided by using spte_can_be_writable(spte)
> instead of is_writable_pte(spte), right?

Well, yes, but it becomes confusing: this optimization is always going
to consider sptes that can be locklessly updated as dirty, even though 
they are read-only. Is that what is wanted?

Ok, if you/Avi want to avoid an atomic read-and-update, please
introduce it later an as optimization patch.