Re: [PATCH v3 5/9] KVM: MMU: introduce SPTE_WRITE_PROTECT bit

[Marcelo Tosatti <mtosatti@redhat.com>]

On Sat, Apr 21, 2012 at 11:47:46AM +0800, Xiao Guangrong wrote:
> On 04/21/2012 05:52 AM, Marcelo Tosatti wrote:
> 
> > On Fri, Apr 20, 2012 at 04:19:17PM +0800, Xiao Guangrong wrote:
> >> If this bit is set, it means the W bit of the spte is cleared due
> >> to shadow page table protection
> >>
> >> Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
> >> ---
> >>  arch/x86/kvm/mmu.c |   56 ++++++++++++++++++++++++++++++++++-----------------
> >>  1 files changed, 37 insertions(+), 19 deletions(-)
> >>
> >> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> >> index dd984b6..eb02fc4 100644
> >> --- a/arch/x86/kvm/mmu.c
> >> +++ b/arch/x86/kvm/mmu.c
> >> @@ -147,6 +147,7 @@ module_param(dbg, bool, 0644);
> >>
> >>  #define SPTE_HOST_WRITEABLE	(1ULL << PT_FIRST_AVAIL_BITS_SHIFT)
> >>  #define SPTE_ALLOW_WRITE	(1ULL << (PT_FIRST_AVAIL_BITS_SHIFT + 1))
> >> +#define SPTE_WRITE_PROTECT	(1ULL << (PT_FIRST_AVAIL_BITS_SHIFT + 2))
> >>
> >>  #define SHADOW_PT_INDEX(addr, level) PT64_INDEX(addr, level)
> >>
> >> @@ -1042,36 +1043,51 @@ static void drop_spte(struct kvm *kvm, u64 *sptep)
> >>  		rmap_remove(kvm, sptep);
> >>  }
> >>
> >> +static bool spte_wp_by_dirty_log(u64 spte)
> >> +{
> >> +	WARN_ON(is_writable_pte(spte));
> >> +
> >> +	return (spte & SPTE_ALLOW_WRITE) && !(spte & SPTE_WRITE_PROTECT);
> >> +}
> > 
> > Is the information accurate? Say:
> > 
> > - dirty log write protect, set SPTE_ALLOW_WRITE, clear WRITABLE.
> > - shadow gfn, rmap_write_protect finds page not WRITABLE.
> > - spte points to shadow gfn, but SPTE_WRITE_PROTECT is not set.
> 
> 
> It can not happen, rmap_write_protect will set SPTE_WRITE_PROTECT
> even if the spte is not WRITABLE, please see:
> 
> +	if (page_table_protect && spte_wp_by_dirty_log(spte))
> +		goto reset_spte;
> +
> +	return false;
> +
> +reset_spte:
>  	rmap_printk("rmap_write_protect: spte %p %llx\n", spte, *spte);
>  	spte = spte & ~PT_WRITABLE_MASK;
> +	if (page_table_protect)
> +		spte |= SPTE_WRITE_PROTECT;
>  	mmu_spte_update(sptep, spte);
> -
>  	return false;
>  }

Right. What about sync path, fault path, prefault path, do they update
SPTE_WRITE_PROTECT properly?

> > BTW,
> > 
> > "introduce SPTE_ALLOW_WRITE bit
> > 
> > This bit indicates whether the spte is allow to be writable that
> > means the gpte of this spte is writable and the pfn pointed by
> > this spte is writable on host"
> > 
> > Other than the fact that each bit should have one meaning, how
> > can this bit be accurate without write protection of the gpte?
> > 
> 
> Above explanation can ensure the meaning of this bit is accurate?
> Or it has another case? :)

No, it is out of sync with guest pte.

> > As soon as guest writes to gpte, information in bit is outdated.
> > 
> 
> 
> The bit will be updated when spte is updated.
> 
> When the guest write gpte, the spte is not updated immediately,
> yes, the bit is outdated at that time, but it is ok since tlb is
> not flushed.

Page faults cause TLB flushes.

>From Intel manual:

"In addition to the instructions identified above, page faults
invalidate entries in the TLBs and paging-structure caches. In
particular, a page-fault exception resulting from an attempt to use
a linear address will invalidate any TLB entries that are for a page
number corresponding to that linear address and that are associated with
the current PCID."

> After tlb flush, the bit can be coincident with gpte.

You must read the gpte before updating from ro->rw, unless you write
protect gpte. IIRC you were doing that in previous patches?