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@xxxxxxxxxxxxxxxxxx>
> >> ---
> >> 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?
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