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

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On Sat, Apr 21, 2012 at 03:25:47PM +0800, Xiao Guangrong wrote:
> On 04/21/2012 12:38 PM, Marcelo Tosatti wrote:
> 
> > 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?
> > 
> 
> 
> All of these case can call set_spte() to update the spte,
> 
> @@ -2346,6 +2363,7 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
>  			ret = 1;
>  			pte_access &= ~ACC_WRITE_MASK;
>  			spte &= ~PT_WRITABLE_MASK;
> +			spte |= SPTE_WRITE_PROTECT;
> 
> SPTE_WRITE_PROTECT bit is set if the page is write-protected.
> 
> >>> 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."
> > 
> 
> 
> Yes, the fault tlb entries is removed _after_ page fault. On page fault
> path, the spte is correctly updated (clear SPTE_ALLOW_WRITABLE bit),
> and the fast page fault path is fail to update the spte (Since
> SPTE_ALLOW_WRITABLE is not set or cmpxchg is fail.)
> 
> There is windows that is between guest write and shadow page page fault,
> it this windows, fast page fault can make the spte to be writable, it is
> ok, since the guest write instruction is not completed.

Yes, the TLB flush on pagefault is after page fault.

> >> 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?
> > 
> 
> 
> Not need. Please see the below sequence:
> 
> gpte.W = 1
> spte is the shadow page entry of gpte.
> spte.W = 0
> 
> 
>       VCPU 0                         VCPU 1
> guest write gpte.W = 0
>                                 guest write memory through gpte
>                                 fast page fault:
>                                      cmpxchg spte + W
> 
> SPTE.SPTE_ALLOW_WRITE = 0 when
> host emulate the write or sync shadow pages
> (spte is zapped or read-only)
> 
> flush_tlb
> 
> return to guest
> the guest write operation is completed.
> 
> It does not break anything.
> 
> Marcelo, i guess you missed "gpte to be written" and "access through gpte",
> yes? A write operation changes the page which the pte points to, not change
> the pte.

No, but the TLB flush on page-fault is irrelevant because no software
should rely on it.

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