Re: [PATCH v5 5/5] KVM: x86: improve reexecute_instruction

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On Fri, Jan 11, 2013 at 02:05:33AM +0800, Xiao Guangrong wrote:
> On 01/11/2013 01:26 AM, Marcelo Tosatti wrote:
> > On Tue, Jan 08, 2013 at 02:38:36PM +0800, Xiao Guangrong wrote:
> >> The current reexecute_instruction can not well detect the failed instruction
> >> emulation. It allows guest to retry all the instructions except it accesses
> >> on error pfn
> >>
> >> For example, some cases are nested-write-protect - if the page we want to
> >> write is used as PDE but it chains to itself. Under this case, we should
> >> stop the emulation and report the case to userspace
> >>
> >> Signed-off-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx>
> >> ---
> >>  arch/x86/include/asm/kvm_host.h |    7 +++++++
> >>  arch/x86/kvm/paging_tmpl.h      |   27 ++++++++++++++++++++-------
> >>  arch/x86/kvm/x86.c              |    8 +++++++-
> >>  3 files changed, 34 insertions(+), 8 deletions(-)
> >>
> >> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> >> index c431b33..d6ab8d2 100644
> >> --- a/arch/x86/include/asm/kvm_host.h
> >> +++ b/arch/x86/include/asm/kvm_host.h
> >> @@ -502,6 +502,13 @@ struct kvm_vcpu_arch {
> >>  		u64 msr_val;
> >>  		struct gfn_to_hva_cache data;
> >>  	} pv_eoi;
> >> +
> >> +	/*
> >> +	 * Indicate whether the access faults on its page table in guest
> >> +	 * which is set when fix page fault and used to detect unhandeable
> >> +	 * instruction.
> >> +	 */
> >> +	bool write_fault_to_shadow_pgtable;
> >>  };
> >>
> >>  struct kvm_lpage_info {
> >> diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
> >> index 67b390d..df50560 100644
> >> --- a/arch/x86/kvm/paging_tmpl.h
> >> +++ b/arch/x86/kvm/paging_tmpl.h
> >> @@ -497,26 +497,34 @@ out_gpte_changed:
> >>   * created when kvm establishes shadow page table that stop kvm using large
> >>   * page size. Do it early can avoid unnecessary #PF and emulation.
> >>   *
> >> + * @write_fault_to_shadow_pgtable will return true if the fault gfn is
> >> + * currently used as its page table.
> >> + *
> >>   * Note: the PDPT page table is not checked for PAE-32 bit guest. It is ok
> >>   * since the PDPT is always shadowed, that means, we can not use large page
> >>   * size to map the gfn which is used as PDPT.
> >>   */
> >>  static bool
> >>  FNAME(is_self_change_mapping)(struct kvm_vcpu *vcpu,
> >> -			      struct guest_walker *walker, int user_fault)
> >> +			      struct guest_walker *walker, int user_fault,
> >> +			      bool *write_fault_to_shadow_pgtable)
> >>  {
> >>  	int level;
> >>  	gfn_t mask = ~(KVM_PAGES_PER_HPAGE(walker->level) - 1);
> >> +	bool self_changed = false;
> >>
> >>  	if (!(walker->pte_access & ACC_WRITE_MASK ||
> >>  	      (!is_write_protection(vcpu) && !user_fault)))
> >>  		return false;
> >>
> >> -	for (level = walker->level; level <= walker->max_level; level++)
> >> -		if (!((walker->gfn ^ walker->table_gfn[level - 1]) & mask))
> >> -			return true;
> >> +	for (level = walker->level; level <= walker->max_level; level++) {
> >> +		gfn_t gfn = walker->gfn ^ walker->table_gfn[level - 1];
> >> +
> >> +		self_changed |= !(gfn & mask);
> >> +		*write_fault_to_shadow_pgtable |= !gfn;
> >> +	}
> >>
> >> -	return false;
> >> +	return self_changed;
> >>  }
> >>
> >>  /*
> >> @@ -544,7 +552,7 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gva_t addr, u32 error_code,
> >>  	int level = PT_PAGE_TABLE_LEVEL;
> >>  	int force_pt_level;
> >>  	unsigned long mmu_seq;
> >> -	bool map_writable;
> >> +	bool map_writable, is_self_change_mapping;
> >>
> >>  	pgprintk("%s: addr %lx err %x\n", __func__, addr, error_code);
> >>
> >> @@ -572,9 +580,14 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gva_t addr, u32 error_code,
> >>  		return 0;
> >>  	}
> >>
> >> +	vcpu->arch.write_fault_to_shadow_pgtable = false;
> >> +
> >> +	is_self_change_mapping = FNAME(is_self_change_mapping)(vcpu,
> >> +	      &walker, user_fault, &vcpu->arch.write_fault_to_shadow_pgtable);
> >> +
> >>  	if (walker.level >= PT_DIRECTORY_LEVEL)
> >>  		force_pt_level = mapping_level_dirty_bitmap(vcpu, walker.gfn)
> >> -		   || FNAME(is_self_change_mapping)(vcpu, &walker, user_fault);
> >> +		   || is_self_change_mapping;
> >>  	else
> >>  		force_pt_level = 1;
> >>  	if (!force_pt_level) {
> >> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> >> index 6f13e03..2957012 100644
> >> --- a/arch/x86/kvm/x86.c
> >> +++ b/arch/x86/kvm/x86.c
> >> @@ -4810,7 +4810,13 @@ static bool reexecute_instruction(struct kvm_vcpu *vcpu, gva_t cr2)
> >>  	 * guest to let CPU execute the instruction.
> >>  	 */
> >>  	kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(gpa));
> >> -	return true;
> >> +
> >> +	/*
> >> +	 * If the access faults on its page table, it can not
> >> +	 * be fixed by unprotecting shadow page and it should
> >> +	 * be reported to userspace.
> >> +	 */
> >> +	return !vcpu->arch.write_fault_to_shadow_pgtable;
> >>  }
> > 
> > This sounds wrong: only reporting emulation failure in case 
> > of a write fault to shadow pagetable? 
> 
> We suppose unprotecting target-gfn can avoid emulation, the same
> as current code. :(

Current code treats access to non-mapped guest address as indication to
exit reporting emulation failure.

The patch above restricts emulation failure reporting to when
write_fault_to_shadow_pgtable = true.

> > The current pattern is sane:
> > 
> > if (condition_1 which allows reexecution is true)
> > 	return true;
> > 
> > if (condition_2 which allows reexecution is true)
> > 	return true;
> > ...
> > 	return false;
> 
> Unfortunately, the current code reports failure only when the access
> fault on error pfn:
> 
>         pfn = gfn_to_pfn(vcpu->kvm, gpa_to_gfn(gpa));
>         if (!is_error_pfn(pfn)) {
>                 kvm_release_pfn_clean(pfn);
>                 return true;
>         }
> 
>         return false;
> 
> All !is_rror_pfn returns true.



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