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

[Gleb Natapov <gleb@redhat.com>]

On Thu, Jan 10, 2013 at 03:30:36PM -0200, 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@linux.vnet.ibm.com>
> > ---
> >  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;
> >  }
> > 
> >  static bool retry_instruction(struct x86_emulate_ctxt *ctxt,
> 
> Also should make sure vcpu->arch.write_fault_to_shadow_pgtable is never
> reused. Say, clean when exiting x86_emulate_instruction?
Clear it right here for clarity.

--
			Gleb.