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

Gleb Natapov
2013-01-10T17:38:23Z

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.