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

Marcelo Tosatti
2013-01-11T13:52:57Z

On Fri, Jan 11, 2013 at 02:16:11AM +0800, Xiao Guangrong wrote:
> On 01/11/2013 01:30 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@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?
>
> Yes, it is more clear.
>
> But i am thinking if it is really needed because 'cr2' is only valid when it
> is called on page fault path, vcpu->arch.write_fault_to_shadow_pgtable is reset
> at the beginning of page-fault path.
>
> For other paths, cr2 is always 0 which is always 'NULL' pointer and not mapped
> on guest, reexecute_instruction will always return true:
>
> gpa = kvm_mmu_gva_to_gpa_write(vcpu, cr2, NULL);
>
> /*
> * If the mapping is invalid in guest, let cpu retry
> * it to generate fault.
> */
> if (gpa == UNMAPPED_GVA)
> return true;

This is cryptic. Its not obvious at all for someone modifying the code,
for example.

Can you please clear it explicitly?