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

Marcelo Tosatti
2013-01-10T17:32:42Z

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?