Re: [PATCH 3/3] KVM: x86: improve reexecute_instruction

Marcelo Tosatti
2012-11-26T22:41:05Z

On Tue, Nov 20, 2012 at 07:59:53AM +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 | 2 +
> arch/x86/kvm/paging_tmpl.h | 2 +
> arch/x86/kvm/x86.c | 54 ++++++++++++++++++++++++++++-----------
> 3 files changed, 43 insertions(+), 15 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index b2e11f4..c5eb52f 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -566,6 +566,8 @@ struct kvm_arch {
> u64 hv_guest_os_id;
> u64 hv_hypercall;
>
> + /* synchronizing reexecute_instruction and page fault path. */
> + u64 page_fault_count;
> #ifdef CONFIG_KVM_MMU_AUDIT
> int audit_point;
> #endif
> diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
> index 891eb6d..d55ad89 100644
> --- a/arch/x86/kvm/paging_tmpl.h
> +++ b/arch/x86/kvm/paging_tmpl.h
> @@ -568,6 +568,8 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gva_t addr, u32 error_code,
> if (mmu_notifier_retry(vcpu->kvm, mmu_seq))
> goto out_unlock;
>
> + vcpu->kvm->arch.page_fault_count++;
> +
> kvm_mmu_audit(vcpu, AUDIT_PRE_PAGE_FAULT);
> kvm_mmu_free_some_pages(vcpu);
> if (!force_pt_level)
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 5fe72cc..2fe484b 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4473,37 +4473,61 @@ static bool reexecute_instruction(struct kvm_vcpu *vcpu, unsigned long cr2)
> {
> gpa_t gpa = cr2;
> pfn_t pfn;
> -
> - if (!ACCESS_ONCE(vcpu->kvm->arch.indirect_shadow_pages))
> - return false;
> + u64 page_fault_count;
> + int emulate;
>
> if (!vcpu->arch.mmu.direct_map) {
> gpa = kvm_mmu_gva_to_gpa_read(vcpu, cr2, NULL);
> + /*
> + * If the mapping is invalid in guest, let cpu retry
> + * it to generate fault.
> + */
> if (gpa == UNMAPPED_GVA)
> - return true; /* let cpu generate fault */
> + return true;
> }
>
> /*
> - * if emulation was due to access to shadowed page table
> - * and it failed try to unshadow page and re-enter the
> - * guest to let CPU execute the instruction.
> - */
> - if (kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(gpa)))
> - return true;
> -
> - /*
> * Do not retry the unhandleable instruction if it faults on the
> * readonly host memory, otherwise it will goto a infinite loop:
> * retry instruction -> write #PF -> emulation fail -> retry
> * instruction -> ...
> */
> pfn = gfn_to_pfn(vcpu->kvm, gpa_to_gfn(gpa));
> - if (!is_error_noslot_pfn(pfn)) {
> - kvm_release_pfn_clean(pfn);
> +
> + /*
> + * If the instruction failed on the error pfn, it can not be fixed,
> + * report the error to userspace.
> + */
> + if (is_error_noslot_pfn(pfn))
> + return false;
> +
> + kvm_release_pfn_clean(pfn);
> +
> + /* The instructions are well-emulated on direct mmu. */
> + if (vcpu->arch.mmu.direct_map) {
> + if (ACCESS_ONCE(vcpu->kvm->arch.indirect_shadow_pages))
> + kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(gpa));
> +
> return true;
> }
>
> - return false;
> +again:
> + page_fault_count = ACCESS_ONCE(vcpu->kvm->arch.page_fault_count);
> +
> + /*
> + * if emulation was due to access to shadowed page table
> + * and it failed try to unshadow page and re-enter the
> + * guest to let CPU execute the instruction.
> + */
> + kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(gpa));
> + emulate = vcpu->arch.mmu.page_fault(vcpu, cr3, PFERR_WRITE_MASK, false);

Can you explain what is the objective here?

> + /* The page fault path called above can increase the count. */
> + if (page_fault_count + 1 !=
> + ACCESS_ONCE(vcpu->kvm->arch.page_fault_count))
> + goto again;
> +
> + return !emulate;
> }
>
> static bool retry_instruction(struct x86_emulate_ctxt *ctxt,