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Date: Sun, 23 Dec 2012 17:02:17 +0200
From: Gleb Natapov <gleb@...hat.com>
To: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc: Marcelo Tosatti <mtosatti@...hat.com>,
LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH v3 5/5] KVM: x86: improve reexecute_instruction
On Sat, Dec 15, 2012 at 03:01:12PM +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@...ux.vnet.ibm.com>
> ---
> arch/x86/include/asm/kvm_host.h | 7 +++++
> arch/x86/kvm/paging_tmpl.h | 23 +++++++++++-----
> arch/x86/kvm/x86.c | 58 +++++++++++++++++++++++++--------------
> 3 files changed, 60 insertions(+), 28 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index dc87b65..487f0a1 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;
> +
> + /*
> + * Cache the access info when fix page fault then use
> + * them to detect unhandeable instruction.
> + */
> + gva_t fault_addr;
> + bool target_gfn_is_pt;
> };
>
> struct kvm_lpage_info {
> diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
> index 0453fa0..b67fab3 100644
> --- a/arch/x86/kvm/paging_tmpl.h
> +++ b/arch/x86/kvm/paging_tmpl.h
> @@ -506,21 +506,27 @@ out_gpte_changed:
> * size to map the gfn which is used as PDPT.
> */
> static bool
> -FNAME(is_self_change_mapping)(struct kvm_vcpu *vcpu,
> +FNAME(is_self_change_mapping)(struct kvm_vcpu *vcpu, gva_t addr,
> struct guest_walker *walker, int user_fault)
> {
> 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;
> + vcpu->arch.fault_addr = addr;
>
> - return false;
> + for (level = walker->level; level <= walker->max_level; level++) {
> + gfn_t gfn = walker->gfn ^ walker->table_gfn[level - 1];
> +
> + self_changed |= !(gfn & mask);
> + vcpu->arch.target_gfn_is_pt |= !gfn;
> + }
> +
> + return self_changed;
> }
>
> /*
> @@ -548,7 +554,7 @@ 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);
>
> @@ -576,9 +582,12 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gva_t addr, u32 error_code,
> return 0;
> }
>
> + is_self_change_mapping = FNAME(is_self_change_mapping)(vcpu, addr,
> + &walker, user_fault);
> +
is_self_change_mapping() has a subtle side-effect by setting
vcpu->arch.target_gfn_is_pt. From reading the page_fault() function
you cannot guess why is_self_change_mapping() is not called inside "if
(walker.level >= PT_DIRECTORY_LEVEL)" since this is the only place where
its output is used. May be pass it pointer to target_gfn_is_pt as a
parameter to make it clear that return value is not the only output of
the function.
> 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 bf66169..fc33563 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4756,29 +4756,25 @@ static int handle_emulation_failure(struct kvm_vcpu *vcpu)
> static bool reexecute_instruction(struct kvm_vcpu *vcpu, unsigned long cr2)
> {
> gpa_t gpa = cr2;
> + gfn_t gfn;
> pfn_t pfn;
> - unsigned int indirect_shadow_pages;
> -
> - spin_lock(&vcpu->kvm->mmu_lock);
> - indirect_shadow_pages = vcpu->kvm->arch.indirect_shadow_pages;
> - spin_unlock(&vcpu->kvm->mmu_lock);
> -
> - if (!indirect_shadow_pages)
> - return false;
>
> if (!vcpu->arch.mmu.direct_map) {
> - gpa = kvm_mmu_gva_to_gpa_read(vcpu, cr2, NULL);
> + /*
> + * Write permission should be allowed since only
> + * write access need to be emulated.
> + */
> + 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; /* let cpu generate fault */
> + return true;
> }
Why not fold this change to if (!vcpu->arch.mmu.direct_map) into
previous patch where it was introduced. This looks independent of
what you are doing in this patch.
>
> - /*
> - * 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;
> + gfn = gpa_to_gfn(gpa);
>
> /*
> * Do not retry the unhandleable instruction if it faults on the
> @@ -4786,13 +4782,33 @@ static bool reexecute_instruction(struct kvm_vcpu *vcpu, unsigned long cr2)
> * 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);
> + pfn = gfn_to_pfn(vcpu->kvm, gfn);
> +
> + /*
> + * 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) {
> + unsigned int indirect_shadow_pages;
> +
> + spin_lock(&vcpu->kvm->mmu_lock);
> + indirect_shadow_pages = vcpu->kvm->arch.indirect_shadow_pages;
> + spin_unlock(&vcpu->kvm->mmu_lock);
> +
> + if (indirect_shadow_pages)
> + kvm_mmu_unprotect_page(vcpu->kvm, gfn);
> +
> return true;
> }
>
> - return false;
> + kvm_mmu_unprotect_page(vcpu->kvm, gfn);
> + return !(vcpu->arch.fault_addr == cr2 && vcpu->arch.target_gfn_is_pt);
Do you store fault_addr only to avoid using stale target_gfn_is_pt? If
yes why not reset target_gfn_is_pt to false at the beginning of a page
fault and get rid of fault_addr?
> }
>
> static bool retry_instruction(struct x86_emulate_ctxt *ctxt,
> --
> 1.7.7.6
--
Gleb.
--
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