Re: [PATCH v4 5/5] KVM: x86: improve reexecute_instruction

Marcelo Tosatti
2013-01-07T20:46:20Z

On Sat, Jan 05, 2013 at 04:16:37PM +0800, Xiao Guangrong wrote:
> On 01/05/2013 06:44 AM, Marcelo Tosatti wrote:
>
> >> index b0a3678..44c6992 100644
> >> --- a/arch/x86/kvm/x86.c
> >> +++ b/arch/x86/kvm/x86.c
> >> @@ -4756,15 +4756,8 @@ 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;
> >
> > This renders the previous patch obsolete, pretty much (please fold).
>
> Will try.
>
> >
> >> if (!vcpu->arch.mmu.direct_map) {
> >> /*
> >> @@ -4781,13 +4774,7 @@ static bool reexecute_instruction(struct kvm_vcpu *vcpu, unsigned long cr2)
> >> 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;
> >> + gfn = gpa_to_gfn(gpa);
> >>
> >> /*
> >> * Do not retry the unhandleable instruction if it faults on the
> >> @@ -4795,13 +4782,38 @@ 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) {
> >
> > !direct_map?
>
> No. This logic is, if it is direct mmu, we just unprotect the page shadowed by
> nested mmu, then let guest retry the instruction, no need to detect unhandlable
> instruction.
>
> >
> >> + 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);
> >> +
> >> + /* If the target gfn is used as page table, the fault can
> >> + * not be avoided by unprotecting shadow page and it will
> >> + * be reported to userspace.
> >> + */
> >> + return !vcpu->arch.target_gfn_is_pt;
> >> }
> >
> > The idea was
> >
> > How about recording the gfn number for shadow pages that have been
> > shadowed in the current pagefault run? (which is cheap, compared to
> > shadowing these pages).
> >
> > If failed instruction emulation is write to one of these gfns, then
> > fail.
>
> If i understood correctly, i do not think it is simpler than the way in this
> patch.
>
> There is the change to apply the idea:
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index c431b33..2163de8 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -502,6 +502,8 @@ struct kvm_vcpu_arch {
> u64 msr_val;
> struct gfn_to_hva_cache data;
> } pv_eoi;
> +
> + gfn_t pt_gfns[4];
> };
>
> struct kvm_lpage_info {
> diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
> index 0453fa0..ac4210f 100644
> --- a/arch/x86/kvm/paging_tmpl.h
> +++ b/arch/x86/kvm/paging_tmpl.h
> @@ -523,6 +523,18 @@ FNAME(is_self_change_mapping)(struct kvm_vcpu *vcpu,
> return false;
> }
>
> +static void FNAME(cache_pt_gfns)(struct kvm_vcpu *vcpu, struct guest_walker *walker)
> +{
> + int level;
> +
> + /* Reset all gfns to -1, then we can detect the levels which is not used in guest. */
> + for (level = 0; level < 4; level++)
> + vcpu->arch.pt_gfns[level] = (gfn_t)(-1);
> +
> + for (level = walker->level; level <= walker->max_level; level++)
> + vcpu->arch.pt_gfns[level - 1] = walker->table_gfn[level - 1];
> +}
> +
> /*
> * Page fault handler. There are several causes for a page fault:
> * - there is no shadow pte for the guest pte
> @@ -576,6 +588,8 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gva_t addr, u32 error_code,
> return 0;
> }
>
> + FNAME(cache_pt_gfns)(vcpu, &walker);
> +
> 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);
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index b0a3678..b86ee24 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4753,18 +4753,25 @@ static int handle_emulation_failure(struct kvm_vcpu *vcpu)
> return r;
> }
>
> +static bool is_gfn_used_as_pt(struct kvm_vcpu *vcpu, gfn_t gfn)
> +{
> + int level;
> +
> + for (level = 0; level < 4; level++) {
> + if (vcpu->arch.pt_gfns[level] == (gfn_t)-1)
> + continue;
> + if (gfn == vcpu->arch.pt_gfns[level])
> + return true;
> + }
> +
> + return false;
> +}
> +
> 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) {
> /*
> @@ -4781,13 +4788,7 @@ static bool reexecute_instruction(struct kvm_vcpu *vcpu, unsigned long cr2)
> 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;
> + gfn = gpa_to_gfn(gpa);
>
> /*
> * Do not retry the unhandleable instruction if it faults on the
> @@ -4795,13 +4796,38 @@ 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);
> +
> + /* If the target gfn is used as page table, the fault can
> + * not be avoided by unprotecting shadow page and it will
> + * be reported to userspace.
> + */
> + return !is_gfn_used_as_pt(vcpu, gfn);
> }
>
> static bool retry_instruction(struct x86_emulate_ctxt *ctxt,
>
>
> You can see we need to record more things in the vcpu struct (bool vs. gfn_t [4])
> and my patch can fold is_gfn_used_as_pt into a existed function FNAME(is_self_change_mapping).
>
> Hmm?

Yes, its not needed. But its not clear where target_gfn_is_pt is reset.
Also please use a more descriptive name, such as
"bool write_fault_to_shadow_pgtable".

Please use coding style which is easier for humans to parse, overall.