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

[Marcelo Tosatti]

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.

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