Re: [PATCH 2/7] KVM: X86: Synchronize the shadow pagetable before link it

[Sean Christopherson]

On Thu, Sep 02, 2021, Sean Christopherson wrote:
> On Tue, Aug 24, 2021, Lai Jiangshan wrote:
> Rather than force the sync here, what about kicking all vCPUs and retrying the
> page fault?  The only gross part is that kvm_mmu_get_page() can now fail :-(
> 
> ---
>  arch/x86/include/asm/kvm_host.h | 3 ++-
>  arch/x86/kvm/mmu/mmu.c          | 9 +++++++--
>  arch/x86/kvm/mmu/paging_tmpl.h  | 4 ++++
>  3 files changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 09b256db394a..332b9fb3454c 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -57,7 +57,8 @@
>  #define KVM_REQ_MIGRATE_TIMER		KVM_ARCH_REQ(0)
>  #define KVM_REQ_REPORT_TPR_ACCESS	KVM_ARCH_REQ(1)
>  #define KVM_REQ_TRIPLE_FAULT		KVM_ARCH_REQ(2)
> -#define KVM_REQ_MMU_SYNC		KVM_ARCH_REQ(3)
> +#define KVM_REQ_MMU_SYNC \
> +	KVM_ARCH_REQ_FLAGS(3, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)
>  #define KVM_REQ_CLOCK_UPDATE		KVM_ARCH_REQ(4)
>  #define KVM_REQ_LOAD_MMU_PGD		KVM_ARCH_REQ(5)
>  #define KVM_REQ_EVENT			KVM_ARCH_REQ(6)
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 4853c033e6ce..03293cd3c7ae 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -2143,8 +2143,10 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
>  			kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu);
>  		}
> 
> -		if (sp->unsync_children)
> -			kvm_make_request(KVM_REQ_MMU_SYNC, vcpu);
> +		if (sp->unsync_children) {
> +			kvm_make_all_cpus_request(KVM_REQ_MMU_SYNC, vcpu);
> +			return NULL;
> +		}
> 
>  		__clear_sp_write_flooding_count(sp);
> 
> @@ -2999,6 +3001,7 @@ static int __direct_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> 
>  		sp = kvm_mmu_get_page(vcpu, base_gfn, it.addr,
>  				      it.level - 1, true, ACC_ALL);
> +		BUG_ON(!sp);
> 
>  		link_shadow_page(vcpu, it.sptep, sp);
>  		if (fault->is_tdp && fault->huge_page_disallowed &&
> @@ -3383,6 +3386,8 @@ static hpa_t mmu_alloc_root(struct kvm_vcpu *vcpu, gfn_t gfn, gva_t gva,
>  	struct kvm_mmu_page *sp;
> 
>  	sp = kvm_mmu_get_page(vcpu, gfn, gva, level, direct, ACC_ALL);
> +	BUG_ON(!sp);

Gah, this is obviously wrong when allocating an indirect root.  On the happy side,
it points out a cleaner approach.  I think this is what we want?

---
 arch/x86/kvm/mmu/mmu.c         | 3 ---
 arch/x86/kvm/mmu/paging_tmpl.h | 4 ++++
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 4853c033e6ce..f24e8088192c 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -2143,9 +2143,6 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
 			kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu);
 		}

-		if (sp->unsync_children)
-			kvm_make_request(KVM_REQ_MMU_SYNC, vcpu);
-
 		__clear_sp_write_flooding_count(sp);

 trace_get_page:
diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
index 50ade6450ace..5b13918a55c2 100644
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -704,6 +704,10 @@ static int FNAME(fetch)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
 			access = gw->pt_access[it.level - 2];
 			sp = kvm_mmu_get_page(vcpu, table_gfn, fault->addr,
 					      it.level-1, false, access);
+			if (sp->unsync_children) {
+				kvm_make_all_cpus_request(KVM_REQ_MMU_SYNC, vcpu);
+				return RET_PF_RETRY;
+			}
 		}

 		/*
--