From accaefe07ddbeb12c0de4cec1d62dba6a0ea1605 Mon Sep 17 00:00:00 2001 From: Xiao Guangrong Date: Wed, 19 Jun 2013 17:09:20 +0800 Subject: KVM: MMU: document clear_spte_count Document it to Documentation/virtual/kvm/mmu.txt Signed-off-by: Xiao Guangrong Signed-off-by: Paolo Bonzini --- arch/x86/include/asm/kvm_host.h | 4 ++++ arch/x86/kvm/mmu.c | 17 ++++++++++++++--- 2 files changed, 18 insertions(+), 3 deletions(-) (limited to 'arch') diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h index 966f2650b6ab..5d28c11d5e21 100644 --- a/arch/x86/include/asm/kvm_host.h +++ b/arch/x86/include/asm/kvm_host.h @@ -226,6 +226,10 @@ struct kvm_mmu_page { DECLARE_BITMAP(unsync_child_bitmap, 512); #ifdef CONFIG_X86_32 + /* + * Used out of the mmu-lock to avoid reading spte values while an + * update is in progress; see the comments in __get_spte_lockless(). + */ int clear_spte_count; #endif diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c index 7113a0fb544c..f385a4cf4bfd 100644 --- a/arch/x86/kvm/mmu.c +++ b/arch/x86/kvm/mmu.c @@ -466,9 +466,20 @@ static u64 __update_clear_spte_slow(u64 *sptep, u64 spte) /* * The idea using the light way get the spte on x86_32 guest is from * gup_get_pte(arch/x86/mm/gup.c). - * The difference is we can not catch the spte tlb flush if we leave - * guest mode, so we emulate it by increase clear_spte_count when spte - * is cleared. + * + * An spte tlb flush may be pending, because kvm_set_pte_rmapp + * coalesces them and we are running out of the MMU lock. Therefore + * we need to protect against in-progress updates of the spte. + * + * Reading the spte while an update is in progress may get the old value + * for the high part of the spte. The race is fine for a present->non-present + * change (because the high part of the spte is ignored for non-present spte), + * but for a present->present change we must reread the spte. + * + * All such changes are done in two steps (present->non-present and + * non-present->present), hence it is enough to count the number of + * present->non-present updates: if it changed while reading the spte, + * we might have hit the race. This is done using clear_spte_count. */ static u64 __get_spte_lockless(u64 *sptep) { -- cgit 1.4.1