Commit accaefe0 authored by Xiao Guangrong's avatar Xiao Guangrong Committed by Gleb Natapov

KVM: MMU: document clear_spte_count

Document it to Documentation/virtual/kvm/mmu.txt
Signed-off-by: default avatarXiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent a8eca9dc
......@@ -210,6 +210,11 @@ Shadow pages contain the following information:
A bitmap indicating which sptes in spt point (directly or indirectly) at
pages that may be unsynchronized. Used to quickly locate all unsychronized
pages reachable from a given page.
clear_spte_count:
Only present on 32-bit hosts, where a 64-bit spte cannot be written
atomically. The reader uses this while running out of the MMU lock
to detect in-progress updates and retry them until the writer has
finished the write.
Reverse map
===========
......
......@@ -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
......
......@@ -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)
{
......
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