Il 19/06/2013 11:09, Xiao Guangrong ha scritto:
> Document it to Documentation/virtual/kvm/mmu.txt
While reviewing the docs, I looked at the code.
Why can't this happen?
CPU 1: __get_spte_lockless CPU 2: __update_clear_spte_slow
------------------------------------------------------------------------------
write low
read count
read low
read high
write high
check low and count
update count
The check passes, but CPU 1 read a "torn" SPTE.
It seems like this is the same reason why seqlocks do two version
updates, one before and one after, and make the reader check "version &
~1". But maybe I'm wrong.
Paolo
> Signed-off-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx>
> ---
> Documentation/virtual/kvm/mmu.txt | 4 ++++
> arch/x86/include/asm/kvm_host.h | 5 +++++
> arch/x86/kvm/mmu.c | 7 ++++---
> 3 files changed, 13 insertions(+), 3 deletions(-)
>
> diff --git a/Documentation/virtual/kvm/mmu.txt b/Documentation/virtual/kvm/mmu.txt
> index 869abcc..ce6df51 100644
> --- a/Documentation/virtual/kvm/mmu.txt
> +++ b/Documentation/virtual/kvm/mmu.txt
> @@ -210,6 +210,10 @@ 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:
> + It is only used on 32bit host which helps us to detect whether updating the
> + 64bit spte is complete so that we can avoid reading the truncated value out
> + of mmu-lock.
>
> Reverse map
> ===========
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 966f265..1dac2c1 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -226,6 +226,11 @@ struct kvm_mmu_page {
> DECLARE_BITMAP(unsync_child_bitmap, 512);
>
> #ifdef CONFIG_X86_32
> + /*
> + * Count after the page's spte has been cleared to avoid
> + * the truncated value is read out of mmu-lock.
> + * please 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 c87b19d..77d516c 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -464,9 +464,10 @@ 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.
> + * The difference is we can not immediately catch the spte tlb since
> + * kvm may collapse tlb flush some times. Please see kvm_set_pte_rmapp.
> + *
> + * We emulate it by increase clear_spte_count when spte is cleared.
> */
> static u64 __get_spte_lockless(u64 *sptep)
> {
>
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