linux-kernel - Re: [PATCH 2/7] KVM: MMU: document clear_spte

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Wed, 19 Jun 2013 13:32:57 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
CC:	gleb@...hat.com, avi.kivity@...il.com, mtosatti@...hat.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH 2/7] KVM: MMU: document clear_spte_count

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@...ux.vnet.ibm.com>
> ---
>  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)
>  {
> 

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/