From: Andy Lutomirski <luto@kernel.org>
To: Andy Lutomirski <luto@kernel.org>
Cc: LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
kvm list <kvm@vger.kernel.org>,
Paolo Bonzini <pbonzini@redhat.com>,
stable <stable@vger.kernel.org>
Subject: Re: [PATCH v2] x86/kvm: Disable KVM_ASYNC_PF_SEND_ALWAYS
Date: Sat, 7 Mar 2020 07:03:24 -0800 [thread overview]
Message-ID: <CALCETrVmsF9JSMLSd44-3GGWEz6siJQxudeaYiVnvv__YDT1BQ@mail.gmail.com> (raw)
In-Reply-To: <ed71d0967113a35f670a9625a058b8e6e0b2f104.1583547991.git.luto@kernel.org>
On Fri, Mar 6, 2020 at 6:26 PM Andy Lutomirski <luto@kernel.org> wrote:
>
> The ABI is broken and we cannot support it properly. Turn it off.
>
> If this causes a meaningful performance regression for someone, KVM
> can introduce an improved ABI that is supportable.
>
> Cc: stable@vger.kernel.org
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> ---
> arch/x86/kernel/kvm.c | 21 ++++++++++++++++++---
> 1 file changed, 18 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 93ab0cbd304e..e6f2aefa298b 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -318,11 +318,26 @@ static void kvm_guest_cpu_init(void)
>
> pa = slow_virt_to_phys(this_cpu_ptr(&apf_reason));
>
> -#ifdef CONFIG_PREEMPTION
> - pa |= KVM_ASYNC_PF_SEND_ALWAYS;
> -#endif
> pa |= KVM_ASYNC_PF_ENABLED;
>
> + /*
> + * We do not set KVM_ASYNC_PF_SEND_ALWAYS. With the current
> + * KVM paravirt ABI, the following scenario is possible:
> + *
> + * #PF: async page fault (KVM_PV_REASON_PAGE_NOT_PRESENT)
> + * NMI before CR2 or KVM_PF_REASON_PAGE_NOT_PRESENT
> + * NMI accesses user memory, e.g. due to perf
> + * #PF: normal page fault
> + * #PF reads CR2 and apf_reason -- apf_reason should be 0
> + *
> + * outer #PF reads CR2 and apf_reason -- apf_reason should be
> + * KVM_PV_REASON_PAGE_NOT_PRESENT
> + *
> + * There is no possible way that both reads of CR2 and
> + * apf_reason get the correct values. Fixing this would
> + * require paravirt ABI changes.
> + */
> +
Upon re-reading my own comment, I think the problem is real, but I
don't think my patch fixes it. The outer #PF could just as easily
have come from user mode. We may actually need the NMI code (and
perhaps MCE and maybe #DB too) to save, clear, and restore apf_reason.
If we do this, then maybe CPL0 async PFs are actually okay, but the
semantics are so poorly defined that I'm not very confident about
that.
next prev parent reply other threads:[~2020-03-07 15:03 UTC|newest]
Thread overview: 55+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-07 2:26 [PATCH v2] x86/kvm: Disable KVM_ASYNC_PF_SEND_ALWAYS Andy Lutomirski
2020-03-07 15:03 ` Andy Lutomirski [this message]
2020-03-07 15:47 ` Thomas Gleixner
2020-03-07 15:59 ` Andy Lutomirski
2020-03-07 19:01 ` Thomas Gleixner
2020-03-07 19:34 ` Andy Lutomirski
2020-03-08 7:23 ` Thomas Gleixner
2020-03-09 6:57 ` Thomas Gleixner
2020-03-09 8:40 ` Paolo Bonzini
2020-03-09 9:09 ` Thomas Gleixner
2020-03-09 18:14 ` Andy Lutomirski
2020-03-09 19:05 ` Thomas Gleixner
2020-03-09 20:22 ` Peter Zijlstra
2020-04-06 19:09 ` Vivek Goyal
2020-04-06 20:25 ` Peter Zijlstra
2020-04-06 20:32 ` Andy Lutomirski
2020-04-06 20:42 ` Andy Lutomirski
2020-04-07 17:21 ` Vivek Goyal
2020-04-07 17:38 ` Andy Lutomirski
2020-04-07 20:20 ` Thomas Gleixner
2020-04-07 21:41 ` Andy Lutomirski
2020-04-07 22:07 ` Paolo Bonzini
2020-04-07 22:29 ` Andy Lutomirski
2020-04-08 0:30 ` Paolo Bonzini
2020-05-21 15:55 ` Vivek Goyal
2020-04-07 22:48 ` Thomas Gleixner
2020-04-08 4:48 ` Andy Lutomirski
2020-04-08 9:32 ` Borislav Petkov
2020-04-08 10:12 ` Thomas Gleixner
2020-04-08 18:23 ` Vivek Goyal
2020-04-07 22:49 ` Vivek Goyal
2020-04-08 10:01 ` Borislav Petkov
2020-04-07 22:04 ` Paolo Bonzini
2020-04-07 23:21 ` Thomas Gleixner
2020-04-08 8:23 ` Paolo Bonzini
2020-04-08 13:01 ` Thomas Gleixner
2020-04-08 15:38 ` Peter Zijlstra
2020-04-08 16:41 ` Thomas Gleixner
2020-04-09 9:03 ` Paolo Bonzini
2020-04-08 15:34 ` Sean Christopherson
2020-04-08 16:50 ` Paolo Bonzini
2020-04-08 18:01 ` Thomas Gleixner
2020-04-08 20:34 ` Vivek Goyal
2020-04-08 23:06 ` Thomas Gleixner
2020-04-08 23:14 ` Thomas Gleixner
2020-04-09 4:50 ` Andy Lutomirski
2020-04-09 9:43 ` Paolo Bonzini
2020-04-09 11:36 ` Andrew Cooper
2020-04-09 12:47 ` Paolo Bonzini
2020-04-09 14:13 ` Andrew Cooper
2020-04-09 14:32 ` Paolo Bonzini
2020-04-09 15:03 ` Andy Lutomirski
2020-04-09 15:17 ` Paolo Bonzini
2020-04-09 17:32 ` Andy Lutomirski
2020-04-06 21:32 ` Thomas Gleixner
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