linux-kernel - Re: [PATCH] x86/kvm: Disable KVM_ASYNC_PF_SEND

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Date:   Sat, 7 Mar 2020 06:36:35 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        kvm list <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>, stable@...r.kernel.org
Subject: Re: [PATCH] x86/kvm: Disable KVM_ASYNC_PF_SEND_ALWAYS


> On Mar 7, 2020, at 2:09 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> Andy Lutomirski <luto@...nel.org> writes:
> 
>> 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@...r.kernel.org
>> Signed-off-by: Andy Lutomirski <luto@...nel.org>
>> ---
>> arch/x86/kernel/kvm.c | 11 ++++++++---
>> 1 file changed, 8 insertions(+), 3 deletions(-)
>> 
>> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
>> index 93ab0cbd304e..71f9f39f93da 100644
>> --- a/arch/x86/kernel/kvm.c
>> +++ b/arch/x86/kernel/kvm.c
>> @@ -318,11 +318,16 @@ 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, if an async page fault occurs on an early
>> +         * memory access in the normal (sync) #PF path or in an NMI
>> +         * that happens early in the #PF code, the combination of CR2
>> +         * and the APF reason field will be corrupted.
> 
> I don't think this can happen. In both cases IF == 0 and that async
> (think host side) page fault will be completely handled on the
> host. There is no injection happening in such a case ever. If it does,
> then yes the host side implementation is buggered, but AFAICT this is
> not the case.

Indeed. But read v2 please.

> 
> See also my reply in the other thread:
> 
>  https://lore.kernel.org/r/87r1y4a3gw.fsf@nanos.tec.linutronix.de
> 
> Thanks,
> 
>        tglx