[PATCH] x86/kvm: Disable KVM_ASYNC_PF_SEND_ALWAYS

[PATCH] x86/kvm: Disable KVM_ASYNC_PF_SEND_ALWAYS

[Andy Lutomirski]

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 | 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.
+		 */
+
 		if (kvm_para_has_feature(KVM_FEATURE_ASYNC_PF_VMEXIT))
 			pa |= KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT;
 
-- 
2.24.1

Re: [PATCH] x86/kvm: Disable KVM_ASYNC_PF_SEND_ALWAYS

[Andy Lutomirski]

On Fri, Mar 6, 2020 at 6:16 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.

Ugh, disregard this patch.  I flubbed the comment, and I'll send v2 shortly.

Re: [PATCH] x86/kvm: Disable KVM_ASYNC_PF_SEND_ALWAYS

[Thomas Gleixner]

Andy Lutomirski <luto@kernel.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@vger.kernel.org
> Signed-off-by: Andy Lutomirski <luto@kernel.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.

See also my reply in the other thread:

  https://lore.kernel.org/r/87r1y4a3gw.fsf@nanos.tec.linutronix.de

Thanks,

        tglx

Re: [PATCH] x86/kvm: Disable KVM_ASYNC_PF_SEND_ALWAYS

[Andy Lutomirski]

> On Mar 7, 2020, at 2:09 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
> 
> Andy Lutomirski <luto@kernel.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@vger.kernel.org
>> Signed-off-by: Andy Lutomirski <luto@kernel.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