linux-kernel - Re: [PATCH 2/2] KVM: async_pf: Inject 'page ready' event only if 'page not present' was previously injected

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Date:   Thu, 11 Jun 2020 10:14:11 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Vivek Goyal <vgoyal@...hat.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] KVM: async_pf: Inject 'page ready' event only if 'page not present' was previously injected

Vivek Goyal <vgoyal@...hat.com> writes:

> On Wed, Jun 10, 2020 at 07:55:32PM +0200, Vitaly Kuznetsov wrote:
>> 'Page not present' event may or may not get injected depending on
>> guest's state. If the event wasn't injected, there is no need to
>> inject the corresponding 'page ready' event as the guest may get
>> confused. E.g. Linux thinks that the corresponding 'page not present'
>> event wasn't delivered *yet* and allocates a 'dummy entry' for it.
>> This entry is never freed.
>> 
>> Note, 'wakeup all' events have no corresponding 'page not present'
>> event and always get injected.
>> 
>> s390 seems to always be able to inject 'page not present', the
>> change is effectively a nop.
>> 
>> Suggested-by: Vivek Goyal <vgoyal@...hat.com>
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>> ---
>>  arch/s390/include/asm/kvm_host.h | 2 +-
>>  arch/s390/kvm/kvm-s390.c         | 4 +++-
>>  arch/x86/include/asm/kvm_host.h  | 2 +-
>>  arch/x86/kvm/x86.c               | 7 +++++--
>>  include/linux/kvm_host.h         | 1 +
>>  virt/kvm/async_pf.c              | 2 +-
>>  6 files changed, 12 insertions(+), 6 deletions(-)
>> 
>> diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h
>> index 3d554887794e..cee3cb6455a2 100644
>> --- a/arch/s390/include/asm/kvm_host.h
>> +++ b/arch/s390/include/asm/kvm_host.h
>> @@ -978,7 +978,7 @@ bool kvm_arch_can_dequeue_async_page_present(struct kvm_vcpu *vcpu);
>>  void kvm_arch_async_page_ready(struct kvm_vcpu *vcpu,
>>  			       struct kvm_async_pf *work);
>>  
>> -void kvm_arch_async_page_not_present(struct kvm_vcpu *vcpu,
>> +bool kvm_arch_async_page_not_present(struct kvm_vcpu *vcpu,
>>  				     struct kvm_async_pf *work);
>
> Hi Vitaly,
>
> A minor nit. Using return code to figure out if exception was injected
> or not is little odd. How about we pass a pointer instead as parameter
> and kvm_arch_async_page_not_present() sets it to true if page not
> present exception was injected. This probably will be easier to
> read.
>
> If for some reason you don't like above, atleats it warrants a comment
> explaining what do 0 and 1 mean.
>

I think it's the 'kvm_arch_async_page_not_present' name which is a bit
misleading now, if we rename it to something like
kvm_arch_inject_apf_not_present() then it becomes a bit more clear
what's going on. We may as well write the code as

    if (kvm_arch_inject_apf_not_present())
        work->notpresent_injected = true;

or change the return type to int so it'll be

    if (!kvm_arch_inject_apf_not_present())
        work->notpresent_injected = true;

> Otherwise both the patches look good to me. I tested and I can confirm
> that now page ready events are not being delivered to guest if page
> not present was not injected.

Thank you for testing!

-- 
Vitaly