linux-kernel - Re: [PATCH 2/8] KVM: x86: extend struct kvm_vcpu_pv_apf

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Date:   Tue, 12 May 2020 11:27:09 -0400
From:   Vivek Goyal <vgoyal@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Jim Mattson <jmattson@...gle.com>,
        Gavin Shan <gshan@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/8] KVM: x86: extend struct kvm_vcpu_pv_apf_data with
 token info

On Mon, May 11, 2020 at 06:47:46PM +0200, Vitaly Kuznetsov wrote:
> Currently, APF mechanism relies on the #PF abuse where the token is being
> passed through CR2. If we switch to using interrupts to deliver page-ready
> notifications we need a different way to pass the data. Extent the existing
> 'struct kvm_vcpu_pv_apf_data' with token information for page-ready
> notifications.
> 
> The newly introduced apf_put_user_ready() temporary puts both reason
> and token information, this will be changed to put token only when we
> switch to interrupt based notifications.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>  arch/x86/include/uapi/asm/kvm_para.h |  3 ++-
>  arch/x86/kvm/x86.c                   | 17 +++++++++++++----
>  2 files changed, 15 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
> index 2a8e0b6b9805..e3602a1de136 100644
> --- a/arch/x86/include/uapi/asm/kvm_para.h
> +++ b/arch/x86/include/uapi/asm/kvm_para.h
> @@ -113,7 +113,8 @@ struct kvm_mmu_op_release_pt {
>  
>  struct kvm_vcpu_pv_apf_data {
>  	__u32 reason;
> -	__u8 pad[60];
> +	__u32 pageready_token;

How about naming this just "token". That will allow me to deliver error
as well. pageready_token name seems to imply that this will always be
successful with page being ready.

And reason will tell whether page could successfully be ready or
it was an error. And token will help us identify the task which
is waiting for the event.

Thanks
Vivek