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

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Date:   Thu, 21 May 2020 14:38:32 -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;

Hi Vitaly,

Given we are redoing it, can we convert "reason" into a flag instead
and use bit 0 for signalling "page not present" Then rest of the 31
bits can be used for other purposes. I potentially want to use one bit to
signal error (if it is known at the time of injecting #PF).

> -	__u8 pad[60];
> +	__u32 pageready_token;
> +	__u8 pad[56];

Given token is 32 bit, for returning error in "page ready" type messages,
I will probably use padding bytes and create pagready_flag and use one
of the bits to signal error.

Thanks
Vivek