Re: [PATCH 2/8] KVM: x86: extend struct kvm_vcpu_pv_apf_data with token info

Vivek Goyal
2020-05-21T18:38:40Z

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@redhat.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