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

From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Vivek Goyal <vgoyal@redhat.com>
Cc: kvm@vger.kernel.org, x86@kernel.org,
	Paolo Bonzini <pbonzini@redhat.com>,
	Andy Lutomirski <luto@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Sean Christopherson <sean.j.christopherson@intel.com>,
	Jim Mattson <jmattson@google.com>, Gavin Shan <gshan@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/8] KVM: x86: extend struct kvm_vcpu_pv_apf_data with token info
Date: Sat, 23 May 2020 18:34:17 +0200	[thread overview]
Message-ID: <87sgfq8vau.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <20200521183832.GB46035@redhat.com>

Vivek Goyal <vgoyal@redhat.com> writes:

> 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).

Yes, I think we can do that. The existing KVM_PV_REASON_PAGE_READY and
KVM_PV_REASON_PAGE_NOT_PRESENT are mutually exclusive and can be
converted to flags (we'll only have KVM_PV_REASON_PAGE_NOT_PRESENT in
use when this series is merged).

>
>> -	__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.

In case we're intended to pass more data in synchronous notifications,
shall we leave some blank space after 'flags' ('reason' previously) and
before 'token'?

-- 
Vitaly