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

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Date:   Tue, 12 May 2020 10:50:17 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Vivek Goyal <vgoyal@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>, 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>,
        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 Tue, May 12, 2020 at 11:53:39AM -0400, Vivek Goyal wrote:
> On Tue, May 12, 2020 at 05:40:10PM +0200, Vitaly Kuznetsov wrote:
> > Vivek Goyal <vgoyal@...hat.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@...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.
> > 
> > I added 'pageready_' prefix to make it clear this is not used for 'page
> > not present' notifications where we pass token through CR2. (BTW
> > 'reason' also becomes a misnomer because we can only see
> > 'KVM_PV_REASON_PAGE_NOT_PRESENT' there.)
> 
> Sure. I am just trying to keep names in such a way so that we could
> deliver more events and not keep it too tightly coupled with only
> two events (page not present, page ready).
> 
> > 
> > I have no strong opinion, can definitely rename this to 'token' and add
> > a line to the documentation to re-state that this is not used for type 1
> > events.
> 
> I don't even know why are we calling "type 1" and "type 2" event. Calling
> it KVM_PV_REASON_PAGE_NOT_PRESENT  and KVM_PV_REASON_PAGE_READY event
> is much more intuitive. If somebody is confused about how event will
> be delivered, that could be part of documentation. And "type1" and "type2"
> does not say anything about delivery method anyway.
> 
> Also, type of event should not necessarily be tied to delivery method.
> For example if we end up introducing say, "KVM_PV_REASON_PAGE_ERROR", then
> I would think that event can be injected both using exception (#PF or #VE)
> as well as interrupt (depending on state of system).

Why bother preserving backwards compatibility?  AIUI, both KVM and guest
will support async #PF iff interrupt delivery is enabled.  Why not make
the interrupt delivery approach KVM_ASYNC_PF_V2 and completely redefine the
ABI?  E.g. to make it compatible with reflecting !PRESENT faults without a
VM-Exit via Intel's EPT Violation #VE?