linux-kernel - Re: [PATCH RFC 04/15] KVM: Implement ring-based dirty memory tracking

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Date:   Wed, 18 Dec 2019 14:24:20 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        "Tian, Kevin" <kevin.tian@...el.com>
Subject: Re: [PATCH RFC 04/15] KVM: Implement ring-based dirty memory tracking

On Wed, Dec 18, 2019 at 04:58:57PM -0500, Peter Xu wrote:
> On Tue, Dec 17, 2019 at 05:28:54PM +0100, Paolo Bonzini wrote:
> > On 17/12/19 17:24, Peter Xu wrote:
> > >> No, please pass it all the way down to the [&] functions but not to
> > >> kvm_write_guest_page.  Those should keep using vcpu->kvm.
> > > Actually I even wanted to refactor these helpers.  I mean, we have two
> > > sets of helpers now, kvm_[vcpu]_{read|write}*(), so one set is per-vm,
> > > the other set is per-vcpu.  IIUC the only difference of these two are
> > > whether we should consider ((vcpu)->arch.hflags & HF_SMM_MASK) or we
> > > just write to address space zero always.
> > 
> > Right.
> > 
> > > Could we unify them into a
> > > single set of helper (I'll just drop the *_vcpu_* helpers because it's
> > > longer when write) but we always pass in vcpu* as the first parameter?
> > > Then we add another parameter "vcpu_smm" to show whether we want to
> > > consider the HF_SMM_MASK flag.
> > 
> > You'd have to check through all KVM implementations whether you always
> > have the vCPU.  Also non-x86 doesn't have address spaces, and by the
> > time you add ", true" or ", false" it's longer than the "_vcpu_" you
> > have removed.  So, not a good idea in my opinion. :D
> 
> Well, now I've changed my mind. :) (considering that we still have
> many places that will not have vcpu*...)
> 
> I can simply add that "vcpu_smm" parameter to kvm_vcpu_write_*()
> without removing the kvm_write_*() helpers.  Then I'll be able to
> convert most of the kvm_write_*() (or its family) callers to
> kvm_vcpu_write*(..., vcpu_smm=false) calls where proper.
> 
> Would that be good?

I've lost track of the problem you're trying to solve, but if you do
something like "vcpu_smm=false", explicitly pass an address space ID
instead of hardcoding x86 specific SMM crud, e.g.

	kvm_vcpu_write*(..., as_id=0);