Subject: Re: [PATCH 10/13] KVM: Provide common implementation for generic dirty log functions

From: Sean Christopherson <[email-masked]>
Date: Thu, 19 Sep 2019 12:39:15 -0700

On Thu, Sep 19, 2019 at 10:22:42AM +1000, Paul Mackerras wrote:
> On Wed, Sep 11, 2019 at 11:50:35AM -0700, Sean Christopherson wrote:
> > Move the implementations of KVM_GET_DIRTY_LOG and KVM_CLEAR_DIRTY_LOG
> > for CONFIG_KVM_GENERIC_DIRTYLOG_READ_PROTECT into common KVM code.
> > The arch specific implemenations are extremely similar, differing
> > only in whether the dirty log needs to be sync'd from hardware (x86)
> > and how the TLBs are flushed.  Add new arch hooks to handle sync
> > and TLB flush; the sync will also be used for non-generic dirty log
> > support in a future patch (s390).
> > 
> > The ulterior motive for providing a common implementation is to
> > eliminate the dependency between arch and common code with respect to
> > the memslot referenced by the dirty log, i.e. to make it obvious in the
> > code that the validity of the memslot is guaranteed, as a future patch
> > will rework memslot handling such that id_to_memslot() can return NULL.
> 
> I notice you add empty definitions of kvm_arch_sync_dirty_log() for
> PPC, both Book E and Book 3S.  Given that PPC doesn't select
> CONFIG_KVM_GENERIC_DIRTYLOG_READ_PROTECT, why is this necessary?

s390 has a non-empty kvm_arch_sync_dirty_log() but doesn't select
CONFIG_KVM_GENERIC_DIRTYLOG_READ_PROTECT.  Patch 11/13 moves s390's call
of kvm_arch_sync_dirty_log() from s390's kvm_vm_ioctl_get_dirty_log() into
the common (but not "generic") kvm_get_dirty_log() so that it's obvious
that kvm_vm_ioctl_get_dirty_log() and kvm_get_dirty_log() are operating on
the same memslot, i.e. aren't independently querying id_to_memslot().

I originally made kvm_arch_sync_dirty_log() opt-in with a __KVM_HAVE_ARCH
macro, but the resulting #ifdeffery felt uglier than having PPC and ARM
provide empty functions.

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