linux-kernel - Re: [PATCH v3 02/15] KVM: arm64: Add "struct kvm_page

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Message-ID: <aFMs9_GBVsiRmgz7@google.com>
Date: Wed, 18 Jun 2025 14:17:43 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Oliver Upton <oliver.upton@...ux.dev>
Cc: James Houghton <jthoughton@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	Jonathan Corbet <corbet@....net>, Marc Zyngier <maz@...nel.org>, Yan Zhao <yan.y.zhao@...el.com>, 
	Nikita Kalyazin <kalyazin@...zon.com>, Anish Moorthy <amoorthy@...gle.com>, 
	Peter Gonda <pgonda@...gle.com>, Peter Xu <peterx@...hat.com>, 
	David Matlack <dmatlack@...gle.com>, wei.w.wang@...el.com, kvm@...r.kernel.org, 
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev
Subject: Re: [PATCH v3 02/15] KVM: arm64: Add "struct kvm_page_fault" to
 gather common fault variables

On Wed, Jun 18, 2025, Oliver Upton wrote:
> On Wed, Jun 18, 2025 at 04:24:11AM +0000, James Houghton wrote:
> > From: Sean Christopherson <seanjc@...gle.com>
> > 
> > Introduce "struct kvm_page_fault" and use it in user_mem_abort() in lieu
> > of a collection of local variables.  Providing "struct kvm_page_fault"
> > will allow common KVM to provide APIs to take in said structure, e.g. when
> > preparing memory fault exits.
> > 
> > No functional change intended.
> > 
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > Signed-off-by: James Houghton <jthoughton@...gle.com>
> > ---
> >  arch/arm64/include/asm/kvm_host.h |  9 +++++++++
> >  arch/arm64/kvm/mmu.c              | 32 +++++++++++++++++--------------
> >  2 files changed, 27 insertions(+), 14 deletions(-)
> > 
> > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > index 6ce2c51734820..ae83d95d11b74 100644
> > --- a/arch/arm64/include/asm/kvm_host.h
> > +++ b/arch/arm64/include/asm/kvm_host.h
> > @@ -413,6 +413,15 @@ struct kvm_vcpu_fault_info {
> >  	u64 disr_el1;		/* Deferred [SError] Status Register */
> >  };
> >  
> > +struct kvm_page_fault {
> > +	const bool exec;
> > +	const bool write;
> > +	const bool is_private;
> > +
> > +	gfn_t gfn;
> > +	struct kvm_memory_slot *slot;
> > +};
> > +
> 
> So this seems to cherry-pick "interesting" values into the structure but

s/interesting/necessary.  I literally grabbed only the fields that were needed
to handle the userfault :-)

> leaves the rest of the abort context scattered about in locals. If we're
> going to do something like this I'd rather have a wholesale refactoring
> than just the bits to intersect with x86 (more on that later...)

Definitely no objection from me.  How about I send a standalone patch so that we
can iterate on just that without James having to respin the entire series (for
changes that have no meaningful impact)?  I'm anticipating we'll need a few rounds
to strike the right balance between what was done here and "throw everything into
kvm_page_fault".  E.g. we probably don't want "vma" in kvm_page_fault.

Hmm, yeah, and I suspect/hope that moving more things into kvm_page_fault will
allow for encapsulating the mmap_read_lock() section in a helper without having
a bajillion boolean flags being passed around.  That would obviate the need to
nullify vma, because it would simply go out of scope.