linux-kernel - Re: [PATCH v3 1/1] KVM: arm64: Allow cacheable stage 2 mapping using VMA flags

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Message-ID: <Z_QAxiEWEyMpfLgL@google.com>
Date: Mon, 7 Apr 2025 09:43:50 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Marc Zyngier <maz@...nel.org>, Ankit Agrawal <ankita@...dia.com>, 
	Catalin Marinas <catalin.marinas@....com>, Oliver Upton <oliver.upton@...ux.dev>, 
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	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v3 1/1] KVM: arm64: Allow cacheable stage 2 mapping using
 VMA flags

On Mon, Apr 07, 2025, Jason Gunthorpe wrote:
> On Mon, Apr 07, 2025 at 08:20:09AM -0700, Sean Christopherson wrote:
> > On Mon, Mar 31, 2025, Jason Gunthorpe wrote:
> > > On Wed, Mar 26, 2025 at 11:24:32AM -0700, Sean Christopherson wrote:
> > > > > I don't know how you reconcile the lack of host mapping and cache
> > > > > maintenance. The latter cannot take place without the former.
> > > > 
> > > > I assume cache maintenance only requires _a_ mapping to the physical memory.
> > > > With guest_memfd, KVM has the pfn (which happens to always be struct page memory
> > > > today), and so can establish a VA=>PA mapping as needed.
> > > 
> > > This is why we are forcing FWB in this work, because we don't have a
> > > VA mapping and KVM doesn't have the code to create one on demand.
> > 
> > I don't follow.  As it exists today, guest_memfd doesn't touch the direct map,
> > i.e. there's already a kernel mapping, KVM doesn't need to create one.
> 
> This is not about guest_memfd..

Heh, my part of the thread was.  I was responding to Marc's comment:

 : Remind me how this work with stuff such as guestmemfd, which, by
 : definition, doesn't have a userspace mapping?

I'm pretty sure we're on the same page.