linux-kernel - Re: [PATCH 4/6] KVM: arm64: Move hyp state to hyp

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Message-ID: <865xk9nsh5.wl-maz@kernel.org>
Date: Sun, 16 Mar 2025 11:08:06 +0000
From: Marc Zyngier <maz@...nel.org>
To: Quentin Perret <qperret@...gle.com>
Cc: Oliver Upton <oliver.upton@...ux.dev>,
	Joey Gouly <joey.gouly@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Zenghui Yu <yuzenghui@...wei.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>,
	Vincent Donnefort <vdonnefort@...gle.com>,
	linux-arm-kernel@...ts.infradead.org,
	kvmarm@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/6] KVM: arm64: Move hyp state to hyp_vmemmap

On Fri, 14 Mar 2025 14:06:48 +0000,
Quentin Perret <qperret@...gle.com> wrote:
> 
> On Friday 14 Mar 2025 at 11:31:36 (+0000), Marc Zyngier wrote:
> > On Thu, 27 Feb 2025 00:33:08 +0000,
> > Quentin Perret <qperret@...gle.com> wrote:
> > > @@ -698,13 +697,13 @@ int __pkvm_host_share_hyp(u64 pfn)
> > >  	if (ret)
> > >  		goto unlock;
> > >  	if (IS_ENABLED(CONFIG_NVHE_EL2_DEBUG)) {
> > > -		ret = __hyp_check_page_state_range((u64)virt, size, PKVM_NOPAGE);
> > > +		ret = __hyp_check_page_state_range(phys, size, PKVM_NOPAGE);
> > 
> > OK, I think I finally clicked here. Does it mean that all the tracking
> > is now done in terms of PAs instead of VAs?
> 
> Yep, that's exactly that. The hyp_vmemmap is indexed by pfn, so I felt
> that the conversion to a PA-based tracking made sense. That also make it
> clear that the 'hyp state' is not a property of a mapping, but really of
> the underlying physical page.

It indeed makes sense. It is just that it took me some time to realise
the extent of the change.

> 
> > >  		if (ret)
> > >  			goto unlock;
> > >  	}
> > >  
> > > -	prot = pkvm_mkstate(PAGE_HYP, PKVM_PAGE_SHARED_BORROWED);
> > > -	WARN_ON(pkvm_create_mappings_locked(virt, virt + size, prot));
> > > +	__hyp_set_page_state_range(phys, size, PKVM_PAGE_SHARED_BORROWED);
> > > +	WARN_ON(pkvm_create_mappings_locked(virt, virt + size, PAGE_HYP));
> > 
> > And this is the split between the state now being kept in the on a PA
> > base and the actual mapping that is now only takes the page attributes
> > and no SW bits?
> 
> Precisely, and the next patch in this series takes advantage of the
> fact that we're now de-correlating the hyp state from the presence of a
> hyp s1 mapping in the linear map range. In the future there'll be more
> use-cases for this I think (e.g. the hyp allocator where we'll have
> pages owned by the hypervisor but only mapped in the 'private' range,
> things like that).

Yup, that's probably the correct direction of travel. The hypervisor
shouldn't need to map everything -- quite the opposite actually.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.