linux-kernel - Re: [PATCH v5 29/36] KVM: arm64: Use page-table to track page ownership

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Date:   Mon, 15 Mar 2021 16:36:19 +0000
From:   Will Deacon <will@...nel.org>
To:     Quentin Perret <qperret@...gle.com>
Cc:     catalin.marinas@....com, maz@...nel.org, james.morse@....com,
        julien.thierry.kdev@...il.com, suzuki.poulose@....com,
        android-kvm@...gle.com, seanjc@...gle.com, mate.toth-pal@....com,
        linux-kernel@...r.kernel.org, robh+dt@...nel.org,
        linux-arm-kernel@...ts.infradead.org, kernel-team@...roid.com,
        kvmarm@...ts.cs.columbia.edu, tabba@...gle.com, ardb@...nel.org,
        mark.rutland@....com, dbrazdil@...gle.com
Subject: Re: [PATCH v5 29/36] KVM: arm64: Use page-table to track page
 ownership

On Mon, Mar 15, 2021 at 02:35:29PM +0000, Quentin Perret wrote:
> As the host stage 2 will be identity mapped, all the .hyp memory regions
> and/or memory pages donated to protected guestis will have to marked
> invalid in the host stage 2 page-table. At the same time, the hypervisor
> will need a way to track the ownership of each physical page to ensure
> memory sharing or donation between entities (host, guests, hypervisor) is
> legal.
> 
> In order to enable this tracking at EL2, let's use the host stage 2
> page-table itself. The idea is to use the top bits of invalid mappings
> to store the unique identifier of the page owner. The page-table owner
> (the host) gets identifier 0 such that, at boot time, it owns the entire
> IPA space as the pgd starts zeroed.
> 
> Provide kvm_pgtable_stage2_set_owner() which allows to modify the
> ownership of pages in the host stage 2. It re-uses most of the map()
> logic, but ends up creating invalid mappings instead. This impacts
> how we do refcount as we now need to count invalid mappings when they
> are used for ownership tracking.
> 
> Signed-off-by: Quentin Perret <qperret@...gle.com>
> ---
>  arch/arm64/include/asm/kvm_pgtable.h |  21 +++++
>  arch/arm64/kvm/hyp/pgtable.c         | 127 ++++++++++++++++++++++-----
>  2 files changed, 124 insertions(+), 24 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> index 4ae19247837b..683e96abdc24 100644
> --- a/arch/arm64/include/asm/kvm_pgtable.h
> +++ b/arch/arm64/include/asm/kvm_pgtable.h
> @@ -238,6 +238,27 @@ int kvm_pgtable_stage2_map(struct kvm_pgtable *pgt, u64 addr, u64 size,
>  			   u64 phys, enum kvm_pgtable_prot prot,
>  			   void *mc);
>  
> +/**
> + * kvm_pgtable_stage2_set_owner() - Annotate invalid mappings with metadata
> + *				    encoding the ownership of a page in the
> + *				    IPA space.

The function does more than this, though, as it will also go ahead and unmap
existing valid mappings which I think should be mentioned here, no?

> +int kvm_pgtable_stage2_set_owner(struct kvm_pgtable *pgt, u64 addr, u64 size,
> +				 void *mc, u8 owner_id)
> +{
> +	int ret;
> +	struct stage2_map_data map_data = {
> +		.phys		= KVM_PHYS_INVALID,
> +		.mmu		= pgt->mmu,
> +		.memcache	= mc,
> +		.mm_ops		= pgt->mm_ops,
> +		.owner_id	= owner_id,
> +	};
> +	struct kvm_pgtable_walker walker = {
> +		.cb		= stage2_map_walker,
> +		.flags		= KVM_PGTABLE_WALK_TABLE_PRE |
> +				  KVM_PGTABLE_WALK_LEAF |
> +				  KVM_PGTABLE_WALK_TABLE_POST,
> +		.arg		= &map_data,
> +	};
> +
> +	if (owner_id > KVM_MAX_OWNER_ID)
> +		return -EINVAL;
> +
> +	ret = kvm_pgtable_walk(pgt, addr, size, &walker);
> +	dsb(ishst);

Why is the DSB needed here? afaict, we only ever unmap a valid entry (which
will have a DSB as part of the TLBI sequence) or we update the owner for an
existing invalid entry, in which case the walker doesn't care.

Will