linux-kernel - Re: [PATCH 08/14] KVM: arm64: Add support for tagging shared pages in page-table

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Message-ID: <YPWe+W3QmeYHqre/@google.com>
Date:   Mon, 19 Jul 2021 16:49:13 +0100
From:   Quentin Perret <qperret@...gle.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     james.morse@....com, alexandru.elisei@....com,
        suzuki.poulose@....com, catalin.marinas@....com, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org, ardb@...nel.org, qwandor@...gle.com,
        tabba@...gle.com, dbrazdil@...gle.com, kernel-team@...roid.com
Subject: Re: [PATCH 08/14] KVM: arm64: Add support for tagging shared pages
 in page-table

On Monday 19 Jul 2021 at 15:43:34 (+0100), Marc Zyngier wrote:
> On Mon, 19 Jul 2021 11:47:29 +0100,
> Quentin Perret <qperret@...gle.com> wrote:
> > 
> > The hypervisor will soon be in charge of tracking ownership of all
> > memory pages in the system. The current page-tracking infrastructure at
> > EL2 only allows binary states: a page is either owned or not by an
> > entity. But a number of use-cases will require more complex states for
> > pages that are shared between two entities (host, hypervisor, or guests).
> > 
> > In preparation for supporting these use-cases, introduce in the KVM
> > page-table library some infrastructure allowing to tag shared pages
> > using ignored bits (a.k.a. software bits) in PTEs.
> > 
> > Signed-off-by: Quentin Perret <qperret@...gle.com>
> > ---
> >  arch/arm64/include/asm/kvm_pgtable.h |  5 +++++
> >  arch/arm64/kvm/hyp/pgtable.c         | 25 +++++++++++++++++++++++++
> >  2 files changed, 30 insertions(+)
> > 
> > diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> > index dd72653314c7..f6d3d5c8910d 100644
> > --- a/arch/arm64/include/asm/kvm_pgtable.h
> > +++ b/arch/arm64/include/asm/kvm_pgtable.h
> > @@ -81,6 +81,8 @@ enum kvm_pgtable_stage2_flags {
> >   * @KVM_PGTABLE_PROT_W:		Write permission.
> >   * @KVM_PGTABLE_PROT_R:		Read permission.
> >   * @KVM_PGTABLE_PROT_DEVICE:	Device attributes.
> > + * @KVM_PGTABLE_STATE_SHARED:	Page shared with another entity.
> > + * @KVM_PGTABLE_STATE_BORROWED:	Page borrowed from another entity.
> >   */
> >  enum kvm_pgtable_prot {
> >  	KVM_PGTABLE_PROT_X			= BIT(0),
> > @@ -88,6 +90,9 @@ enum kvm_pgtable_prot {
> >  	KVM_PGTABLE_PROT_R			= BIT(2),
> >  
> >  	KVM_PGTABLE_PROT_DEVICE			= BIT(3),
> > +
> > +	KVM_PGTABLE_STATE_SHARED		= BIT(4),
> > +	KVM_PGTABLE_STATE_BORROWED		= BIT(5),
> 
> I'd rather have some indirection here, as we have other potential
> users for the SW bits outside of pKVM (see the NV series, which uses
> some of these SW bits as the backend for TTL-based TLB invalidation).
> 
> Can we instead only describe the SW bit states in this enum, and let
> the users map the semantic they require onto that state? See [1] for
> what I carry in the NV branch.

Works for me -- I just wanted to make sure we don't have users in
different places that use the same bits without knowing, but no strong
opinions, so happy to change.

> >  };
> >  
> >  #define KVM_PGTABLE_PROT_RW	(KVM_PGTABLE_PROT_R | KVM_PGTABLE_PROT_W)
> > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> > index 5bdbe7a31551..51598b79dafc 100644
> > --- a/arch/arm64/kvm/hyp/pgtable.c
> > +++ b/arch/arm64/kvm/hyp/pgtable.c
> > @@ -211,6 +211,29 @@ static kvm_pte_t kvm_init_invalid_leaf_owner(u8 owner_id)
> >  	return FIELD_PREP(KVM_INVALID_PTE_OWNER_MASK, owner_id);
> >  }
> >  
> > +static kvm_pte_t pte_ignored_bit_prot(enum kvm_pgtable_prot prot)
> 
> Can we call these sw rather than ignored?

Sure.

> > +{
> > +	kvm_pte_t ignored_bits = 0;
> > +
> > +	/*
> > +	 * Ignored bits 0 and 1 are reserved to track the memory ownership
> > +	 * state of each page:
> > +	 *   00: The page is owned solely by the page-table owner.
> > +	 *   01: The page is owned by the page-table owner, but is shared
> > +	 *       with another entity.
> > +	 *   10: The page is shared with, but not owned by the page-table owner.
> > +	 *   11: Reserved for future use (lending).
> > +	 */
> > +	if (prot & KVM_PGTABLE_STATE_SHARED) {
> > +		if (prot & KVM_PGTABLE_STATE_BORROWED)
> > +			ignored_bits |= BIT(1);
> > +		else
> > +			ignored_bits |= BIT(0);
> > +	}
> > +
> > +	return FIELD_PREP(KVM_PTE_LEAF_ATTR_IGNORED, ignored_bits);
> > +}
> > +
> >  static int kvm_pgtable_visitor_cb(struct kvm_pgtable_walk_data *data, u64 addr,
> >  				  u32 level, kvm_pte_t *ptep,
> >  				  enum kvm_pgtable_walk_flags flag)
> > @@ -357,6 +380,7 @@ static int hyp_set_prot_attr(enum kvm_pgtable_prot prot, kvm_pte_t *ptep)
> >  	attr |= FIELD_PREP(KVM_PTE_LEAF_ATTR_LO_S1_AP, ap);
> >  	attr |= FIELD_PREP(KVM_PTE_LEAF_ATTR_LO_S1_SH, sh);
> >  	attr |= KVM_PTE_LEAF_ATTR_LO_S1_AF;
> > +	attr |= pte_ignored_bit_prot(prot);
> >  	*ptep = attr;
> >  
> >  	return 0;
> > @@ -558,6 +582,7 @@ static int stage2_set_prot_attr(struct kvm_pgtable *pgt, enum kvm_pgtable_prot p
> >  
> >  	attr |= FIELD_PREP(KVM_PTE_LEAF_ATTR_LO_S2_SH, sh);
> >  	attr |= KVM_PTE_LEAF_ATTR_LO_S2_AF;
> > +	attr |= pte_ignored_bit_prot(prot);
> >  	*ptep = attr;
> >  
> >  	return 0;
> 
> How about kvm_pgtable_stage2_relax_perms()?

It should leave SW bits untouched, and it really felt like a path were
we want to change permissions and nothing else. What did you have in
mind?

Cheers,
Quentin