linux-kernel - Re: [PATCH v4 28/34] KVM: arm64: Use page-table to track page ownership

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Date:   Fri, 12 Mar 2021 11:18:05 +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, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com, kvmarm@...ts.cs.columbia.edu,
        linux-arm-kernel@...ts.infradead.org, tabba@...gle.com,
        mark.rutland@....com, dbrazdil@...gle.com, mate.toth-pal@....com,
        seanjc@...gle.com, robh+dt@...nel.org, ardb@...nel.org
Subject: Re: [PATCH v4 28/34] KVM: arm64: Use page-table to track page
 ownership

On Fri, Mar 12, 2021 at 10:13:26AM +0000, Quentin Perret wrote:
> On Friday 12 Mar 2021 at 09:32:06 (+0000), Will Deacon wrote:
> > I'm not saying to use the VMID directly, just that allocating half of the
> > pte feels a bit OTT given that the state of things after this patch series
> > is that we're using exactly 1 bit.
> 
> Right, and that was the reason for the PROT_NONE approach in the
> previous version, but we agreed it'd be worth generalizing to allow for
> future use-cases :-)

Yeah, just generalising to 32 bits feels like going too far! I dunno,
make it a u8 for now, or define the hypervisor owner ID as 1 and reject
owners greater than that? We can easily extend it later.

> > > > > @@ -517,28 +543,36 @@ static int stage2_map_walker_try_leaf(u64 addr, u64 end, u32 level,
> > > > >  	if (!kvm_block_mapping_supported(addr, end, phys, level))
> > > > >  		return -E2BIG;
> > > > >  
> > > > > -	new = kvm_init_valid_leaf_pte(phys, data->attr, level);
> > > > > -	if (kvm_pte_valid(old)) {
> > > > > +	if (kvm_pte_valid(data->attr))
> > > > 
> > > > This feels like a bit of a hack to me: the 'attr' field in stage2_map_data
> > > > is intended to correspond directly to the lower/upper attributes of the
> > > > descriptor as per the architecture, so tagging the valid bit in there is
> > > > pretty grotty. However, I can see the significant advantage in being able
> > > > to re-use the stage2_map_walker functionality, so about instead of nobbling
> > > > attr, you set phys to something invalid instead, e.g.:
> > > > 
> > > > 	#define KVM_PHYS_SET_OWNER	(-1ULL)
> > > 
> > > That'll confuse kvm_block_mapping_supported() and friends I think, at
> > > least in their current form. If you _really_ don't like this, maybe we
> > > could have an extra 'flags' field in stage2_map_data?
> > 
> > I was pondering this last night and I thought of two ways to do it:
> > 
> > 1. Add a 'bool valid' and then stick the owner and the phys in a union.
> >    (yes, you'll need to update the block mapping checks to look at the
> >     valid flag)
> 
> Right, though that is also used for the hyp s1 which doesn't use any of
> this ATM. That shouldn't be too bad to change, I'll have a look.

Oh, I meant stick the bool in the stage2_map_data so that should be limited
to the stage2 path.

Will