Re: [PATCH v3 15/15] KVM: arm64: pkvm: Unshare guest structs during teardown

Quentin Perret
2021-12-10T14:50:43Z

On Thursday 09 Dec 2021 at 11:22:33 (+0000), Will Deacon wrote:
> On Wed, Dec 01, 2021 at 05:04:09PM +0000, Quentin Perret wrote:
> > Make use of the newly introduced unshare hypercall during guest teardown
> > to unmap guest-related data structures from the hyp stage-1.
> >
> > Signed-off-by: Quentin Perret <qperret@google.com>
> > ---
> > arch/arm64/include/asm/kvm_host.h | 2 ++
> > arch/arm64/include/asm/kvm_mmu.h | 1 +
> > arch/arm64/kvm/arm.c | 2 ++
> > arch/arm64/kvm/fpsimd.c | 34 ++++++++++++++++++++++---
> > arch/arm64/kvm/mmu.c | 42 +++++++++++++++++++++++++++++++
> > arch/arm64/kvm/reset.c | 8 +++++-
> > 6 files changed, 85 insertions(+), 4 deletions(-)
>
> [...]
>
> > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> > index d72566896755..8e506ba8988e 100644
> > --- a/arch/arm64/kvm/mmu.c
> > +++ b/arch/arm64/kvm/mmu.c
> > @@ -344,6 +344,32 @@ static int share_pfn_hyp(u64 pfn)
> > return ret;
> > }
> >
> > +static int unshare_pfn_hyp(u64 pfn)
> > +{
> > + struct rb_node **node, *parent;
> > + struct hyp_shared_pfn *this;
> > + int ret = 0;
> > +
> > + mutex_lock(&hyp_shared_pfns_lock);
> > + this = find_shared_pfn(pfn, &node, &parent);
> > + if (WARN_ON(!this)) {
> > + ret = -EINVAL;
>
> -ENOENT?

Sure.

> > + goto unlock;
> > + }
> > +
> > + this->count--;
> > + if (this->count)
> > + goto unlock;
>
> Again, if we did an RCU lookup then this could be converted to a refcount_t
> to take the mutex only when it hits zero. But for now I think it's fine.

No objection to do this in the future, but yeah I think we might as well
start simple :)

> > +
> > + rb_erase(&this->node, &hyp_shared_pfns);
> > + kfree(this);
> > + ret = kvm_call_hyp_nvhe(__pkvm_host_unshare_hyp, pfn, 1);
> > +unlock:
> > + mutex_unlock(&hyp_shared_pfns_lock);
> > +
> > + return ret;
> > +}
> > +
> > int kvm_share_hyp(void *from, void *to)
> > {
> > phys_addr_t start, end, cur;
> > @@ -376,6 +402,22 @@ int kvm_share_hyp(void *from, void *to)
> > return 0;
> > }
> >
> > +void kvm_unshare_hyp(void *from, void *to)
> > +{
> > + phys_addr_t start, end, cur;
> > + u64 pfn;
> > +
> > + if (is_kernel_in_hyp_mode() || kvm_host_owns_hyp_mappings() || !from)
>
> I don't think you need the is_kernel_in_hyp_mode() check any more not that
> you've moved that into kvm_host_owns_hyp_mappings().

The logic is a little odd, but I think I still do as
kvm_host_owns_hyp_mappings() will return false if is_kernel_in_hyp_mode()
is true.