Re: [PATCH v3 08/15] KVM: arm64: pkvm: Refcount the pages shared with EL2

Will Deacon
2021-12-09T11:17:38Z

On Wed, Dec 01, 2021 at 05:04:02PM +0000, Quentin Perret wrote:
> In order to simplify the page tracking infrastructure at EL2 in nVHE
> protected mode, move the responsibility of refcounting pages that are
> shared multiple times on the host. In order to do so, let's create a
> red-black tree tracking all the PFNs that have been shared, along with
> a refcount.
>
> Signed-off-by: Quentin Perret <qperret@google.com>
> ---
> arch/arm64/kvm/mmu.c | 78 ++++++++++++++++++++++++++++++++++++++------
> 1 file changed, 68 insertions(+), 10 deletions(-)
>
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index fd868fb9d922..d72566896755 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -284,23 +284,72 @@ static phys_addr_t kvm_kaddr_to_phys(void *kaddr)
> }
> }
>
> -static int pkvm_share_hyp(phys_addr_t start, phys_addr_t end)
> +struct hyp_shared_pfn {
> + u64 pfn;
> + int count;
> + struct rb_node node;
> +};
> +
> +static DEFINE_MUTEX(hyp_shared_pfns_lock);
> +static struct rb_root hyp_shared_pfns = RB_ROOT;
> +
> +static struct hyp_shared_pfn *find_shared_pfn(u64 pfn, struct rb_node ***node,
> + struct rb_node **parent)
> {
> - phys_addr_t addr;
> - int ret;
> + struct hyp_shared_pfn *this;
> +
> + *node = &hyp_shared_pfns.rb_node;
> + *parent = NULL;
> + while (**node) {
> + this = container_of(**node, struct hyp_shared_pfn, node);
> + *parent = **node;
> + if (this->pfn < pfn)
> + *node = &((**node)->rb_left);
> + else if (this->pfn > pfn)
> + *node = &((**node)->rb_right);
> + else
> + return this;
> + }
>
> - for (addr = ALIGN_DOWN(start, PAGE_SIZE); addr < end; addr += PAGE_SIZE) {
> - ret = kvm_call_hyp_nvhe(__pkvm_host_share_hyp,
> - __phys_to_pfn(addr));
> - if (ret)
> - return ret;
> + return NULL;
> +}
> +
> +static int share_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);

I don't think this is a fast-path at the moment, but in the future we might
consider using RCU to do the lookup outside of the mutex.

But as-is:

Acked-by: Will Deacon <will@kernel.org>

Will