Re: [RFC PATCH v2 03/10] KVM: Implement kvm_(read|/write)_guest_page for private memory slots

[Sean Christopherson <seanjc@google.com>]

On Thu, Aug 01, 2024, Fuad Tabba wrote:
> Make __kvm_read_guest_page/__kvm_write_guest_page capable of
> accessing guest memory if no userspace address is available.
> Moreover, check that the memory being accessed is shared with the
> host before attempting the access.
> 
> KVM at the host might need to access shared memory that is not
> mapped in the host userspace but is in fact shared with the host,
> e.g., when accounting for stolen time. This allows the access
> without relying on the slot's userspace_addr being set.

Why?  As evidenced by the amount of code below, special casing guest_memfd isn't
trivial, and taking kvm->slots_lock is likely a complete non-starter.  In the
happy case, uaccess is about as fast as can be, and has no inherent scaling issues.

> This does not circumvent protection, since the access is only
> attempted if the memory is mappable by the host, which implies
> shareability.
> 
> Signed-off-by: Fuad Tabba <tabba@google.com>
> ---
>  virt/kvm/kvm_main.c | 127 ++++++++++++++++++++++++++++++++++++++------
>  1 file changed, 111 insertions(+), 16 deletions(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index f4b4498d4de6..ec6255c7325e 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -3385,20 +3385,108 @@ int kvm_gmem_clear_mappable(struct kvm *kvm, gfn_t start, gfn_t end)
>  	return kvm_gmem_toggle_mappable(kvm, start, end, false);
>  }
>  
> +static int __kvm_read_private_guest_page(struct kvm *kvm,

The changelog says this is for accessing memory that is shared, but this says
"private".

> +					 struct kvm_memory_slot *slot,
> +					 gfn_t gfn, void *data, int offset,
> +					 int len)