Hi Sean, On Fri, 16 Aug 2024 at 20:32, Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > 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@xxxxxxxxxx> > > --- > > 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". This is bad naming on my part. Instead, I should call this function something like, read_guestmem_page (and similar for the write one). Thanks for pointing this out. Cheers, /fuad > > + struct kvm_memory_slot *slot, > > + gfn_t gfn, void *data, int offset, > > + int len)
