Fuad Tabba <tabba@xxxxxxxxxx> writes:
> Make kvm_(read|/write)_guest_page() capable of accessing guest
> memory for slots that don't have a userspace address, but only if
> the memory is mappable, which also indicates that it is
> accessible by the host.
Fuad explained to me that this patch removes the need for userspace to
mmap a guest_memfd fd just to provide userspace_addr when only a limited
range of shared pages are required, e.g. for kvm_clock.
Questions to anyone who might be more familiar:
1. Should we let userspace save the trouble of providing userspace_addr
if only KVM (and not userspace) needs to access the shared pages?
2. Other than kvm_{read,write}_guest_page, are there any other parts of
KVM that may require updates so that guest_memfd can be used directly
from the kernel?
Patrick, does this help to answer the question of "how does KVM
internally access guest_memfd for non-CoCo VMs" that you brought up in
this other thread [*]?
[*] https://lore.kernel.org/all/6bca3ad4-3eca-4a75-a775-5f8b0467d7a3@xxxxxxxxxxxx/
>
> Signed-off-by: Fuad Tabba <tabba@xxxxxxxxxx>
> ---
> virt/kvm/kvm_main.c | 137 ++++++++++++++++++++++++++++++++++++++------
> 1 file changed, 118 insertions(+), 19 deletions(-)
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index aed9cf2f1685..77e6412034b9 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -3399,23 +3399,114 @@ 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_guest_memfd_page(struct kvm *kvm,
> + struct kvm_memory_slot *slot,
> + gfn_t gfn, void *data, int offset,
> + int len)
> +{
> + struct page *page;
> + u64 pfn;
> + int r;
> +
> + /*
> + * Holds the folio lock until after checking whether it can be faulted
> + * in, to avoid races with paths that change a folio's mappability.
> + */
> + r = kvm_gmem_get_pfn_locked(kvm, slot, gfn, &pfn, NULL);
> + if (r)
> + return r;
> +
> + page = pfn_to_page(pfn);
> +
> + if (!kvm_gmem_is_mappable(kvm, gfn, gfn + 1)) {
> + r = -EPERM;
> + goto unlock;
> + }
> + memcpy(data, page_address(page) + offset, len);
> +unlock:
> + if (r)
> + put_page(page);
> + else
> + kvm_release_pfn_clean(pfn);
> + unlock_page(page);
> +
> + return r;
> +}
> +
> +static int __kvm_write_guest_memfd_page(struct kvm *kvm,
> + struct kvm_memory_slot *slot,
> + gfn_t gfn, const void *data,
> + int offset, int len)
> +{
> + struct page *page;
> + u64 pfn;
> + int r;
> +
> + /*
> + * Holds the folio lock until after checking whether it can be faulted
> + * in, to avoid races with paths that change a folio's mappability.
> + */
> + r = kvm_gmem_get_pfn_locked(kvm, slot, gfn, &pfn, NULL);
> + if (r)
> + return r;
> +
> + page = pfn_to_page(pfn);
> +
> + if (!kvm_gmem_is_mappable(kvm, gfn, gfn + 1)) {
> + r = -EPERM;
> + goto unlock;
> + }
> + memcpy(page_address(page) + offset, data, len);
> +unlock:
> + if (r)
> + put_page(page);
> + else
> + kvm_release_pfn_dirty(pfn);
> + unlock_page(page);
> +
> + return r;
> +}
> +#else
> +static int __kvm_read_guest_memfd_page(struct kvm *kvm,
> + struct kvm_memory_slot *slot,
> + gfn_t gfn, void *data, int offset,
> + int len)
> +{
> + WARN_ON_ONCE(1);
> + return -EIO;
> +}
> +
> +static int __kvm_write_guest_memfd_page(struct kvm *kvm,
> + struct kvm_memory_slot *slot,
> + gfn_t gfn, const void *data,
> + int offset, int len)
> +{
> + WARN_ON_ONCE(1);
> + return -EIO;
> +}
> #endif /* CONFIG_KVM_GMEM_MAPPABLE */
>
> /* Copy @len bytes from guest memory at '(@gfn * PAGE_SIZE) + @offset' to @data */
> -static int __kvm_read_guest_page(struct kvm_memory_slot *slot, gfn_t gfn,
> - void *data, int offset, int len)
> +
> +static int __kvm_read_guest_page(struct kvm *kvm, struct kvm_memory_slot *slot,
> + gfn_t gfn, void *data, int offset, int len)
> {
> - int r;
> unsigned long addr;
>
> if (WARN_ON_ONCE(offset + len > PAGE_SIZE))
> return -EFAULT;
>
> + if (IS_ENABLED(CONFIG_KVM_GMEM_MAPPABLE) &&
> + kvm_slot_can_be_private(slot) &&
> + !slot->userspace_addr) {
> + return __kvm_read_guest_memfd_page(kvm, slot, gfn, data,
> + offset, len);
> + }
> +
> addr = gfn_to_hva_memslot_prot(slot, gfn, NULL);
> if (kvm_is_error_hva(addr))
> return -EFAULT;
> - r = __copy_from_user(data, (void __user *)addr + offset, len);
> - if (r)
> + if (__copy_from_user(data, (void __user *)addr + offset, len))
> return -EFAULT;
> return 0;
> }
> @@ -3425,7 +3516,7 @@ int kvm_read_guest_page(struct kvm *kvm, gfn_t gfn, void *data, int offset,
> {
> struct kvm_memory_slot *slot = gfn_to_memslot(kvm, gfn);
>
> - return __kvm_read_guest_page(slot, gfn, data, offset, len);
> + return __kvm_read_guest_page(kvm, slot, gfn, data, offset, len);
> }
> EXPORT_SYMBOL_GPL(kvm_read_guest_page);
>
> @@ -3434,7 +3525,7 @@ int kvm_vcpu_read_guest_page(struct kvm_vcpu *vcpu, gfn_t gfn, void *data,
> {
> struct kvm_memory_slot *slot = kvm_vcpu_gfn_to_memslot(vcpu, gfn);
>
> - return __kvm_read_guest_page(slot, gfn, data, offset, len);
> + return __kvm_read_guest_page(vcpu->kvm, slot, gfn, data, offset, len);
> }
> EXPORT_SYMBOL_GPL(kvm_vcpu_read_guest_page);
>
> @@ -3511,22 +3602,30 @@ EXPORT_SYMBOL_GPL(kvm_vcpu_read_guest_atomic);
>
> /* Copy @len bytes from @data into guest memory at '(@gfn * PAGE_SIZE) + @offset' */
> static int __kvm_write_guest_page(struct kvm *kvm,
> - struct kvm_memory_slot *memslot, gfn_t gfn,
> - const void *data, int offset, int len)
> + struct kvm_memory_slot *slot, gfn_t gfn,
> + const void *data, int offset, int len)
> {
> - int r;
> - unsigned long addr;
> -
> if (WARN_ON_ONCE(offset + len > PAGE_SIZE))
> return -EFAULT;
>
> - addr = gfn_to_hva_memslot(memslot, gfn);
> - if (kvm_is_error_hva(addr))
> - return -EFAULT;
> - r = __copy_to_user((void __user *)addr + offset, data, len);
> - if (r)
> - return -EFAULT;
> - mark_page_dirty_in_slot(kvm, memslot, gfn);
> + if (IS_ENABLED(CONFIG_KVM_GMEM_MAPPABLE) &&
> + kvm_slot_can_be_private(slot) &&
> + !slot->userspace_addr) {
> + int r = __kvm_write_guest_memfd_page(kvm, slot, gfn, data,
> + offset, len);
> +
> + if (r)
> + return r;
> + } else {
> + unsigned long addr = gfn_to_hva_memslot(slot, gfn);
> +
> + if (kvm_is_error_hva(addr))
> + return -EFAULT;
> + if (__copy_to_user((void __user *)addr + offset, data, len))
> + return -EFAULT;
> + }
> +
> + mark_page_dirty_in_slot(kvm, slot, gfn);
> return 0;
> }
