On 11/01/2023 11.00, Janis Schoetterl-Glausch wrote:
On Wed, 2023-01-11 at 08:59 +0100, Thomas Huth wrote:
On 10/01/2023 21.26, Janis Schoetterl-Glausch wrote:
User space can use the MEM_OP ioctl to make storage key checked reads
and writes to the guest, however, it has no way of performing atomic,
key checked, accesses to the guest.
Extend the MEM_OP ioctl in order to allow for this, by adding a cmpxchg
mode. For now, support this mode for absolute accesses only.
This mode can be use, for example, to set the device-state-change
indicator and the adapter-local-summary indicator atomically.
Signed-off-by: Janis Schoetterl-Glausch <scgl@xxxxxxxxxxxxx>
---
include/uapi/linux/kvm.h | 7 +++
arch/s390/kvm/gaccess.h | 3 ++
arch/s390/kvm/gaccess.c | 102 +++++++++++++++++++++++++++++++++++++++
arch/s390/kvm/kvm-s390.c | 41 +++++++++++++++-
4 files changed, 151 insertions(+), 2 deletions(-)
[...]
+/**
+ * cmpxchg_guest_abs_with_key() - Perform cmpxchg on guest absolute address.
+ * @kvm: Virtual machine instance.
+ * @gpa: Absolute guest address of the location to be changed.
+ * @len: Operand length of the cmpxchg, required: 1 <= len <= 16. Providing a
+ * non power of two will result in failure.
+ * @old_addr: Pointer to old value. If the location at @gpa contains this value, the
+ * exchange will succeed. After calling cmpxchg_guest_abs_with_key() *@old
+ * contains the value at @gpa before the attempt to exchange the value.
+ * @new: The value to place at @gpa.
+ * @access_key: The access key to use for the guest access.
+ *
+ * Atomically exchange the value at @gpa by @new, if it contains *@old.
+ * Honors storage keys.
+ *
+ * Return: * 0: successful exchange
+ * * 1: exchange unsuccessful
+ * * a program interruption code indicating the reason cmpxchg could
+ * not be attempted
PGM_OPERATION has also the value 1 ... can we be sure that it never happens
here?
Currently yes, only program errors are those explicit in the code,
PGM_ADDRESSING and PGM_PROTECTION.
... maybe it would make sense to use KVM_S390_MEMOP_R_NO_XCHG for
return value here instead of 1, too, just to be on the safe side?
I didn't like that idea because I consider KVM_S390_MEMOP_R_NO_XCHG to be
part of the KVM's api surface and cmpxchg_guest_abs_with_key is an internal
function that shouldn't concern itself with that.
But being unclear on PGM_OPERATION is indeed ugly.
Maybe I should just replace "a program interruption code ..." with the specific ones?
Yes, that would help to avoid this confusion. With such a change feel free
to add:
Reviewed-by: Thomas Huth <thuth@xxxxxxxxxx>
