Messages in this thread | | | Date | Wed, 11 Jan 2023 11:21:14 +0100 | Subject | Re: [PATCH v5 01/10] KVM: s390: Extend MEM_OP ioctl by storage key checked cmpxchg | From | Thomas Huth <> |
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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@linux.ibm.com> >>> --- >>> 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@redhat.com>
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