Re: [PATCH v5 01/10] KVM: s390: Extend MEM_OP ioctl by storage key checked cmpxchg

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?

Apart from that, patch looks fine to me.

Thomas

...

• •     * -EINVAL: address misaligned or len not power of two
    

• •     * -EAGAIN: transient failure (len 1 or 2)
    

• •     * -EOPNOTSUPP: read-only memslot (should never occur)
    

• */

+int cmpxchg_guest_abs_with_key(struct kvm *kvm, gpa_t gpa, int len,

• 	       __uint128_t *old_addr, __uint128_t new,
  

• 	       u8 access_key)
  

+{

• gfn_t gfn = gpa >> PAGE_SHIFT;
• struct kvm_memory_slot *slot = gfn_to_memslot(kvm, gfn);
• bool writable;
• hva_t hva;
• int ret;
• if (!IS_ALIGNED(gpa, len))

• return -EINVAL;
  

• hva = gfn_to_hva_memslot_prot(slot, gfn, &writable);
• if (kvm_is_error_hva(hva))

• return PGM_ADDRESSING;
  

• /*

• * Check if it's a read-only memslot, even though that cannot occur
  

• * since those are unsupported.
  

• * Don't try to actually handle that case.
  

• */
  

• if (!writable)

• return -EOPNOTSUPP;
  

• hva += offset_in_page(gpa);
• switch (len) {
• case 1: {

• u8 old;
  

• ret = cmpxchg_user_key((u8 *)hva, &old, *old_addr, new, access_key);
  

• ret = ret < 0 ? ret : old != *old_addr;
  

• *old_addr = old;
  

• break;
  

• }
• case 2: {

• u16 old;
  

• ret = cmpxchg_user_key((u16 *)hva, &old, *old_addr, new, access_key);
  

• ret = ret < 0 ? ret : old != *old_addr;
  

• *old_addr = old;
  

• break;
  

• }
• case 4: {

• u32 old;
  

• ret = cmpxchg_user_key((u32 *)hva, &old, *old_addr, new, access_key);
  

• ret = ret < 0 ? ret : old != *old_addr;
  

• *old_addr = old;
  

• break;
  

• }
• case 8: {

• u64 old;
  

• ret = cmpxchg_user_key((u64 *)hva, &old, *old_addr, new, access_key);
  

• ret = ret < 0 ? ret : old != *old_addr;
  

• *old_addr = old;
  

• break;
  

• }
• case 16: {

• __uint128_t old;
  

• ret = cmpxchg_user_key((__uint128_t *)hva, &old, *old_addr, new, access_key);
  

• ret = ret < 0 ? ret : old != *old_addr;
  

• *old_addr = old;
  

• break;
  

• }
• default:

• return -EINVAL;
  

• }
• mark_page_dirty_in_slot(kvm, slot, gfn);
• /*

• * Assume that the fault is caused by protection, either key protection
  

• * or user page write protection.
  

• */
  

• if (ret == -EFAULT)

• ret = PGM_PROTECTION;
  

• return ret;

+}

[...]