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[v2,0/2] Dirtying, failing memop: don't indicate suppression

Message ID 20220425100147.1755340-1-scgl@linux.ibm.com
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Series Dirtying, failing memop: don't indicate suppression | expand

Message

Janis Schoetterl-Glausch April 25, 2022, 10:01 a.m. UTC 
If a memop fails due to key checked protection, after already having
written to the guest, don't indicate suppression to the guest, as that
would imply that memory wasn't modified.

This could be considered a fix to the code introducing storage key
support, however this is a bug in KVM only if we emulate an
instructions writing to an operand spanning multiple pages, which I
don't believe we do.

v1 -> v2
 * Reword commit message of patch 1

Janis Schoetterl-Glausch (2):
  KVM: s390: Don't indicate suppression on dirtying, failing memop
  KVM: s390: selftest: Test suppression indication on key prot exception

 arch/s390/kvm/gaccess.c                   | 47 ++++++++++++++---------
 tools/testing/selftests/kvm/s390x/memop.c | 43 ++++++++++++++++++++-
 2 files changed, 70 insertions(+), 20 deletions(-)


base-commit: af2d861d4cd2a4da5137f795ee3509e6f944a25b

Comments

Christian Borntraeger April 26, 2022, 6:19 a.m. UTC | #1 
Am 25.04.22 um 19:29 schrieb Janis Schoetterl-Glausch:
> On 4/25/22 18:30, Christian Borntraeger wrote:
>> Am 25.04.22 um 12:01 schrieb Janis Schoetterl-Glausch:
>>> If a memop fails due to key checked protection, after already having
>>> written to the guest, don't indicate suppression to the guest, as that
>>> would imply that memory wasn't modified.
>>>
>>> This could be considered a fix to the code introducing storage key
>>> support, however this is a bug in KVM only if we emulate an
>>> instructions writing to an operand spanning multiple pages, which I
>>> don't believe we do.
>>>
>>
>> Thanks applied. I think it makes sense for 5.18 nevertheless.
> 
> Janosch had some concerns because the protection code being 000 implies
> that the effective address in the TEID is unpredictable.
> Let's see if he chimes in.

z/VM does exactly the same on key protection crossing a page boundary. The
architecture was written in a way to allow all zeros exactly for this case.
(hypervisor emulation of key protection crossing pages).
This is even true for ESOP-2. See Figure 3-5 or figure 3-8 (the first line)
which allows to NOT have a valid address in the TEID for key controlled
protection.

The only question is, do we need to change the suppression parameter in
access_guest_with_key

  (mode != GACC_STORE) || (idx == 0)

to also check for prot != PROT_TYPE_KEYC
? I think we do not need this as we have checked other reasons before.
Christian Borntraeger May 2, 2022, 7:58 a.m. UTC | #2 
Am 26.04.22 um 09:25 schrieb Janosch Frank:
> 
> To me this measure looks like a last resort option and the POP doesn't state a 100% what is to be done. Some instructions can mandate suppression instead of termination according to the architects.
> 
> My intuition tells me that if we are in a situation where this would happen then we would be much better off just doing it by hand (i.e. in the instruction emulation code) and not letting this function decide.
> 
> So I'm not entirely sure if we're replacing something that is not correct with something that also won't be correct for all cases.
> 
> But to summarize this: I'm not entirely sure even after reading the POP for more than an hour and consulting an architect

According to Damian, the definition in the POP is exactly the way it is to cover for z/VMs way of handling key protection for long operatings in a terminating fashion since the 70ies or 80ies.
As it is fine for z/VM (and then also for z/OS and zVSE under z/VM) I guess we can (and should) mimic that behaviour.