linux-kernel - Re: [PATCH v2 0/2] Dirtying, failing memop: don't indicate suppression

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Message-ID: <1ccb1333-2233-8832-4102-a6c082b29108@linux.ibm.com>
Date:   Tue, 26 Apr 2022 08:19:53 +0200
From:   Christian Borntraeger <borntraeger@...ux.ibm.com>
To:     Janis Schoetterl-Glausch <scgl@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        Shuah Khan <shuah@...nel.org>, linux-kselftest@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org
Subject: Re: [PATCH v2 0/2] Dirtying, failing memop: don't indicate
 suppression



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.