linux-kernel - Re: [PATCH v1 2/9] KVM: s390: Extend MEM_OP ioctl by storage key checked cmpxchg

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Message-ID: <37197cfe-d109-332f-089b-266d7e8e23f8@redhat.com>
Date:   Wed, 5 Oct 2022 08:32:33 +0200
From:   Thomas Huth <thuth@...hat.com>
To:     Janis Schoetterl-Glausch <scgl@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Jonathan Corbet <corbet@....net>, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-s390@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Sven Schnelle <svens@...ux.ibm.com>
Subject: Re: [PATCH v1 2/9] KVM: s390: Extend MEM_OP ioctl by storage key
 checked cmpxchg

On 30/09/2022 23.07, 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@...ux.ibm.com>
> ---
> 
> 
> The return value of MEM_OP is:
>    0 on success,
>    < 0 on generic error (e.g. -EFAULT or -ENOMEM),
>    > 0 if an exception occurred while walking the page tables
> A cmpxchg failing because the old value doesn't match is neither an
> error nor an exception, so the question is how best to signal that
> condition. This is not strictly necessary since user space can compare
> the value of old after the MEM_OP with the value it set. If they're
> different the cmpxchg failed. It might be a better user interface if
> there is an easier way to see if the cmpxchg failed.
> This patch sets the cmpxchg flag bit to 0 on a successful cmpxchg.
> This way you can compare against a constant instead of the old old
> value.
> This has the disadvantage of being a bit weird, other suggestions
> welcome.

This also breaks the old API of defining the ioctl as _IOW only ... with 
your change to the flags field, it effectively gets IOWR instead.

Maybe it would be better to put all the new logic into a new struct and only 
pass a pointer to that struct in kvm_s390_mem_op, so that the ioctl stays 
IOW ? ... or maybe even introduce a completely new ioctl for this 
functionality instead?

  Thomas