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

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Message-ID: <839f7ee6c273f8542f06d3330622e545b86b8abc.camel@linux.ibm.com>
Date:   Wed, 11 Jan 2023 16:29:51 +0100
From:   Janis Schoetterl-Glausch <scgl@...ux.ibm.com>
To:     Janosch Frank <frankja@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...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 v5 01/10] KVM: s390: Extend MEM_OP ioctl by storage key
 checked cmpxchg

On Wed, 2023-01-11 at 10:38 +0100, Janosch Frank wrote:
> On 1/10/23 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@...ux.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(-)
> > 
> > diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> > index 55155e262646..452f43c1cc34 100644
> > --- a/include/uapi/linux/kvm.h
> > +++ b/include/uapi/linux/kvm.h
> > @@ -583,6 +583,8 @@ struct kvm_s390_mem_op {
> >   		struct {
> >   			__u8 ar;	/* the access register number */
> >   			__u8 key;	/* access key, ignored if flag unset */
> > +			__u8 pad1[6];	/* ignored */
> > +			__u64 old_addr;	/* ignored if flag unset */
> 
> These 3 are only used for flag values >=4, no?
> They aren't used for all flag values but for specific ones, right?

key is used with the skey flag, old_addr with the cmpxchg flag,
so yes only used with specific flags.
ar is used for logical accesses.