Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:18 as permitted sender) client-ip=2620:137:e000::1:18;
Message-ID: <701033df-49c5-987e-b316-40835ad83d16@redhat.com>
Date:   Thu, 12 May 2022 17:50:50 +0200
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.8.0
Subject: Re: [PATCH v3 1/2] KVM: s390: Don't indicate suppression on dirtying,
 failing memop
Content-Language: en-US
To:     Christian Borntraeger <borntraeger@linux.ibm.com>,
        Janis Schoetterl-Glausch <scgl@linux.ibm.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Janosch Frank <frankja@linux.ibm.com>,
        Claudio Imbrenda <imbrenda@linux.ibm.com>,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Alexander Gordeev <agordeev@linux.ibm.com>
Cc:     Sven Schnelle <svens@linux.ibm.com>, kvm@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-s390@vger.kernel.org
References: <20220512131019.2594948-1-scgl@linux.ibm.com>
 <20220512131019.2594948-2-scgl@linux.ibm.com>
 <77f6f5e7-5945-c478-0e41-affed62252eb@redhat.com>
 <4a06e3e8-4453-9204-eb66-d435860c5714@linux.ibm.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <4a06e3e8-4453-9204-eb66-d435860c5714@linux.ibm.com>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 12.05.22 15:51, Christian Borntraeger wrote:
> 
> 
> Am 12.05.22 um 15:22 schrieb David Hildenbrand:
>> On 12.05.22 15:10, Janis Schoetterl-Glausch wrote:
>>> If user space uses a memop to emulate an instruction and that
>>> memop fails, the execution of the instruction ends.
>>> Instruction execution can end in different ways, one of which is
>>> suppression, which requires that the instruction execute like a no-op.
>>> A writing memop that spans multiple pages and fails due to key
>>> protection may have modified guest memory, as a result, the likely
>>> correct ending is termination. Therefore, do not indicate a
>>> suppressing instruction ending in this case.
>>
>> I think that is possibly problematic handling.
>>
>> In TCG we stumbled in similar issues in the past for MVC when crossing
>> page boundaries. Failing after modifying the first page already
>> seriously broke some user space, because the guest would retry the
>> instruction after fixing up the fault reason on the second page: if
>> source and destination operands overlap, you'll be in trouble because
>> the input parameters already changed.
>>
>> For this reason, in TCG we make sure that all accesses are valid before
>> starting modifications.
>>
>> See target/s390x/tcg/mem_helper.c:do_helper_mvc with access_prepare()
>> and friends as an example.
>>
>> Now, I don't know how to tackle that for KVM, I just wanted to raise
>> awareness that injecting an interrupt after modifying page content is
>> possible dodgy and dangerous.
> 
> this is really special and only for key protection crossing pages.
> Its been done since the 70ies in that way on z/VM. The architecture
> is and was always written in a way to allow termination for this
> case for hypervisors.

Just so I understand correctly: all instructions that a hypervisor with
hardware virtualization is supposed to emulate are "written in a way to
allow termination", correct? That makes things a lot easier.

-- 
Thanks,

David / dhildenb