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[2003:cb:c701:d200:ee5d:1275:f171:136d]) by smtp.gmail.com with ESMTPSA id d13-20020a5d4f8d000000b0020c5253d911sm4326778wru.93.2022.05.12.08.50.50 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 12 May 2022 08:50:51 -0700 (PDT) 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 , Janis Schoetterl-Glausch , Paolo Bonzini , Jonathan Corbet , Janosch Frank , Claudio Imbrenda , Heiko Carstens , Vasily Gorbik , Alexander Gordeev Cc: Sven Schnelle , 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 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 List-ID: X-Mailing-List: linux-doc@vger.kernel.org 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