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[2003:cb:c707:5c00:e650:bcd7:e2a0:54fe]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4282bb64952sm46172355e9.37.2024.07.31.23.56.43 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 31 Jul 2024 23:56:44 -0700 (PDT) Message-ID: Date: Thu, 1 Aug 2024 08:56:42 +0200 Precedence: bulk X-Mailing-List: devicetree@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v17 19/35] arch/mm: Export direct {un,}map functions To: Elliot Berman , Christoph Hellwig , Will Deacon , Quentin Perret , Chris Goldsworthy , Android KVM , Patrick Daly , Alex Elder , Srinivas Kandagatla , Murali Nalajal , Trilok Soni , Srivatsa Vaddagiri , Carl van Schaik , Philip Derrin , Prakruthi Deepak Heragu , Jonathan Corbet , Rob Herring , Krzysztof Kozlowski , Conor Dooley , Catalin Marinas , Konrad Dybcio , Bjorn Andersson , Dmitry Baryshkov , Fuad Tabba , Sean Christopherson , Andrew Morton , linux-arm-msm@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, devicetree@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org References: <20240222-gunyah-v17-0-1e9da6763d38@quicinc.com> <20240222-gunyah-v17-19-1e9da6763d38@quicinc.com> <20240223071006483-0800.eberman@hu-eberman-lv.qualcomm.com> <2f4c44ad-b309-4baa-ac21-2ae19efd31fb@redhat.com> <20240226092020370-0800.eberman@hu-eberman-lv.qualcomm.com> <49d14780-56f4-478d-9f5f-0857e788c667@redhat.com> <20240229170329275-0800.eberman@hu-eberman-lv.qualcomm.com> <20240731140323693-0700.eberman@hu-eberman-lv.qualcomm.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 01.08.24 00:21, Elliot Berman wrote: > I wanted to revive this thread based on the mm alignment discussion for > guest_memfd. > > Gunyah's guest_memfd allocates memory via filemap_alloc_folio, identical > to KVM's guest_memfd. There's a possiblity of a stage-2 fault when > memory is donated to guest VM and Linux incidentally tries to access the > donated memory with an unaligned access. This access will cause kernel > to panic as it expects to be able to access all memory which has been > mapped in stage 1. We don't want to disallow unaligned access simply > because Gunyah drivers are enabled. > > There are two options I see to prevent the stage-2 fault from crashing > the kernel: we can fix up the stage-2 fault or ensure that Linux has a > S1 table consistent with S2. > > To do the latter, the obvious solution seemed to be using the > set_direct_map functions, but you and Christoph have valid concerns > about exporting this to modules since it's a low-level API. One way to > avoid exporting the symbols is to make Gunyah a built-in, but I'd like > to find a better solution. > > One way I can think of is to create a "guest_memfd library" that both > KVM and Gunyah can use. It abstracts the common bits between the 2 into > a built-in module and can be the one to call the set_direct_map > functions. I also think the abstraction will also help keep KVM > guest_memfd cleaner once we start supporting huge folios (and splitting > them). Do KVM and mm folks also see value to using a library-fied > guest_memfd? Without knowing about any details, this sounds like the right approach to me! -- Cheers, David / dhildenb