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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 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 Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Queue-Id: 10DEC8000E X-Rspamd-Server: rspam01 X-Stat-Signature: gxokd7bqrwocej667i74rc9h3qk71db8 X-HE-Tag: 1722495408-238270 X-HE-Meta: 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 lbq0cdkV J6UnirgLJGWutPNVNyTTnHekyAP34yQGxjryR75SzSxHN9uR74T7XAVOvxVozdoCt5gQiFkbC24bBw0jqpSbA2w5FKyZNExEUpSawWwXi+pn+maNgifHpTQ/TFj8fjFQlA5cNiVP8nJWxnsF9AOm3zce6/zxqxQeBW77w1+XUucQ5sdcfB86q/Sfp8+sGSmmWDpgZINqXiRdWhn4LpSFrpein3RpQERk7/9735CXDc9oSZDdHvHmoyqH+oKyXCdmaFKbrwPkIfqbQEi3BntAZWcZjyFeRDqVV0ppheKf1By+0cuf61IQrb7mjE8E0OGp4AA9hmU4QB06iqT4ha6VSztPy9Y0NKkVi+2NBjkrI+4GjHmT/DqbF98+uQx8sKRzcXEKe1LQxdZyDQhILKDMKRFVUwMCXfLRPg7Oo+yX/owJLRS7OjXQ69T/BZupJkqRvzpMZ X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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