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Wed, 10 Jul 2024 14:56:05 -0700 (PDT) Received: from [10.35.209.243] ([208.115.86.75]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-1fbb6a2be97sm38150205ad.102.2024.07.10.14.56.04 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 10 Jul 2024 14:56:05 -0700 (PDT) Message-ID: <3affa4a0-c930-45d3-927c-c81b38920c53@redhat.com> Date: Wed, 10 Jul 2024 23:56:03 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 8/8] kvm: gmem: Allow restricted userspace mappings To: Sean Christopherson Cc: Patrick Roy , Fuad Tabba , pbonzini@redhat.com, akpm@linux-foundation.org, dwmw@amazon.co.uk, rppt@kernel.org, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, willy@infradead.org, graf@amazon.com, derekmn@amazon.com, kalyazin@amazon.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, dmatlack@google.com, chao.p.peng@linux.intel.com, xmarcalx@amazon.co.uk, James Gowans References: <20240709132041.3625501-1-roypat@amazon.co.uk> <20240709132041.3625501-9-roypat@amazon.co.uk> <47ce1b10-e031-4ac1-b88f-9d4194533745@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: pdyxhycpgz8jmuaj68ij8cmsrrerkx89 X-Rspamd-Queue-Id: 7E7EF180008 X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1720648571-291856 X-HE-Meta: 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 x56u4Z6A ZGZCpsMJFn0ky48J2v+3pHJpYxSFmiZaGB/SlHhZEmkG/LhjnFc8JAQn7bBCzgfbM/ajDrFb8bP5ChPQkqwXXa5aJ/fKmck9S6Uj1zXSX5ysTVqoXEHgGZCnfC4OO29Nm6yE+t5OSH7uoRR+I5Ug4VA9La5tx9pWzhuqSqso3naX802sCIrFAC0/1Vz1SF7zBYXr04aa6CFphmlMvjC7VsK6IcQR7HTc9U2sE3nMBJ5SvUzF5btfVpWh8KxPBjMN2SWKhVL+uuJBYWhuRaUcZNScWCrcc9RoshU9JmqRDaB5ejca/XtHQBp4GTB83aAazA01HxxTlK7rd4Mzp/I8SM368jfu/ZSfQbbib 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 10.07.24 23:53, Sean Christopherson wrote: > On Wed, Jul 10, 2024, David Hildenbrand wrote: >> On 10.07.24 11:51, Patrick Roy wrote: >>> On 7/9/24 22:13, David Hildenbrand wrote: >>>> Note that just from staring at this commit, I don't understand the >>>> motivation *why* we would want to do that. >>> >>> Fair - I admittedly didn't get into that as much as I probably should >>> have. In our usecase, we do not have anything that pKVM would (I think) >>> call "guest-private" memory. I think our memory can be better described >>> as guest-owned, but always shared with the VMM (e.g. userspace), but >>> ideally never shared with the host kernel. This model lets us do a lot >>> of simplifying assumptions: Things like I/O can be handled in userspace >>> without the guest explicitly sharing I/O buffers (which is not exactly >>> what we would want long-term anyway, as sharing in the guest_memfd >>> context means sharing with the host kernel), we can easily do VM >>> snapshotting without needing things like TDX's TDH.EXPORT.MEM APIs, etc. >> >> Okay, so essentially you would want to use guest_memfd to only contain shard >> memory and disallow any pinning like for secretmem. >> >> If so, I wonder if it wouldn't be better to simply add KVM support to >> consume *real* secretmem memory? IIRC so far there was only demand to >> probably remove the directmap of private memory in guest_memfd, not of >> shared memory. > > It's also desirable to remove shared memory from the directmap, e.g. to prevent > using the directmap in a cross-VM attack. > > I don't think we want to allow KVM to consume secretmem. That would require > letting KVM gup() secretmem, which AIUI defeats the entire purpose of secretmem, > and I don't think KVM should be special. I would mean consuming secretmem via the fd, *not* via page tables / gup. But if we also want to have the option of directmap modifications for shared memory in guest_memfd, we could make that indeed a guest_memfd feature. -- Cheers, David / dhildenb