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[35.189.192.250]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-49fe0c3ed96sm1697075e9.2.2026.09.23.08.21.12 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 23 Sep 2026 08:21:13 -0700 (PDT) Date: Wed, 23 Sep 2026 15:20:43 +0000 From: Mostafa Saleh To: Jason Gunthorpe Cc: Catalin Marinas , "Aneesh Kumar K.V" , linux-coco@lists.linux.dev, kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, iommu@lists.linux.dev, Andrew Morton , christian.koenig@amd.com, Joerg Roedel , Marc Zyngier , Marek Szyprowski , Robin Murphy , Steven Price , Sumit Semwal , Suzuki K Poulose , Thomas Gleixner , Will Deacon , dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org, linux-media@vger.kernel.org, linux-mm@kvack.org Subject: Re: [RFC PATCH v7 02/13] mm: Add an allocator for CoCo shared memory Message-ID: References: <20260921144847.501151-1-aneesh.kumar@kernel.org> <20260921144847.501151-3-aneesh.kumar@kernel.org> <20260923130031.GE1540250@ziepe.ca> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260923130031.GE1540250@ziepe.ca> X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 3E34C140007 X-Stat-Signature: mt4sct9bkhgs3ijm6ib11qp7rkykigrn X-HE-Tag: 1790176878-231919 X-HE-Meta: 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 NGus9EqP fC2W4O9WVZjqTcSl+1bt719bKGuZWvQkwPXo0GlSCY7sxuyyda0z3fHqn9jlmCCrpp2c+D5zbxQJhpNpC6x8nsRkujt/501SEGOzS87srjGJqcWbDFL1yE7uoMpWUsA38LYWpDu6rjZMfeTGgmvpNx2OSesAHzks6mrQyjJY0P4zPSmQWf6kbTqGt7N9hnag6JyHDw/w27Ynh+NLV3zufVu2GXjY35DZWY+oLkecaQaUSL3Bql+T6rugM4keMJKncJaEaOGmfoskAErY3YaaxbZ3AFVMTC71+lqESmUkTg36MlZ1CGhhaWwGAtaYbI1M+vlGYSM/aUXaCglLRM1n40Lle9B3ZB8MuaNiv89YzMWOAKhI= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Sep 23, 2026 at 10:00:31AM -0300, Jason Gunthorpe wrote: > On Wed, Sep 23, 2026 at 10:42:39AM +0100, Catalin Marinas wrote: > > > On pKVM, we want set_memory_decrypted() to zero the buffer > > before the host can access it (I guess currently relying on __GFP_ZERO > > allocations). Since no cryptographic encryption takes place, there's not > > much point in memset'ing again after the operation as the content was > > already zeroed. > > It sounds like this a pkvm unique thing, since RMM always makes the > memory hidden, pkvm flow should do it too. Either inside its guest > set_memory_decrypted() or inside the hypervisor like RMM. > pKVM doesn't clear memory on sharing (see __pkvm_guest_share_host()), and that is not a documented behaviour for ARM_SMCCC_KVM_FUNC_MEM_SHARE (see Documentation/virt/kvm/arm/hypercalls.rst) which implements set_memory_decrypted(). The guest is definitely the one knowing what is inside the pages and whether it can be shared or not. For example, a while ago I was experimenting with inline decryption to avoid bouncing, and I imagine it might be used to build on in the future. Also as these hypercalls are ABI, I'd argue we shouldn't change their semantics. > Hypervisor doing it is the right place, hypervisor always needs to > ensure guest memory is cleansed whenever the guest gives it up. Either > via a private to shared, unmap or just exiting. > "gives it up" is different than sharing, for example: - When the guest dies pKVM will zero the memory before returning it back to the host. - At the moment the guest can not relinquish memory (ballooning) but that is supported in Android which can zero the page. Thanks, Mostafa > > I don't think cc_make_shared() has the right information on how to > > safely and efficiently do the zeroing. That's only known to the > > set_memory_* backend. So you'd have to propagate the flag down. > > +1 > > Jason >