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Wed, 10 Jul 2024 23:26:31 -0700 (PDT) X-Google-Smtp-Source: AGHT+IEpxFjj7NiMI1xmtWy0TI38wJOXY2YEJ3Jj7DSBm0Wl5Hb9DKO4dJlV8vOZlAWOvZPr1hgngg== X-Received: by 2002:a05:6000:2cf:b0:363:1c9d:d853 with SMTP id ffacd0b85a97d-367f0505632mr1555547f8f.32.1720679190949; Wed, 10 Jul 2024 23:26:30 -0700 (PDT) Received: from [192.168.10.81] ([151.95.101.29]) by smtp.googlemail.com with ESMTPSA id 5b1f17b1804b1-4264a1f22acsm274224945e9.24.2024.07.10.23.26.11 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 10 Jul 2024 23:26:30 -0700 (PDT) Message-ID: <74e92b31-e48d-484f-b819-ef7f07faad63@redhat.com> Date: Thu, 11 Jul 2024 08:25:39 +0200 Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 6/8] kvm: gmem: Temporarily restore direct map entries when needed To: Patrick Roy , seanjc@google.com, akpm@linux-foundation.org, dwmw@amazon.co.uk, rppt@kernel.org, david@redhat.com Cc: 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, tabba@google.com, chao.p.peng@linux.intel.com, xmarcalx@amazon.co.uk References: <20240709132041.3625501-1-roypat@amazon.co.uk> <20240709132041.3625501-7-roypat@amazon.co.uk> From: Paolo Bonzini Content-Language: en-US Autocrypt: addr=pbonzini@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 7/9/24 15:20, Patrick Roy wrote: > If KVM_GMEM_NO_DIRECT_MAP is set, and KVM tries to internally access > guest-private memory inside kvm_{read,write}_guest, or via a > gfn_to_pfn_cache, temporarily restore the direct map entry. > > To avoid race conditions between two threads restoring or zapping direct > map entries for the same page and potentially interfering with each > other (e.g. unfortune interweavings of map->read->unmap in the form of > map(A)->map(B)->read(A)->unmap(A)->read(B) [BOOM]), the following > invariant is upheld in this patch: > > - Only a single gfn_to_pfn_cache can exist for any given pfn, and I think this is not ensured. You can however use set_page_private()/page_private() to count the number of references. Paolo > - All non-gfn_to_pfn_cache code paths that temporarily restore direct > map entries complete the entire map->access->unmap critical section > while holding the folio lock.