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Wed, 20 Mar 2024 09:26:03 -0700 (PDT) X-Google-Smtp-Source: AGHT+IECLA18nqUOxI+h+v1n/cHVmjwkTvIzsjJjO+lGm4cMUYzEvTsYkEVQt6PI24bSP3DzE+CZ8Q== X-Received: by 2002:a17:906:5387:b0:a46:9f12:ca2a with SMTP id g7-20020a170906538700b00a469f12ca2amr131895ejo.22.1710951962682; Wed, 20 Mar 2024 09:26:02 -0700 (PDT) Received: from [192.168.10.118] ([151.95.49.219]) by smtp.googlemail.com with ESMTPSA id bx21-20020a170906a1d500b00a4655976025sm7391435ejb.82.2024.03.20.09.26.00 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 20 Mar 2024 09:26:01 -0700 (PDT) Message-ID: Date: Wed, 20 Mar 2024 17:26:00 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 19/49] kvm: Make kvm_convert_memory() obey ram_block_discard_is_enabled() Content-Language: en-US To: Michael Roth , qemu-devel@nongnu.org Cc: kvm@vger.kernel.org, Tom Lendacky , =?UTF-8?Q?Daniel_P_=2E_Berrang=C3=A9?= , Markus Armbruster , Pankaj Gupta , Xiaoyao Li , Isaku Yamahata References: <20240320083945.991426-1-michael.roth@amd.com> <20240320083945.991426-20-michael.roth@amd.com> From: Paolo Bonzini Autocrypt: addr=pbonzini@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=170.10.133.124; envelope-from=pbonzini@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -24 X-Spam_score: -2.5 X-Spam_bar: -- X-Spam_report: (-2.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.417, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On 3/20/24 09:39, Michael Roth wrote: > Some subsystems like VFIO might disable ram block discard for > uncoordinated cases. Since kvm_convert_memory()/guest_memfd don't > implement a RamDiscardManager handler to convey discard operations to > various listeners like VFIO. > Because of this, sequences like the > following can result due to stale IOMMU mappings: Alternatively, should guest-memfd memory regions call ram_block_discard_require(true)? This will prevent VFIO from operating, but it will avoid consuming twice the memory. If desirable, guest-memfd support can be changed to implement an extension of RamDiscardManager that notifies about private/shared memory changes, and then guest-memfd would be able to support coordinated discard. But I wonder if that's doable at all - how common are shared<->private flips, and is it feasible to change the IOMMU page tables every time? If the real solution is SEV-TIO (which means essentially guest_memfd support for VFIO), calling ram_block_discard_require(true) may be the simplest stopgap solution. Paolo > - convert page shared->private > - discard shared page > - convert page private->shared > - new page is allocated > - issue DMA operations against that shared page > > Address this by taking ram_block_discard_is_enabled() into account when > deciding whether or not to discard pages. > > Signed-off-by: Michael Roth > --- > accel/kvm/kvm-all.c | 8 ++++++-- > 1 file changed, 6 insertions(+), 2 deletions(-) > > diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c > index 53ce4f091e..6ae03c880f 100644 > --- a/accel/kvm/kvm-all.c > +++ b/accel/kvm/kvm-all.c > @@ -2962,10 +2962,14 @@ static int kvm_convert_memory(hwaddr start, hwaddr size, bool to_private) > */ > return 0; > } else { > - ret = ram_block_discard_range(rb, offset, size); > + ret = ram_block_discard_is_disabled() > + ? ram_block_discard_range(rb, offset, size) > + : 0; > } > } else { > - ret = ram_block_discard_guest_memfd_range(rb, offset, size); > + ret = ram_block_discard_is_disabled() > + ? ram_block_discard_guest_memfd_range(rb, offset, size) > + : 0; > } > } else { > error_report("Convert non guest_memfd backed memory region "