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[2003:cb:c709:6200:16ba:af70:999d:6a1a]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-437c74c4e38sm227863345e9.21.2025.01.21.00.05.36 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 21 Jan 2025 00:05:36 -0800 (PST) Message-ID: <5fd5f718-ec08-4b38-827f-99d13bc7e225@redhat.com> Date: Tue, 21 Jan 2025 09:05:35 +0100 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: [PATCH 6/7] RAMBlock: make guest_memfd require coordinate discard To: Chenyi Qiang , Paolo Bonzini , Peter Xu , =?UTF-8?Q?Philippe_Mathieu-Daud=C3=A9?= , Michael Roth Cc: qemu-devel@nongnu.org, kvm@vger.kernel.org, Williams Dan J , Peng Chao P , Gao Chao , Xu Yilun References: <20241213070852.106092-1-chenyi.qiang@intel.com> <20241213070852.106092-7-chenyi.qiang@intel.com> <3e23b5b0-963c-4ca1-a26b-dd5f247a3a60@redhat.com> <46fcd4fd-999a-46ac-a268-e3651b94ef49@intel.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 21.01.25 07:26, Chenyi Qiang wrote: > > > On 1/20/2025 9:11 PM, David Hildenbrand wrote: >> On 14.01.25 02:38, Chenyi Qiang wrote: >>> >>> >>> On 1/13/2025 6:56 PM, David Hildenbrand wrote: >>>> On 13.12.24 08:08, Chenyi Qiang wrote: >>>>> As guest_memfd is now managed by guest_memfd_manager with >>>>> RamDiscardManager, only block uncoordinated discard. >>>>> >>>>> Signed-off-by: Chenyi Qiang >>>>> --- >>>>>    system/physmem.c | 2 +- >>>>>    1 file changed, 1 insertion(+), 1 deletion(-) >>>>> >>>>> diff --git a/system/physmem.c b/system/physmem.c >>>>> index 532182a6dd..585090b063 100644 >>>>> --- a/system/physmem.c >>>>> +++ b/system/physmem.c >>>>> @@ -1872,7 +1872,7 @@ static void ram_block_add(RAMBlock *new_block, >>>>> Error **errp) >>>>>            assert(kvm_enabled()); >>>>>            assert(new_block->guest_memfd < 0); >>>>>    -        ret = ram_block_discard_require(true); >>>>> +        ret = ram_block_coordinated_discard_require(true); >>>>>            if (ret < 0) { >>>>>                error_setg_errno(errp, -ret, >>>>>                                 "cannot set up private guest memory: >>>>> discard currently blocked"); >>>> >>>> Would that also unlock virtio-mem by accident? >>> >>> Hum, that's true. At present, the rdm in MR can only point to one >>> instance, thus if we unlock virtio-mem and try to use it with >>> guest_memfd, it would trigger assert in >>> memory_region_set_ram_discard_manager(). >>> >>> Maybe we need to add some explicit check in virtio-mem to exclude it >>> with guest_memfd at present? >> >> Likely we should make memory_region_set_ram_discard_manager() fail if >> there is already something, and handle it in the callers? >> >> In case of virtio-mem, we'd have to undo what we did and fail realize(). >> >> In case of CC, we'd have to bail out in a different way. >> >> >> Then, I think if we see new_block->guest_memfd here, that we can assume >> that any coordinated discard corresponds to only the guest_memfd one, >> not to anything else? > > LGTM. In case of CC, I think we can also check the > memory_region_set_ram_discard_manager() failure, undo what we did and > make the ram_block_add() fail (set errno). As we have memory_region_has_ram_discard_manager(), we could also check that instead of failing memory_region_set_ram_discard_manager(). But failing memory_region_set_ram_discard_manager() will force everybody to handle that, so it might be the better choice. Of course, setting it to "NULL" should be guaranteed to never fail. -- Cheers, David / dhildenb