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On this issue, I'm with Microsoft: it isn't Microsoft's responsibility to check if every third party AV product has not done silly things to the OS that would make systems fail when the Meltdown patch is applied. MS have simply said to the AV companies "This is what the Meltdown patch does. You know what you products do, so you are the ones to decide whether your products are compatible with the Meltdown patch". Short of not issuing any patch, I don't see what other choice MS have.