[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH v10 13/17] vpci: add initial support for virtual PCI bus topology
Hi Volodymyr, On 17/11/2023 14:09, Volodymyr Babchuk wrote: Hi Stefano, Stefano Stabellini <sstabellini@xxxxxxxxxx> writes:On Fri, 17 Nov 2023, Volodymyr Babchuk wrote:I still think, no matter the BDF allocation scheme, that we should try to avoid as much as possible to have two different PCI Root Complex emulators. Ideally we would have only one PCI Root Complex emulated by Xen. Having 2 PCI Root Complexes both of them emulated by Xen would be tolerable but not ideal.But what is exactly wrong with this setup?[...]The worst case I would like to avoid is to have two PCI Root Complexes, one emulated by Xen and one emulated by QEMU.This is how our setup works right now.If we have: - a single PCI Root Complex emulated in Xen - Xen is safety certified - individual Virtio devices emulated by QEMU with grants for memory We can go very far in terms of being able to use Virtio in safety use-cases. We might even be able to use Virtio (frontends) in a SafeOS. On the other hand if we put an additional Root Complex in QEMU: - we pay a price in terms of complexity of the codebase - we pay a price in terms of resource utilization - we have one additional problem in terms of using this setup with a SafeOS (one more device emulated by a non-safe component) Having 2 PCI Root Complexes both emulated in Xen is a middle ground solution because: - we still pay a price in terms of resource utilization - the code complexity goes up a bit but hopefully not by much - there is no impact on safety compared to the ideal scenario This is why I wrote that it is tolerable.Ah, I see now. Yes, I am agree with this. Also I want to add some more points: - There is ongoing work on implementing virtio backends as a separate applications, written in Rust. Linaro are doing this part. Right now they are implementing only virtio-mmio, but if they want to provide virtio-pci as well, they will need a mechanism to plug only virtio-pci, without Root Complex. This is argument for using single Root Complex emulated in Xen. - As far as I know (actually, Oleksandr told this to me), QEMU has no mechanism for exposing virtio-pci backends without exposing PCI root complex as well. Architecturally, there should be a PCI bus to which virtio-pci devices are connected. Or we need to make some changes to QEMU internals to be able to create virtio-pci backends that are not connected to any bus. Also, added benefit that PCI Root Complex emulator in QEMU handles legacy PCI interrupts for us. This is argument for separate Root Complex for QEMU. As right now we have only virtio-pci backends provided by QEMU and this setup is already working, I propose to stick to this solution. Especially, taking into account that it does not require any changes to hypervisor code. I am not against two hostbridge as a temporary solution as long as this is not a one way door decision. I am not concerned about the hypervisor itself, I am more concerned about the interface exposed by the toolstack and QEMU. To clarify, I don't particular want to have to maintain the two hostbridges solution once we can use a single hostbridge. So we need to be able to get rid of it without impacting the interface too much. Cheers, -- Julien Grall
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