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Subject: Re: [virtio-comment] [PATCH v1 1/8] admin: Add theory of operation for device migration
On Wed, Oct 25, 2023 at 3:03âPM Parav Pandit <parav@nvidia.com> wrote: > > > > From: Jason Wang <jasowang@redhat.com> > > Sent: Wednesday, October 25, 2023 6:59 AM > > > For passthrough PASID assignment vq is not needed. > > > > How do you know that? > Because for passthrough, the hypervisor is not involved in dealing with VQ at all. Ok, so if I understand correctly, you are saying your design can't work for the case of PASID assignment. > > > There are works ongoing to make vPASID work for the > > guest like vSVA. Virtio doesn't differ from other devices. > Passthrough do not run like SVA. Great, you find another limitation of "passthrough" by yourself. > Each passthrough device has PASID from its own space fully managed by the guest. > Some cpu required vPASID and SIOV is not going this way anmore. Then how to migrate? Invent a full set of something else through another giant series like this to migrate to the SIOV thing? That's a mess for sure. > > > > > > If at all it is done, it will be done from the guest by the driver using virtio > > interface. > > > > Then you need to trap. Such things couldn't be passed through to guests directly. > > > Only PASID capability is trapped. PASID allocation and usage is directly from guest. How can you achieve this? Assigning a PAISD to a device is completely device(virtio) specific. How can you use a general layer without the knowledge of virtio to trap that? > Regardless it is not relevant to passthrough mode as PASID is yet another resource. > And for some cpu if it is trapped, it is generic layer, that does not require virtio involvement. > So virtio interface asking to trap something because generic facility has done in not the approach. This misses the point of PASID. How to use PASID is totally device specific. > > > > Capabilities of #2 is generic across all pci devices, so it will be handled by the > > HV. > > > ATS/PRI cap is also generic manner handled by the HV and PCI device. > > > > No, ATS/PRI requires the cooperation from the vIOMMU. You can simply do > > ATS/PRI passthrough but with an emulated vIOMMU. > And that is not the reason for virtio device to build trap+emulation for passthrough member devices. vIOMMU is emulated by hypervisor with a PRI queue, how can you pass through a hardware PRI request to a guest directly without trapping it then? What's more, PCIE allows the PRI to be done in a vendor (virtio) specific way, so you want to break this rule? Or you want to blacklist ATS/PRI for virtio? Thanks
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