From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [PATCH v2 1/3] iommu/virtio: Add topology description to virtio-iommu config space Date: Wed, 4 Mar 2020 14:34:33 -0500 Message-ID: <20200304142838-mutt-send-email-mst@kernel.org> References: <20200228172537.377327-1-jean-philippe@linaro.org> <20200228172537.377327-2-jean-philippe@linaro.org> <20200302161611.GD7829@8bytes.org> <9004f814-2f7c-9024-3465-6f9661b97b7a@redhat.com> <20200303130155.GA13185@8bytes.org> <20200303084753-mutt-send-email-mst@kernel.org> <20200303155318.GA3954@8bytes.org> <20200303105523-mutt-send-email-mst@kernel.org> <20200304133707.GB4177@8bytes.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20200304133707.GB4177@8bytes.org> Sender: linux-pci-owner@vger.kernel.org To: Joerg Roedel Cc: Auger Eric , Jean-Philippe Brucker , iommu@lists.linux-foundation.org, virtualization@lists.linux-foundation.org, linux-pci@vger.kernel.org, bhelgaas@google.com, jasowang@redhat.com, kevin.tian@intel.com, sebastien.boeuf@intel.com, jacob.jun.pan@intel.com, robin.murphy@arm.com List-Id: virtualization@lists.linuxfoundation.org On Wed, Mar 04, 2020 at 02:37:08PM +0100, Joerg Roedel wrote: > Hi Michael, > > On Tue, Mar 03, 2020 at 11:09:41AM -0500, Michael S. Tsirkin wrote: > > No. It's coded into the hardware. Which might even be practical > > for bare-metal (e.g. on-board flash), but is very practical > > when the device is part of a hypervisor. > > If its that way on PPC, than fine for them. But since this is enablement > for x86, it should follow the x86 platform best practices, and that > means describing hardware through ACPI. For hardware, sure. Hypervisors aren't hardware though and a bunch of hypervisors don't use ACPI. > > This "hardware" is actually part of hypervisor so there's no > > reason it can't be completely self-descriptive. It's specified > > by the same entity as the "firmware". > > That is just an implementation detail. Yes, QEMU emulates the hardware > and builds the ACPI tables. But it could also be implemented in a way > where the ACPI tables are build by guest firmware. All these extra levels of indirection is one of the reasons hypervisors such as kata try to avoid ACPI. > > I don't see why it would be much faster. The interface isn't that > > different from command queues of VTD. > > VirtIO IOMMU doesn't need to build page-tables that the hypervisor then > has to shadow, which makes things much faster. If you emulate one of the > other IOMMUs (VT-d or AMD-Vi) the code has to shadow the full page-table > at once when device passthrough is used. VirtIO-IOMMU doesn't need that, > and that makes it much faster and efficient. IIUC VT-d at least supports range invalidations. > > > Making ACPI meet the goals of embedded projects such as kata containers > > would be a gigantic task with huge stability implications. By > > comparison this 400-line parser is well contained and does the job. I > > didn't yet see compelling reasons not to merge this, but I'll be > > interested to see some more specific concerns. > > An ACPI table parse wouldn't need more lines of code. It realies on ACPI OSPM itself to handle ACPI bytecode, which is huge. > For embedded > systems there is still the DT way of describing things. For some embedded systems. > Regards, > > Joerg