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    SubjectRe: [RFC PATCH v3 4/7] bus/cdx: add cdx-MSI domain with gic-its domain as parent
    On Wed, Oct 12, 2022 at 03:09:26PM +0000, Radovanovic, Aleksandar wrote:

    > > On Wed, Oct 12, 2022 at 01:37:54PM +0000, Radovanovic, Aleksandar wrote:
    > > > > On Wed, Oct 12, 2022 at 10:34:23AM +0000, Radovanovic, Aleksandar
    > > wrote:
    > > > >
    > > > >
    > > > > > As for GITS_TRANSLATER, we can take up to 4 different IOVAs, which
    > > > > > limits us to 4 CDX devices (should be sufficient for current HW
    > > > > > use-cases). Also, it means that the address part must be the same
    > > > > > for all vectors within a single CDX device. I'm assuming this is
    > > > > > OK as it is going to be a single interrupt and IOMMU domain anyway.
    > > > >
    > > > > This is not at all how MSI is supposed to work.
    > > >
    > > > In the general case, no, they're not.
    > >
    > > I don't mean that you can hack this to work - I mean that in MSI the
    > > addr/data is supposed to come from the end point itself, not from some kind
    > > of shared structure. This is important because the actual act of generating
    > > the write has to be coherent with the DMA the device is doing, as the MSI
    > > write must push any DMA data to visibility to meet the "producer /
    > > consumer" model.
    > >
    >
    > I'm not sure I follow your argument, the limitation here is that the MSI
    > address value is shared between vectors of the same device (requester id
    > or endpoint, whichever way you prefer to call it), not between
    > devices.

    That isn't what you said, you said "we can take up to 4 different
    IOVAs, which limits us to 4 CDX devices" - which sounds like HW being
    shared across devices??

    Jason

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